x86/entry: Preserve PKRS MSR across exceptions

The PKRS MSR is not managed by XSAVE.  It is preserved through a context
switch but this support leaves exception handling code open to memory
accesses during exceptions.

2 possible places for preserving this state were considered,
irqentry_state_t or pt_regs.[1]  pt_regs was much more complicated and
was potentially fraught with unintended consequences.[2]  However, Andy
came up with a way to hide additional values on the stack which could be
accessed as "extended_pt_regs".[3]  This method allows for; any place
which has struct pt_regs can get access to the extra information; no
extra information is added to irq_state; and pt_regs is left intact for
compatibility with outside tools like BPF.

To simplify, the assembly code only adds space on the stack.  The
setting or use of any needed values are left to the C code.  While some
entry points may not use this space it is still added where ever pt_regs
is passed to the C code for consistency.

Each nested exception gets another copy of this extended space allowing
for any number of levels of exception handling.

In the assembly, a macro is defined to allow a central place to add
space for other uses should the need arise.

Finally export pkrs_{save_set|restore}_irq to the common code to allow
it to preserve the current task's PKRS in the new extended pt_regs if
enabled.

Peter, Thomas, Andy, Dave, and Dan all suggested parts of the patch or
aided in the development of the patch..

[1] https://lore.kernel.org/lkml/CALCETrVe1i5JdyzD_BcctxQJn+ZE3T38EFPgjxN1F577M36g+w@mail.gmail.com/
[2] https://lore.kernel.org/lkml/874kpxx4jf.fsf@nanos.tec.linutronix.de/#t
[3] https://lore.kernel.org/lkml/CALCETrUHwZPic89oExMMe-WyDY8-O3W68NcZvse3=PGW+iW5=w@mail.gmail.com/

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

---
Changes from V3:
	Move all extended regs stuff to pks.h
	From Dan Williams
		Move show_extended_regs_oops ifdefery to pks.h
		Remove a bad comment
		s/irq_save_set_pkrs/pkrs_save_set_irq
		s/irq_restore_pkrs/pkrs_restore_irq
		s/ARCH_HAS/ARCH_ENABLE_SUPERVISOR_PKEYS
	From Dave Hansen:
		remove extra macro parameter for most calls
		clarify with comments
		Add BUILD check for extend regs size
		use subq/addq vs push/pop
	Fix 0-day issues
	From Dan Williams and Dave Hansen:
		Use a macro call to wrap the c function calls with
		push/pop extended_pt_regs
	From Dave Hansen:
		Guidance on where to find each of the pt_regs being
		passed to C code
	From Thomas Gleixner:
		Remove unnecessary noinstr's
	From Andy Lutomirski:
		Convert to using the extended pt_regs
		Add in showing pks on fault through the extended pt_regs

Changes from V1
	remove redundant irq_state->pkrs
		This value is only needed for the global tracking.  So
		it should be included in that patch and not in this one.

Changes from RFC V3
	Standardize on 'irq_state' variable name
	Per Dave Hansen
		irq_save_pkrs() -> irq_save_set_pkrs()
	Rebased based on clean up patch by Thomas Gleixner
		This includes moving irq_[save_set|restore]_pkrs() to
		the core as well.

arch/x86/entry/calling.h

@@ -97,6 +97,32 @@ For 32-bit we have the following conventions - kernel is built with
 
 #define SIZEOF_PTREGS	21*8
 
+/*
+ * __call_ext_ptregs - Helper macro to call into C with extended pt_regs
+ * @cfunc:		C function to be called
+ *
+ * This will ensure that extended_ptregs is added and removed as needed during
+ * a call into C code.
+ */
+.macro __call_ext_ptregs cfunc annotate_retpoline_safe:req
+#ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
+	/* add space for extended_pt_regs */
+	subq    $EXTENDED_PT_REGS_SIZE, %rsp
+#endif
+	.if \annotate_retpoline_safe == 1
+		ANNOTATE_RETPOLINE_SAFE
+	.endif
+	call	\cfunc
+#ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
+	/* remove space for extended_pt_regs */
+	addq    $EXTENDED_PT_REGS_SIZE, %rsp
+#endif
+.endm
+
+.macro call_ext_ptregs cfunc
+	__call_ext_ptregs \cfunc, annotate_retpoline_safe=0
+.endm
+
 .macro PUSH_AND_CLEAR_REGS rdx=%rdx rax=%rax save_ret=0
 	.if \save_ret
 	pushq	%rsi		/* pt_regs->si */

arch/x86/entry/common.c

@@ -34,6 +34,7 @@
 #include <asm/io_bitmap.h>
 #include <asm/syscall.h>
 #include <asm/irq_stack.h>
+#include <asm/pks.h>
 
