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    Subject[RFC PATCH v9 13/27] x86/mm: Shadow Stack page fault error checking
    Date
    If a page fault is triggered by a Shadow Stack (SHSTK) access
    (e.g. CALL/RET) or SHSTK management instructions (e.g. WRUSSQ), then bit[6]
    of the page fault error code is set.

    In access_error(), verify a SHSTK page fault is within a SHSTK memory area.
    It is always an error otherwise.

    For a valid SHSTK access, set FAULT_FLAG_WRITE to effect copy-on-write.

    Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    ---
    arch/x86/include/asm/traps.h | 2 ++
    arch/x86/mm/fault.c | 18 ++++++++++++++++++
    2 files changed, 20 insertions(+)

    diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
    index 7ac26bbd0bef..8023d177fcd8 100644
    --- a/arch/x86/include/asm/traps.h
    +++ b/arch/x86/include/asm/traps.h
    @@ -169,6 +169,7 @@ enum {
    * bit 3 == 1: use of reserved bit detected
    * bit 4 == 1: fault was an instruction fetch
    * bit 5 == 1: protection keys block access
    + * bit 6 == 1: shadow stack access fault
    */
    enum x86_pf_error_code {
    X86_PF_PROT = 1 << 0,
    @@ -177,5 +178,6 @@ enum x86_pf_error_code {
    X86_PF_RSVD = 1 << 3,
    X86_PF_INSTR = 1 << 4,
    X86_PF_PK = 1 << 5,
    + X86_PF_SHSTK = 1 << 6,
    };
    #endif /* _ASM_X86_TRAPS_H */
    diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
    index 304d31d8cbbc..9c1243302663 100644
    --- a/arch/x86/mm/fault.c
    +++ b/arch/x86/mm/fault.c
    @@ -1187,6 +1187,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
    (error_code & X86_PF_INSTR), foreign))
    return 1;

    + /*
    + * Verify X86_PF_SHSTK is within a Shadow Stack VMA.
    + * It is always an error if there is a Shadow Stack
    + * fault outside a Shadow Stack VMA.
    + */
    + if (error_code & X86_PF_SHSTK) {
    + if (!(vma->vm_flags & VM_SHSTK))
    + return 1;
    + return 0;
    + }
    +
    if (error_code & X86_PF_WRITE) {
    /* write, present and write, not present: */
    if (unlikely(!(vma->vm_flags & VM_WRITE)))
    @@ -1344,6 +1355,13 @@ void do_user_addr_fault(struct pt_regs *regs,

    perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

    + /*
    + * If the fault is caused by a Shadow Stack access,
    + * i.e. CALL/RET/SAVEPREVSSP/RSTORSSP, then set
    + * FAULT_FLAG_WRITE to effect copy-on-write.
    + */
    + if (hw_error_code & X86_PF_SHSTK)
    + flags |= FAULT_FLAG_WRITE;
    if (hw_error_code & X86_PF_WRITE)
    flags |= FAULT_FLAG_WRITE;
    if (hw_error_code & X86_PF_INSTR)
    --
    2.21.0
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