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entry_64.S
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entry_64.S
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/* SPDX-License-Identifier: GPL-2.0 */
/*
* linux/arch/x86_64/entry.S
*
* Copyright (C) 1991, 1992 Linus Torvalds
* Copyright (C) 2000, 2001, 2002 Andi Kleen SuSE Labs
* Copyright (C) 2000 Pavel Machek <pavel@suse.cz>
*
* entry.S contains the system-call and fault low-level handling routines.
*
* Some of this is documented in Documentation/x86/entry_64.rst
*
* A note on terminology:
* - iret frame: Architecture defined interrupt frame from SS to RIP
* at the top of the kernel process stack.
*
* Some macro usage:
* - SYM_FUNC_START/END:Define functions in the symbol table.
* - idtentry: Define exception entry points.
*/
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/cache.h>
#include <asm/errno.h>
#include <asm/asm-offsets.h>
#include <asm/msr.h>
#include <asm/unistd.h>
#include <asm/thread_info.h>
#include <asm/hw_irq.h>
#include <asm/page_types.h>
#include <asm/irqflags.h>
#include <asm/paravirt.h>
#include <asm/percpu.h>
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/pgtable_types.h>
#include <asm/export.h>
#include <asm/frame.h>
#include <asm/trapnr.h>
#include <asm/nospec-branch.h>
#include <asm/fsgsbase.h>
#include <linux/err.h>
#include "calling.h"
.code64
.section .entry.text, "ax"
/*
* 64-bit SYSCALL instruction entry. Up to 6 arguments in registers.
*
* This is the only entry point used for 64-bit system calls. The
* hardware interface is reasonably well designed and the register to
* argument mapping Linux uses fits well with the registers that are
* available when SYSCALL is used.
*
* SYSCALL instructions can be found inlined in libc implementations as
* well as some other programs and libraries. There are also a handful
* of SYSCALL instructions in the vDSO used, for example, as a
* clock_gettimeofday fallback.
*
* 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
* then loads new ss, cs, and rip from previously programmed MSRs.
* rflags gets masked by a value from another MSR (so CLD and CLAC
* are not needed). SYSCALL does not save anything on the stack
* and does not change rsp.
*
* Registers on entry:
* rax system call number
* rcx return address
* r11 saved rflags (note: r11 is callee-clobbered register in C ABI)
* rdi arg0
* rsi arg1
* rdx arg2
* r10 arg3 (needs to be moved to rcx to conform to C ABI)
* r8 arg4
* r9 arg5
* (note: r12-r15, rbp, rbx are callee-preserved in C ABI)
*
* Only called from user space.
*
* When user can change pt_regs->foo always force IRET. That is because
* it deals with uncanonical addresses better. SYSRET has trouble
* with them due to bugs in both AMD and Intel CPUs.
*/
SYM_CODE_START(entry_SYSCALL_64)
UNWIND_HINT_EMPTY
swapgs
/* tss.sp2 is scratch space. */
movq %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
SYM_INNER_LABEL(entry_SYSCALL_64_safe_stack, SYM_L_GLOBAL)
/* Construct struct pt_regs on stack */
pushq $__USER_DS /* pt_regs->ss */
pushq PER_CPU_VAR(cpu_tss_rw + TSS_sp2) /* pt_regs->sp */
pushq %r11 /* pt_regs->flags */
pushq $__USER_CS /* pt_regs->cs */
pushq %rcx /* pt_regs->ip */
SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
pushq %rax /* pt_regs->orig_ax */
PUSH_AND_CLEAR_REGS rax=$-ENOSYS
/* IRQs are off. */
movq %rax, %rdi
movq %rsp, %rsi
call do_syscall_64 /* returns with IRQs disabled */
/*
* Try to use SYSRET instead of IRET if we're returning to
* a completely clean 64-bit userspace context. If we're not,
* go to the slow exit path.
* In the Xen PV case we must use iret anyway.
*/
ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", \
X86_FEATURE_XENPV
movq RCX(%rsp), %rcx
movq RIP(%rsp), %r11
cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
jne swapgs_restore_regs_and_return_to_usermode
/*
* On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
* in kernel space. This essentially lets the user take over
* the kernel, since userspace controls RSP.
*
* If width of "canonical tail" ever becomes variable, this will need
* to be updated to remain correct on both old and new CPUs.
*
* Change top bits to match most significant bit (47th or 56th bit
* depending on paging mode) in the address.