 #ifdef CONFIG_X86_64
 __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
@@ -213,6 +214,60 @@ SYSCALL_DEFINE0(ni_syscall)
 	return -ENOSYS;
 }
 
+#ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
+
+void show_extended_regs_oops(struct pt_regs *regs, unsigned error_code)
+{
+	struct extended_pt_regs *ept_regs = extended_pt_regs(regs);
+
+	if (cpu_feature_enabled(X86_FEATURE_PKS) && (error_code & X86_PF_PK))
+		pr_alert("PKRS: 0x%x\n", ept_regs->thread_pkrs);
+}
+
+/*
+ * PKRS is a per-logical-processor MSR which overlays additional protection for
+ * pages which have been mapped with a protection key.
+ *
+ * The register is not maintained with XSAVE so we have to maintain the MSR
+ * value in software during context switch and exception handling.
+ *
+ * Context switches save the MSR in the task struct thus taking that value to
+ * other processors if necessary.
+ *
+ * To protect against exceptions having access to this memory we save the
+ * current running value and sets the PKRS value to be used during the
+ * exception.
+ */
+void pkrs_save_set_irq(struct pt_regs *regs, u32 val)
+{
+	struct extended_pt_regs *ept_regs;
+
+	BUILD_BUG_ON(sizeof(struct extended_pt_regs)
+			!= EXTENDED_PT_REGS_SIZE
+				+ sizeof(struct pt_regs));
+
+	if (!cpu_feature_enabled(X86_FEATURE_PKS))
+		return;
+
+	ept_regs = extended_pt_regs(regs);
+	ept_regs->thread_pkrs = current->thread.saved_pkrs;
+	write_pkrs(val);
+}
+
+void pkrs_restore_irq(struct pt_regs *regs)
+{
+	struct extended_pt_regs *ept_regs;
+
+	if (!cpu_feature_enabled(X86_FEATURE_PKS))
+		return;
+
+	ept_regs = extended_pt_regs(regs);
+	write_pkrs(ept_regs->thread_pkrs);
+	current->thread.saved_pkrs = ept_regs->thread_pkrs;
+}
+
+#endif /* CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS */
+
 #ifdef CONFIG_XEN_PV
 #ifndef CONFIG_PREEMPTION
 /*
@@ -269,6 +324,8 @@ __visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
 
 	inhcall = get_and_clear_inhcall();
 	if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
+		/* Normally called by irqentry_exit, we must restore pkrs here */
+		pkrs_restore_irq(regs);
 		instrumentation_begin();
 		irqentry_exit_cond_resched();
 		instrumentation_end();

arch/x86/entry/entry_64.S

@@ -331,7 +331,7 @@ SYM_CODE_END(ret_from_fork)
 		movq	$-1, ORIG_RAX(%rsp)	/* no syscall to restart */
 	.endif
 
-	call	\cfunc
+	call_ext_ptregs \cfunc
 
 	jmp	error_return
 .endm
@@ -434,7 +434,7 @@ SYM_CODE_START(\asmsym)
 
 	movq	%rsp, %rdi		/* pt_regs pointer */
 
-	call	\cfunc
+	call_ext_ptregs \cfunc
 
 	jmp	paranoid_exit
 
@@ -495,7 +495,7 @@ SYM_CODE_START(\asmsym)
 	 * stack.
 	 */
 	movq	%rsp, %rdi		/* pt_regs pointer */
-	call	vc_switch_off_ist
+	call_ext_ptregs vc_switch_off_ist
 	movq	%rax, %rsp		/* Switch to new stack */
 
 	UNWIND_HINT_REGS
@@ -506,7 +506,7 @@ SYM_CODE_START(\asmsym)
 
 	movq	%rsp, %rdi		/* pt_regs pointer */
 
-	call	\cfunc
+	call_ext_ptregs \cfunc
 
 	/*
 	 * No need to switch back to the IST stack. The current stack is either
@@ -541,7 +541,7 @@ SYM_CODE_START(\asmsym)
 	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
+	call_ext_ptregs \cfunc
 
 	jmp	paranoid_exit
 
@@ -780,7 +780,7 @@ SYM_CODE_START_LOCAL(exc_xen_hypervisor_callback)
 	movq	%rdi, %rsp			/* we don't return, adjust the stack frame */
 	UNWIND_HINT_REGS
 
