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x86/tdx: Clarify RIP adjustments in #VE handler
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After successful #VE handling, tdx_handle_virt_exception() has to move
RIP to the next instruction. The handler needs to know the length of the
instruction.

If the #VE happened due to instruction execution, GET_VEINFO TDX module
call provides info on the instruction in R10, including its length.

For #VE due to EPT violation, info in R10 is not usable and kernel has
to decode instruction manually to find out its length.

Restructure the code to make it explicit that the instruction length
depends on the type of #VE. Handler of an exit reason returns
instruction length on success or -errno on failure.

Suggested-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
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@kiryl
kiryl committed Jun 13, 2022
1 parent 8bae373 commit b79b8d6
Showing 1 changed file with 123 additions and 55 deletions.
178 changes: 123 additions & 55 deletions arch/x86/coco/tdx/tdx.c
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Expand Up @@ -124,6 +124,51 @@ static u64 get_cc_mask(void)
return BIT_ULL(gpa_width - 1);
}

/*
* TDX module spec states that #VE may be injected by the Intel TDX module in
* several cases:
*
* - Emulation of the architectural #VE injection on EPT violation;
*
* - As a result of guest TD execution of a disallowed instruction,
* a disallowed MSR access, or CPUID virtualization;
*
* - A notification to the guest TD about anomalous behavior;
*
* The last one is opt-in and is not used by the kernel.
*
* Intel Software Developer's Manual describes cases when instruction length
* field can be used in section "Information for VM Exits Due to Instruction
* Execution".
*
* For TDX, it ultimately means GET_VEINFO provides reliable instruction length
* information if #VE occurred due to instruction execution, but not for EPT
* violations.
*/
static int ve_instr_len(struct ve_info *ve)
{
switch (ve->exit_reason) {
case EXIT_REASON_HLT:
case EXIT_REASON_MSR_READ:
case EXIT_REASON_MSR_WRITE:
case EXIT_REASON_CPUID:
case EXIT_REASON_IO_INSTRUCTION:
/* It is safe to use ve->instr_len for #VE due instructions */
return ve->instr_len;
case EXIT_REASON_EPT_VIOLATION:
/*
* For EPT violations, ve->insn_len is not defined. For those,
* the kernel must decode instructions manually and should not
* be using this function.
*/
WARN_ONCE(1, "ve->instr_len is not defined for EPT violations");
return 0;
default:
WARN_ONCE(1, "Unexpected #VE-type: %d\n", ve->exit_reason);
return ve->instr_len;
}
}

static u64 __cpuidle __halt(const bool irq_disabled, const bool do_sti)
{
struct tdx_hypercall_args args = {
Expand All @@ -147,7 +192,7 @@ static u64 __cpuidle __halt(const bool irq_disabled, const bool do_sti)
return __tdx_hypercall(&args, do_sti ? TDX_HCALL_ISSUE_STI : 0);
}

static bool handle_halt(void)
static int handle_halt(struct ve_info *ve)
{
/*
* Since non safe halt is mainly used in CPU offlining
Expand All @@ -158,9 +203,9 @@ static bool handle_halt(void)
const bool do_sti = false;

if (__halt(irq_disabled, do_sti))
return false;
return -EIO;

return true;
return ve_instr_len(ve);
}

void __cpuidle tdx_safe_halt(void)
Expand All @@ -180,7 +225,7 @@ void __cpuidle tdx_safe_halt(void)
WARN_ONCE(1, "HLT instruction emulation failed\n");
}

static bool read_msr(struct pt_regs *regs)
static int read_msr(struct pt_regs *regs, struct ve_info *ve)
{
struct tdx_hypercall_args args = {
.r10 = TDX_HYPERCALL_STANDARD,
Expand All @@ -194,14 +239,14 @@ static bool read_msr(struct pt_regs *regs)
* (GHCI), section titled "TDG.VP.VMCALL<Instruction.RDMSR>".
*/
if (__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT))
return false;
return -EIO;

regs->ax = lower_32_bits(args.r11);
regs->dx = upper_32_bits(args.r11);
return true;
return ve_instr_len(ve);
}