*/
#ifdef CONFIG_X86_5LEVEL
ALTERNATIVE "shl $(64 - 48), %rcx; sar $(64 - 48), %rcx", \
"shl $(64 - 57), %rcx; sar $(64 - 57), %rcx", X86_FEATURE_LA57
#else
shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
#endif
/* If this changed %rcx, it was not canonical */
cmpq %rcx, %r11
jne swapgs_restore_regs_and_return_to_usermode
cmpq $__USER_CS, CS(%rsp) /* CS must match SYSRET */
jne swapgs_restore_regs_and_return_to_usermode
movq R11(%rsp), %r11
cmpq %r11, EFLAGS(%rsp) /* R11 == RFLAGS */
jne swapgs_restore_regs_and_return_to_usermode
/*
* SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
* restore RF properly. If the slowpath sets it for whatever reason, we
* need to restore it correctly.
*
* SYSRET can restore TF, but unlike IRET, restoring TF results in a
* trap from userspace immediately after SYSRET. This would cause an
* infinite loop whenever #DB happens with register state that satisfies
* the opportunistic SYSRET conditions. For example, single-stepping
* this user code:
*
* movq $stuck_here, %rcx
* pushfq
* popq %r11
* stuck_here:
*
* would never get past 'stuck_here'.
*/
testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
jnz swapgs_restore_regs_and_return_to_usermode
/* nothing to check for RSP */
cmpq $__USER_DS, SS(%rsp) /* SS must match SYSRET */
jne swapgs_restore_regs_and_return_to_usermode
/*
* We win! This label is here just for ease of understanding
* perf profiles. Nothing jumps here.
*/
syscall_return_via_sysret:
/* rcx and r11 are already restored (see code above) */
POP_REGS pop_rdi=0 skip_r11rcx=1
/*
* Now all regs are restored except RSP and RDI.
* Save old stack pointer and switch to trampoline stack.
*/
movq %rsp, %rdi
movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
UNWIND_HINT_EMPTY
pushq RSP-RDI(%rdi) /* RSP */
pushq (%rdi) /* RDI */
/*
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
STACKLEAK_ERASE_NOCLOBBER
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
popq %rdi
popq %rsp
swapgs
sysretq
SYM_CODE_END(entry_SYSCALL_64)
/*
* %rdi: prev task
* %rsi: next task
*/
.pushsection .text, "ax"
SYM_FUNC_START(__switch_to_asm)
/*
* Save callee-saved registers
* This must match the order in inactive_task_frame
*/
pushq %rbp
pushq %rbx
pushq %r12
pushq %r13
pushq %r14
pushq %r15
/* switch stack */
movq %rsp, TASK_threadsp(%rdi)
movq TASK_threadsp(%rsi), %rsp
#ifdef CONFIG_STACKPROTECTOR
movq TASK_stack_canary(%rsi), %rbx
movq %rbx, PER_CPU_VAR(fixed_percpu_data) + stack_canary_offset
#endif
#ifdef CONFIG_RETPOLINE
/*
* When switching from a shallower to a deeper call stack
* the RSB may either underflow or use entries populated
* with userspace addresses. On CPUs where those concerns
* exist, overwrite the RSB with entries which capture
* speculative execution to prevent attack.
*/
FILL_RETURN_BUFFER %r12, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_CTXSW
#endif
/* restore callee-saved registers */
popq %r15
popq %r14
popq %r13
popq %r12
popq %rbx
popq %rbp
jmp __switch_to
SYM_FUNC_END(__switch_to_asm)
.popsection
/*
* A newly forked process directly context switches into this address.
*
* rax: prev task we switched from
* rbx: kernel thread func (NULL for user thread)
* r12: kernel thread arg
*/
.pushsection .text, "ax"
SYM_CODE_START(ret_from_fork)
UNWIND_HINT_EMPTY
movq %rax, %rdi
call schedule_tail /* rdi: 'prev' task parameter */
testq %rbx, %rbx /* from kernel_thread? */
jnz 1f /* kernel threads are uncommon */
2:
UNWIND_HINT_REGS
movq %rsp, %rdi
call syscall_exit_to_user_mode /* returns with IRQs disabled */
jmp swapgs_restore_regs_and_return_to_usermode
1:
/* kernel thread */
UNWIND_HINT_EMPTY
movq %r12, %rdi
CALL_NOSPEC rbx
/*
* A kernel thread is allowed to return here after successfully
* calling kernel_execve(). Exit to userspace to complete the execve()
* syscall.