-	call	xen_pv_evtchn_do_upcall
+	call_ext_ptregs xen_pv_evtchn_do_upcall
 
 	jmp	error_return
 SYM_CODE_END(exc_xen_hypervisor_callback)
@@ -986,7 +986,7 @@ SYM_CODE_START_LOCAL(error_entry)
 	/* Put us onto the real thread stack. */
 	popq	%r12				/* save return addr in %12 */
 	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
-	call	sync_regs
+	call_ext_ptregs sync_regs
 	movq	%rax, %rsp			/* switch stack */
 	ENCODE_FRAME_POINTER
 	pushq	%r12
@@ -1041,7 +1041,7 @@ SYM_CODE_START_LOCAL(error_entry)
 	 * as if we faulted immediately after IRET.
 	 */
 	mov	%rsp, %rdi
-	call	fixup_bad_iret
+	call_ext_ptregs fixup_bad_iret
 	mov	%rax, %rsp
 	jmp	.Lerror_entry_from_usermode_after_swapgs
 SYM_CODE_END(error_entry)
@@ -1147,7 +1147,7 @@ SYM_CODE_START(asm_exc_nmi)
 
 	movq	%rsp, %rdi
 	movq	$-1, %rsi
-	call	exc_nmi
+	call_ext_ptregs exc_nmi
 
 	/*
 	 * Return back to user mode.  We must *not* do the normal exit
@@ -1183,6 +1183,8 @@ SYM_CODE_START(asm_exc_nmi)
 	 * +---------------------------------------------------------+
 	 * | pt_regs                                                 |
 	 * +---------------------------------------------------------+
+	 * | (Optionally) extended_pt_regs                           |
+	 * +---------------------------------------------------------+
 	 *
 	 * The "original" frame is used by hardware.  Before re-enabling
 	 * NMIs, we need to be done with it, and we need to leave enough
@@ -1359,7 +1361,7 @@ end_repeat_nmi:
 
 	movq	%rsp, %rdi
 	movq	$-1, %rsi
-	call	exc_nmi
+	call_ext_ptregs exc_nmi
 
 	/* Always restore stashed CR3 value (see paranoid_entry) */
 	RESTORE_CR3 scratch_reg=%r15 save_reg=%r14

arch/x86/entry/entry_64_compat.S

@@ -136,7 +136,7 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 .Lsysenter_flags_fixed:
 
 	movq	%rsp, %rdi
-	call	do_SYSENTER_32
+	call_ext_ptregs do_SYSENTER_32
 	/* XEN PV guests always use IRET path */
 	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
 		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
@@ -251,7 +251,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
 	UNWIND_HINT_REGS
 
 	movq	%rsp, %rdi
-	call	do_fast_syscall_32
+	call_ext_ptregs do_fast_syscall_32
 	/* XEN PV guests always use IRET path */
 	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
 		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
@@ -408,6 +408,6 @@ SYM_CODE_START(entry_INT80_compat)
 	cld
 
 	movq	%rsp, %rdi
-	call	do_int80_syscall_32
+	call_ext_ptregs do_int80_syscall_32
 	jmp	swapgs_restore_regs_and_return_to_usermode
 SYM_CODE_END(entry_INT80_compat)

arch/x86/include/asm/pks.h

@@ -4,11 +4,27 @@
 
 #ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
 
+struct extended_pt_regs {
+	u32 thread_pkrs;
+	/* Keep stack 8 byte aligned */
+	u32 pad;
+	struct pt_regs pt_regs;
+};
+
 void setup_pks(void);
 
+static inline struct extended_pt_regs *extended_pt_regs(struct pt_regs *regs)
+{
+	return container_of(regs, struct extended_pt_regs, pt_regs);
+}
+
+void show_extended_regs_oops(struct pt_regs *regs, unsigned error_code);
+
 #else /* !CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS */
 
 static inline void setup_pks(void) { }
+static inline void show_extended_regs_oops(struct pt_regs *regs,
+					   unsigned error_code) { }
 
 #endif /* CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS */
 

arch/x86/include/asm/processor-flags.h

@@ -53,4 +53,6 @@
 # define X86_CR3_PTI_PCID_USER_BIT	11
 #endif
 
+#define EXTENDED_PT_REGS_SIZE 8
+
 #endif /* _ASM_X86_PROCESSOR_FLAGS_H */

arch/x86/kernel/head_64.S

@@ -319,8 +319,7 @@ SYM_CODE_START_NOALIGN(vc_boot_ghcb)
 	movq    %rsp, %rdi
 	movq	ORIG_RAX(%rsp), %rsi
 	movq	initial_vc_handler(%rip), %rax
-	ANNOTATE_RETPOLINE_SAFE
-	call	*%rax
+	__call_ext_ptregs *%rax, annotate_retpoline_safe=1
 