static bool write_msr(struct pt_regs *regs)
static int write_msr(struct pt_regs *regs, struct ve_info *ve)
{
struct tdx_hypercall_args args = {
.r10 = TDX_HYPERCALL_STANDARD,
Expand All @@ -215,10 +260,13 @@ static bool write_msr(struct pt_regs *regs)
* can be found in TDX Guest-Host-Communication Interface
* (GHCI) section titled "TDG.VP.VMCALL<Instruction.WRMSR>".
*/
return !__tdx_hypercall(&args, 0);
if (__tdx_hypercall(&args, 0))
return -EIO;

return ve_instr_len(ve);
}

static bool handle_cpuid(struct pt_regs *regs)
static int handle_cpuid(struct pt_regs *regs, struct ve_info *ve)
{
struct tdx_hypercall_args args = {
.r10 = TDX_HYPERCALL_STANDARD,
Expand All @@ -236,7 +284,7 @@ static bool handle_cpuid(struct pt_regs *regs)
*/
if (regs->ax < 0x40000000 || regs->ax > 0x4FFFFFFF) {
regs->ax = regs->bx = regs->cx = regs->dx = 0;
return true;
return ve_instr_len(ve);
}

/*
Expand All @@ -245,7 +293,7 @@ static bool handle_cpuid(struct pt_regs *regs)
* (GHCI), section titled "VP.VMCALL<Instruction.CPUID>".
*/
if (__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT))
return false;
return -EIO;

/*
* As per TDX GHCI CPUID ABI, r12-r15 registers contain contents of
Expand All @@ -257,7 +305,7 @@ static bool handle_cpuid(struct pt_regs *regs)
regs->cx = args.r14;
regs->dx = args.r15;

return true;
return ve_instr_len(ve);
}

static bool mmio_read(int size, unsigned long addr, unsigned long *val)
Expand All @@ -283,7 +331,7 @@ static bool mmio_write(int size, unsigned long addr, unsigned long val)
EPT_WRITE, addr, val);
}

static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
{
char buffer[MAX_INSN_SIZE];
unsigned long *reg, val;
Expand All @@ -294,34 +342,36 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)

/* Only in-kernel MMIO is supported */
if (WARN_ON_ONCE(user_mode(regs)))
return false;
return -EFAULT;

if (copy_from_kernel_nofault(buffer, (void *)regs->ip, MAX_INSN_SIZE))
return false;
return -EFAULT;

if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64))
return false;
return -EINVAL;

mmio = insn_decode_mmio(&insn, &size);
if (WARN_ON_ONCE(mmio == MMIO_DECODE_FAILED))
return false;
return -EINVAL;

if (mmio != MMIO_WRITE_IMM && mmio != MMIO_MOVS) {
reg = insn_get_modrm_reg_ptr(&insn, regs);
if (!reg)
return false;
return -EINVAL;
}

ve->instr_len = insn.length;

/* Handle writes first */
switch (mmio) {
case MMIO_WRITE:
memcpy(&val, reg, size);
return mmio_write(size, ve->gpa, val);
if (!mmio_write(size, ve->gpa, val))
return -EIO;
return insn.length;
case MMIO_WRITE_IMM:
val = insn.immediate.value;
return mmio_write(size, ve->gpa, val);
if (!mmio_write(size, ve->gpa, val))
return -EIO;
return insn.length;
case MMIO_READ:
case MMIO_READ_ZERO_EXTEND:
case MMIO_READ_SIGN_EXTEND:
Expand All @@ -334,15 +384,15 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
* decoded or handled properly. It was likely not using io.h
* helpers or accessed MMIO accidentally.
*/
return false;
return -EINVAL;
default:
WARN_ONCE(1, "Unknown insn_decode_mmio() decode value?");
return false;
return -EINVAL;
}