*/
movq $0, RAX(%rsp)
jmp 2b
SYM_CODE_END(ret_from_fork)
.popsection
.macro DEBUG_ENTRY_ASSERT_IRQS_OFF
#ifdef CONFIG_DEBUG_ENTRY
pushq %rax
SAVE_FLAGS(CLBR_RAX)
testl $X86_EFLAGS_IF, %eax
jz .Lokay_\@
ud2
.Lokay_\@:
popq %rax
#endif
.endm
/**
* idtentry_body - Macro to emit code calling the C function
* @cfunc: C function to be called
* @has_error_code: Hardware pushed error code on stack
*/
.macro idtentry_body cfunc has_error_code:req
call error_entry
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs pointer into 1st argument*/
.if \has_error_code == 1
movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
.endif
call \cfunc
jmp error_return
.endm
/**
* idtentry - Macro to generate entry stubs for simple IDT entries
* @vector: Vector number
* @asmsym: ASM symbol for the entry point
* @cfunc: C function to be called
* @has_error_code: Hardware pushed error code on stack
*
* The macro emits code to set up the kernel context for straight forward
* and simple IDT entries. No IST stack, no paranoid entry checks.
*/
.macro idtentry vector asmsym cfunc has_error_code:req
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS offset=\has_error_code*8
ASM_CLAC
.if \has_error_code == 0
pushq $-1 /* ORIG_RAX: no syscall to restart */
.endif
.if \vector == X86_TRAP_BP
/*
* If coming from kernel space, create a 6-word gap to allow the
* int3 handler to emulate a call instruction.
*/
testb $3, CS-ORIG_RAX(%rsp)
jnz .Lfrom_usermode_no_gap_\@
.rept 6
pushq 5*8(%rsp)
.endr
UNWIND_HINT_IRET_REGS offset=8
.Lfrom_usermode_no_gap_\@:
.endif
idtentry_body \cfunc \has_error_code
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
/*
* Interrupt entry/exit.
*
+ The interrupt stubs push (vector) onto the stack, which is the error_code
* position of idtentry exceptions, and jump to one of the two idtentry points
* (common/spurious).
*
* common_interrupt is a hotpath, align it to a cache line
*/
.macro idtentry_irq vector cfunc
.p2align CONFIG_X86_L1_CACHE_SHIFT
idtentry \vector asm_\cfunc \cfunc has_error_code=1
.endm
/*
* System vectors which invoke their handlers directly and are not
* going through the regular common device interrupt handling code.
*/
.macro idtentry_sysvec vector cfunc
idtentry \vector asm_\cfunc \cfunc has_error_code=0
.endm
/**
* idtentry_mce_db - Macro to generate entry stubs for #MC and #DB
* @vector: Vector number
* @asmsym: ASM symbol for the entry point
* @cfunc: C function to be called
*
* The macro emits code to set up the kernel context for #MC and #DB
*
* If the entry comes from user space it uses the normal entry path
* including the return to user space work and preemption checks on
* exit.
*
* If hits in kernel mode then it needs to go through the paranoid
* entry as the exception can hit any random state. No preemption
* check on exit to keep the paranoid path simple.
*/
.macro idtentry_mce_db vector asmsym cfunc
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS
ASM_CLAC
pushq $-1 /* ORIG_RAX: no syscall to restart */
/*
* If the entry is from userspace, switch stacks and treat it as
* a normal entry.
*/
testb $3, CS-ORIG_RAX(%rsp)
jnz .Lfrom_usermode_switch_stack_\@
/* paranoid_entry returns GS information for paranoid_exit in EBX. */
call paranoid_entry
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs pointer */
call \cfunc
jmp paranoid_exit
/* Switch to the regular task stack and use the noist entry point */
.Lfrom_usermode_switch_stack_\@:
idtentry_body noist_\cfunc, has_error_code=0
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
#ifdef CONFIG_AMD_MEM_ENCRYPT
/**
* idtentry_vc - Macro to generate entry stub for #VC
* @vector: Vector number
* @asmsym: ASM symbol for the entry point
* @cfunc: C function to be called
*
* The macro emits code to set up the kernel context for #VC. The #VC handler
* runs on an IST stack and needs to be able to cause nested #VC exceptions.