 	/* Unwind pt_regs */
 	POP_REGS
@@ -397,7 +396,7 @@ SYM_CODE_START_LOCAL(early_idt_handler_common)
 	UNWIND_HINT_REGS
 
 	movq %rsp,%rdi		/* RDI = pt_regs; RSI is already trapnr */
-	call do_early_exception
+	call_ext_ptregs do_early_exception
 
 	decl early_recursion_flag(%rip)
 	jmp restore_regs_and_return_to_kernel
@@ -421,7 +420,7 @@ SYM_CODE_START_NOALIGN(vc_no_ghcb)
 	/* Call C handler */
 	movq    %rsp, %rdi
 	movq	ORIG_RAX(%rsp), %rsi
-	call    do_vc_no_ghcb
+	call_ext_ptregs do_vc_no_ghcb
 
 	/* Unwind pt_regs */
 	POP_REGS

arch/x86/mm/fault.c

@@ -32,6 +32,7 @@
 #include <asm/pgtable_areas.h>		/* VMALLOC_START, ...		*/
 #include <asm/kvm_para.h>		/* kvm_handle_async_pf		*/
 #include <asm/vdso.h>			/* fixup_vdso_exception()	*/
+#include <asm/pks.h>
 
 #define CREATE_TRACE_POINTS
 #include <asm/trace/exceptions.h>
@@ -547,6 +548,8 @@ show_fault_oops(struct pt_regs *regs, unsigned long error_code, unsigned long ad
 		 (error_code & X86_PF_PK)    ? "protection keys violation" :
 					       "permissions violation");
 
+	show_extended_regs_oops(regs, error_code);
+
 	if (!(error_code & X86_PF_USER) && user_mode(regs)) {
 		struct desc_ptr idt, gdt;
 		u16 ldtr, tr;

include/linux/pkeys.h

@@ -50,4 +50,21 @@ static inline void copy_init_pkru_to_fpregs(void)
 
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
+
+#ifdef CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS
+
+void pkrs_save_set_irq(struct pt_regs *regs, u32 val);
+void pkrs_restore_irq(struct pt_regs *regs);
+
+#else /* !CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS */
+
+#ifndef INIT_PKRS_VALUE
+#define INIT_PKRS_VALUE 0
+#endif
+
+static inline void pkrs_save_set_irq(struct pt_regs *regs, u32 val) { }
+static inline void pkrs_restore_irq(struct pt_regs *regs) { }
+
+#endif /* CONFIG_ARCH_ENABLE_SUPERVISOR_PKEYS */
+
 #endif /* _LINUX_PKEYS_H */

kernel/entry/common.c

@@ -5,6 +5,7 @@
 #include <linux/highmem.h>
 #include <linux/livepatch.h>
 #include <linux/audit.h>
+#include <linux/pkeys.h>
 
 #include "common.h"
 
@@ -363,7 +364,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 		instrumentation_end();
 
 		ret.exit_rcu = true;
-		return ret;
+		goto done;
 	}
 
 	/*
@@ -378,6 +379,8 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 	trace_hardirqs_off_finish();
 	instrumentation_end();
 
+done:
+	pkrs_save_set_irq(regs, INIT_PKRS_VALUE);
 	return ret;
 }
 
@@ -403,7 +406,12 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
 	/* Check whether this returns to user mode */
 	if (user_mode(regs)) {
 		irqentry_exit_to_user_mode(regs);
-	} else if (!regs_irqs_disabled(regs)) {
+		return;
+	}
+
+	pkrs_restore_irq(regs);
+
+	if (!regs_irqs_disabled(regs)) {
 		/*
 		 * If RCU was not watching on entry this needs to be done
 		 * carefully and needs the same ordering of lockdep/tracing
@@ -457,11 +465,13 @@ irqentry_state_t noinstr irqentry_nmi_enter(struct pt_regs *regs)
 	ftrace_nmi_enter();
 	instrumentation_end();
 
+	pkrs_save_set_irq(regs, INIT_PKRS_VALUE);
 	return irq_state;
 }
 
 void noinstr irqentry_nmi_exit(struct pt_regs *regs, irqentry_state_t irq_state)
 {
+	pkrs_restore_irq(regs);
 	instrumentation_begin();
 	ftrace_nmi_exit();
 	if (irq_state.lockdep) {