/* Handle reads */
if (!mmio_read(size, ve->gpa, &val))
return false;
return -EIO;

switch (mmio) {
case MMIO_READ:
Expand All @@ -364,13 +414,13 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
default:
/* All other cases has to be covered with the first switch() */
WARN_ON_ONCE(1);
return false;
return -EINVAL;
}

if (extend_size)
memset(reg, extend_val, extend_size);
memcpy(reg, &val, size);
return true;
return insn.length;
}

static bool handle_in(struct pt_regs *regs, int size, int port)
Expand Down Expand Up @@ -421,23 +471,28 @@ static bool handle_out(struct pt_regs *regs, int size, int port)
*
* Return True on success or False on failure.
*/
static bool handle_io(struct pt_regs *regs, u32 exit_qual)
static int handle_io(struct pt_regs *regs, struct ve_info *ve)
{
u32 exit_qual = ve->exit_qual;
int size, port;
bool in;
bool in, ret;

if (VE_IS_IO_STRING(exit_qual))
return false;
return -EIO;

in = VE_IS_IO_IN(exit_qual);
size = VE_GET_IO_SIZE(exit_qual);
port = VE_GET_PORT_NUM(exit_qual);


if (in)
return handle_in(regs, size, port);
ret = handle_in(regs, size, port);
else
return handle_out(regs, size, port);
ret = handle_out(regs, size, port);
if (!ret)
return -EIO;

return ve_instr_len(ve);
}

/*
Expand All @@ -447,17 +502,19 @@ static bool handle_io(struct pt_regs *regs, u32 exit_qual)
__init bool tdx_early_handle_ve(struct pt_regs *regs)
{
struct ve_info ve;
bool ret;
int insn_len;

tdx_get_ve_info(&ve);

if (ve.exit_reason != EXIT_REASON_IO_INSTRUCTION)
return false;

ret = handle_io(regs, ve.exit_qual);
if (ret)
regs->ip += ve.instr_len;
return ret;
insn_len = handle_io(regs, &ve);
if (insn_len < 0)
return false;

regs->ip += insn_len;
return true;
}

void tdx_get_ve_info(struct ve_info *ve)
Expand Down Expand Up @@ -490,54 +547,65 @@ void tdx_get_ve_info(struct ve_info *ve)
ve->instr_info = upper_32_bits(out.r10);
}

/* Handle the user initiated #VE */
static bool virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
/*
* Handle the user initiated #VE.
*
* On success, returns the number of bytes RIP should be incremented (>=0)
* or -errno on error.
*/
static int virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
{
switch (ve->exit_reason) {
case EXIT_REASON_CPUID:
return handle_cpuid(regs);
return handle_cpuid(regs, ve);
default:
pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
return false;
return -EIO;
}
}

/* Handle the kernel #VE */
static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
/*
* Handle the kernel #VE.
*
* On success, returns the number of bytes RIP should be incremented (>=0)
* or -errno on error.
*/
static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
{
switch (ve->exit_reason) {
case EXIT_REASON_HLT:
return handle_halt();
return handle_halt(ve);
case EXIT_REASON_MSR_READ:
return read_msr(regs);
return read_msr(regs, ve);
case EXIT_REASON_MSR_WRITE:
return write_msr(regs);
return write_msr(regs, ve);
case EXIT_REASON_CPUID:
return handle_cpuid(regs);
return handle_cpuid(regs, ve);
case EXIT_REASON_EPT_VIOLATION:
return handle_mmio(regs, ve);
case EXIT_REASON_IO_INSTRUCTION:
return handle_io(regs, ve->exit_qual);
return handle_io(regs, ve);
default:
pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
return false;
return -EIO;
}
}

bool tdx_handle_virt_exception(struct pt_regs *regs, struct ve_info *ve)
{
bool ret;
int insn_len;

if (user_mode(regs))
ret = virt_exception_user(regs, ve);
insn_len = virt_exception_user(regs, ve);
else
ret = virt_exception_kernel(regs, ve);
insn_len = virt_exception_kernel(regs, ve);
if (insn_len < 0)
return false;

/* After successful #VE handling, move the IP */
if (ret)
regs->ip += ve->instr_len;
regs->ip += insn_len;

return ret;
return true;
}

static bool tdx_tlb_flush_required(bool private)
Expand Down

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