*
* To make this work the #VC entry code tries its best to pretend it doesn't use
* an IST stack by switching to the task stack if coming from user-space (which
* includes early SYSCALL entry path) or back to the stack in the IRET frame if
* entered from kernel-mode.
*
* If entered from kernel-mode the return stack is validated first, and if it is
* not safe to use (e.g. because it points to the entry stack) the #VC handler
* will switch to a fall-back stack (VC2) and call a special handler function.
*
* The macro is only used for one vector, but it is planned to be extended in
* the future for the #HV exception.
*/
.macro idtentry_vc vector asmsym cfunc
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS
ASM_CLAC
/*
* If the entry is from userspace, switch stacks and treat it as
* a normal entry.
*/
testb $3, CS-ORIG_RAX(%rsp)
jnz .Lfrom_usermode_switch_stack_\@
/*
* paranoid_entry returns SWAPGS flag for paranoid_exit in EBX.
* EBX == 0 -> SWAPGS, EBX == 1 -> no SWAPGS
*/
call paranoid_entry
UNWIND_HINT_REGS
/*
* Switch off the IST stack to make it free for nested exceptions. The
* vc_switch_off_ist() function will switch back to the interrupted
* stack if it is safe to do so. If not it switches to the VC fall-back
* stack.
*/
movq %rsp, %rdi /* pt_regs pointer */
call vc_switch_off_ist
movq %rax, %rsp /* Switch to new stack */
UNWIND_HINT_REGS
/* Update pt_regs */
movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
movq %rsp, %rdi /* pt_regs pointer */
call \cfunc
/*
* No need to switch back to the IST stack. The current stack is either
* identical to the stack in the IRET frame or the VC fall-back stack,
* so it is definitly mapped even with PTI enabled.
*/
jmp paranoid_exit
/* Switch to the regular task stack */
.Lfrom_usermode_switch_stack_\@:
idtentry_body safe_stack_\cfunc, has_error_code=1
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
#endif
/*
* Double fault entry. Straight paranoid. No checks from which context
* this comes because for the espfix induced #DF this would do the wrong
* thing.
*/
.macro idtentry_df vector asmsym cfunc
SYM_CODE_START(\asmsym)
UNWIND_HINT_IRET_REGS offset=8
ASM_CLAC
/* paranoid_entry returns GS information for paranoid_exit in EBX. */
call paranoid_entry
UNWIND_HINT_REGS
movq %rsp, %rdi /* pt_regs pointer into first argument */
movq ORIG_RAX(%rsp), %rsi /* get error code into 2nd argument*/
movq $-1, ORIG_RAX(%rsp) /* no syscall to restart */
call \cfunc
jmp paranoid_exit
_ASM_NOKPROBE(\asmsym)
SYM_CODE_END(\asmsym)
.endm
/*
* Include the defines which emit the idt entries which are shared
* shared between 32 and 64 bit and emit the __irqentry_text_* markers
* so the stacktrace boundary checks work.
*/
.align 16
.globl __irqentry_text_start
__irqentry_text_start:
#include <asm/idtentry.h>
.align 16
.globl __irqentry_text_end
__irqentry_text_end:
SYM_CODE_START_LOCAL(common_interrupt_return)
SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
#ifdef CONFIG_DEBUG_ENTRY
/* Assert that pt_regs indicates user mode. */
testb $3, CS(%rsp)
jnz 1f
ud2
1:
#endif
POP_REGS pop_rdi=0
/*
* The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
* Save old stack pointer and switch to trampoline stack.
*/
movq %rsp, %rdi
movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
UNWIND_HINT_EMPTY
/* Copy the IRET frame to the trampoline stack. */
pushq 6*8(%rdi) /* SS */
pushq 5*8(%rdi) /* RSP */
pushq 4*8(%rdi) /* EFLAGS */
pushq 3*8(%rdi) /* CS */
pushq 2*8(%rdi) /* RIP */
/* Push user RDI on the trampoline stack. */
pushq (%rdi)
/*
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
STACKLEAK_ERASE_NOCLOBBER
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
/* Restore RDI. */
popq %rdi
SWAPGS
INTERRUPT_RETURN
SYM_INNER_LABEL(restore_regs_and_return_to_kernel, SYM_L_GLOBAL)
#ifdef CONFIG_DEBUG_ENTRY
/* Assert that pt_regs indicates kernel mode. */
testb $3, CS(%rsp)
jz 1f
ud2
1:
#endif
POP_REGS
addq $8, %rsp /* skip regs->orig_ax */
/*
* ARCH_HAS_MEMBARRIER_SYNC_CORE rely on IRET core serialization
* when returning from IPI handler.
*/
INTERRUPT_RETURN
SYM_INNER_LABEL_ALIGN(native_iret, SYM_L_GLOBAL)
UNWIND_HINT_IRET_REGS
/*
* Are we returning to a stack segment from the LDT? Note: in
* 64-bit mode SS:RSP on the exception stack is always valid.
*/
#ifdef CONFIG_X86_ESPFIX64
testb $4, (SS-RIP)(%rsp)
jnz native_irq_return_ldt
#endif
SYM_INNER_LABEL(native_irq_return_iret, SYM_L_GLOBAL)
/*
* This may fault. Non-paranoid faults on return to userspace are
* handled by fixup_bad_iret. These include #SS, #GP, and #NP.
* Double-faults due to espfix64 are handled in exc_double_fault.
* Other faults here are fatal.
*/
iretq
#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
/*
* We are running with user GSBASE. All GPRs contain their user
* values. We have a percpu ESPFIX stack that is eight slots
* long (see ESPFIX_STACK_SIZE). espfix_waddr points to the bottom
* of the ESPFIX stack.
*
* We clobber RAX and RDI in this code. We stash RDI on the
* normal stack and RAX on the ESPFIX stack.
*
* The ESPFIX stack layout we set up looks like this:
*
* --- top of ESPFIX stack ---
* SS
* RSP
* RFLAGS
* CS
* RIP <-- RSP points here when we're done
* RAX <-- espfix_waddr points here
* --- bottom of ESPFIX stack ---
*/
pushq %rdi /* Stash user RDI */
swapgs /* to kernel GS */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */
movq PER_CPU_VAR(espfix_waddr), %rdi
movq %rax, (0*8)(%rdi) /* user RAX */
movq (1*8)(%rsp), %rax /* user RIP */
movq %rax, (1*8)(%rdi)
movq (2*8)(%rsp), %rax /* user CS */
movq %rax, (2*8)(%rdi)
movq (3*8)(%rsp), %rax /* user RFLAGS */
movq %rax, (3*8)(%rdi)
movq (5*8)(%rsp), %rax /* user SS */
movq %rax, (5*8)(%rdi)
movq (4*8)(%rsp), %rax /* user RSP */
movq %rax, (4*8)(%rdi)
/* Now RAX == RSP. */
andl $0xffff0000, %eax /* RAX = (RSP & 0xffff0000) */
/*
* espfix_stack[31:16] == 0. The page tables are set up such that
* (espfix_stack | (X & 0xffff0000)) points to a read-only alias of
* espfix_waddr for any X. That is, there are 65536 RO aliases of
* the same page. Set up RSP so that RSP[31:16] contains the
* respective 16 bits of the /userspace/ RSP and RSP nonetheless
* still points to an RO alias of the ESPFIX stack.
*/
orq PER_CPU_VAR(espfix_stack), %rax
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi
swapgs /* to user GS */
popq %rdi /* Restore user RDI */
movq %rax, %rsp
UNWIND_HINT_IRET_REGS offset=8
/*
* At this point, we cannot write to the stack any more, but we can
* still read.
*/
popq %rax /* Restore user RAX */
/*
* RSP now points to an ordinary IRET frame, except that the page
* is read-only and RSP[31:16] are preloaded with the userspace
* values. We can now IRET back to userspace.
*/
jmp native_irq_return_iret
#endif
SYM_CODE_END(common_interrupt_return)
_ASM_NOKPROBE(common_interrupt_return)
/*
* Reload gs selector with exception handling
* edi: new selector
*
* Is in entry.text as it shouldn't be instrumented.
*/
SYM_FUNC_START(asm_load_gs_index)
FRAME_BEGIN
swapgs
.Lgs_change:
movl %edi, %gs
2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
swapgs
FRAME_END
ret
SYM_FUNC_END(asm_load_gs_index)
EXPORT_SYMBOL(asm_load_gs_index)
_ASM_EXTABLE(.Lgs_change, .Lbad_gs)
.section .fixup, "ax"
/* running with kernelgs */
SYM_CODE_START_LOCAL_NOALIGN(.Lbad_gs)
swapgs /* switch back to user gs */
.macro ZAP_GS
/* This can't be a string because the preprocessor needs to see it. */
movl $__USER_DS, %eax
movl %eax, %gs
.endm
ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
xorl %eax, %eax
movl %eax, %gs
jmp 2b
SYM_CODE_END(.Lbad_gs)
.previous
#ifdef CONFIG_XEN_PV
/*
* A note on the "critical region" in our callback handler.
* We want to avoid stacking callback handlers due to events occurring
* during handling of the last event. To do this, we keep events disabled
* until we've done all processing. HOWEVER, we must enable events before
* popping the stack frame (can't be done atomically) and so it would still
* be possible to get enough handler activations to overflow the stack.
* Although unlikely, bugs of that kind are hard to track down, so we'd
* like to avoid the possibility.
* So, on entry to the handler we detect whether we interrupted an
* existing activation in its critical region -- if so, we pop the current
* activation and restart the handler using the previous one.
*
* C calling convention: exc_xen_hypervisor_callback(struct *pt_regs)
*/
SYM_CODE_START_LOCAL(exc_xen_hypervisor_callback)
/*
* Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
* see the correct pointer to the pt_regs
*/
UNWIND_HINT_FUNC
movq %rdi, %rsp /* we don't return, adjust the stack frame */
UNWIND_HINT_REGS
call xen_pv_evtchn_do_upcall
jmp error_return
SYM_CODE_END(exc_xen_hypervisor_callback)
/*
* Hypervisor uses this for application faults while it executes.
* We get here for two reasons:
* 1. Fault while reloading DS, ES, FS or GS
* 2. Fault while executing IRET
* Category 1 we do not need to fix up as Xen has already reloaded all segment
* registers that could be reloaded and zeroed the others.
* Category 2 we fix up by killing the current process. We cannot use the
* normal Linux return path in this case because if we use the IRET hypercall
* to pop the stack frame we end up in an infinite loop of failsafe callbacks.
* We distinguish between categories by comparing each saved segment register
* with its current contents: any discrepancy means we in category 1.
*/
SYM_CODE_START(xen_failsafe_callback)
UNWIND_HINT_EMPTY
movl %ds, %ecx
cmpw %cx, 0x10(%rsp)
jne 1f
movl %es, %ecx
cmpw %cx, 0x18(%rsp)
jne 1f
movl %fs, %ecx
cmpw %cx, 0x20(%rsp)
jne 1f
movl %gs, %ecx
cmpw %cx, 0x28(%rsp)
jne 1f
/* All segments match their saved values => Category 2 (Bad IRET). */
movq (%rsp), %rcx
movq 8(%rsp), %r11
addq $0x30, %rsp
pushq $0 /* RIP */
UNWIND_HINT_IRET_REGS offset=8
jmp asm_exc_general_protection
1: /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
movq (%rsp), %rcx
movq 8(%rsp), %r11
addq $0x30, %rsp
UNWIND_HINT_IRET_REGS
pushq $-1 /* orig_ax = -1 => not a system call */
PUSH_AND_CLEAR_REGS
ENCODE_FRAME_POINTER
jmp error_return
SYM_CODE_END(xen_failsafe_callback)
#endif /* CONFIG_XEN_PV */
/*
* Save all registers in pt_regs. Return GSBASE related information
* in EBX depending on the availability of the FSGSBASE instructions:
*
* FSGSBASE R/EBX
* N 0 -> SWAPGS on exit
* 1 -> no SWAPGS on exit
*
* Y GSBASE value at entry, must be restored in paranoid_exit
*/
SYM_CODE_START_LOCAL(paranoid_entry)
UNWIND_HINT_FUNC
cld
PUSH_AND_CLEAR_REGS save_ret=1
ENCODE_FRAME_POINTER 8
/*
* Always stash CR3 in %r14. This value will be restored,
* verbatim, at exit. Needed if paranoid_entry interrupted
* another entry that already switched to the user CR3 value
* but has not yet returned to userspace.
*
* This is also why CS (stashed in the "iret frame" by the
* hardware at entry) can not be used: this may be a return
* to kernel code, but with a user CR3 value.
*
* Switching CR3 does not depend on kernel GSBASE so it can
* be done before switching to the kernel GSBASE. This is
* required for FSGSBASE because the kernel GSBASE has to
* be retrieved from a kernel internal table.
*/
SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
/*
* Handling GSBASE depends on the availability of FSGSBASE.
*
* Without FSGSBASE the kernel enforces that negative GSBASE
* values indicate kernel GSBASE. With FSGSBASE no assumptions
* can be made about the GSBASE value when entering from user
* space.
*/
ALTERNATIVE "jmp .Lparanoid_entry_checkgs", "", X86_FEATURE_FSGSBASE
/*
* Read the current GSBASE and store it in %rbx unconditionally,
* retrieve and set the current CPUs kernel GSBASE. The stored value
* has to be restored in paranoid_exit unconditionally.
*
* The unconditional write to GS base below ensures that no subsequent
* loads based on a mispredicted GS base can happen, therefore no LFENCE
* is needed here.
*/
SAVE_AND_SET_GSBASE scratch_reg=%rax save_reg=%rbx
ret
.Lparanoid_entry_checkgs:
/* EBX = 1 -> kernel GSBASE active, no restore required */
movl $1, %ebx
/*
* The kernel-enforced convention is a negative GSBASE indicates
* a kernel value. No SWAPGS needed on entry and exit.
*/
movl $MSR_GS_BASE, %ecx
rdmsr
testl %edx, %edx
jns .Lparanoid_entry_swapgs
ret
.Lparanoid_entry_swapgs:
swapgs
/*
* The above SAVE_AND_SWITCH_TO_KERNEL_CR3 macro doesn't do an
* unconditional CR3 write, even in the PTI case. So do an lfence
* to prevent GS speculation, regardless of whether PTI is enabled.
*/
FENCE_SWAPGS_KERNEL_ENTRY
/* EBX = 0 -> SWAPGS required on exit */
xorl %ebx, %ebx
ret
SYM_CODE_END(paranoid_entry)
/*
* "Paranoid" exit path from exception stack. This is invoked
* only on return from non-NMI IST interrupts that came
* from kernel space.
*
* We may be returning to very strange contexts (e.g. very early
* in syscall entry), so checking for preemption here would
* be complicated. Fortunately, there's no good reason to try
* to handle preemption here.
*
* R/EBX contains the GSBASE related information depending on the
* availability of the FSGSBASE instructions:
*
* FSGSBASE R/EBX
* N 0 -> SWAPGS on exit
* 1 -> no SWAPGS on exit
*
* Y User space GSBASE, must be restored unconditionally
*/
SYM_CODE_START_LOCAL(paranoid_exit)
UNWIND_HINT_REGS
/*
* The order of operations is important. RESTORE_CR3 requires
* kernel GSBASE.
*
* NB to anyone to try to optimize this code: this code does
* not execute at all for exceptions from user mode. Those
* exceptions go through error_exit instead.
*/
RESTORE_CR3 scratch_reg=%rax save_reg=%r14
/* Handle the three GSBASE cases */
ALTERNATIVE "jmp .Lparanoid_exit_checkgs", "", X86_FEATURE_FSGSBASE
/* With FSGSBASE enabled, unconditionally restore GSBASE */
wrgsbase %rbx
jmp restore_regs_and_return_to_kernel
.Lparanoid_exit_checkgs:
/* On non-FSGSBASE systems, conditionally do SWAPGS */
testl %ebx, %ebx
jnz restore_regs_and_return_to_kernel
/* We are returning to a context with user GSBASE */
swapgs
jmp restore_regs_and_return_to_kernel
SYM_CODE_END(paranoid_exit)
/*
* Save all registers in pt_regs, and switch GS if needed.
*/
SYM_CODE_START_LOCAL(error_entry)
UNWIND_HINT_FUNC
cld
PUSH_AND_CLEAR_REGS save_ret=1
ENCODE_FRAME_POINTER 8
testb $3, CS+8(%rsp)
jz .Lerror_kernelspace
/*
* We entered from user mode or we're pretending to have entered
* from user mode due to an IRET fault.
*/
SWAPGS
FENCE_SWAPGS_USER_ENTRY
/* We have user CR3. Change to kernel CR3. */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
.Lerror_entry_from_usermode_after_swapgs:
/* Put us onto the real thread stack. */
popq %r12 /* save return addr in %12 */
movq %rsp, %rdi /* arg0 = pt_regs pointer */
call sync_regs
movq %rax, %rsp /* switch stack */
ENCODE_FRAME_POINTER
pushq %r12
ret
.Lerror_entry_done_lfence:
FENCE_SWAPGS_KERNEL_ENTRY
.Lerror_entry_done:
ret
/*