[PATCHv8 06/17] x86/vsyscall: Do not require X86_PF_INSTR to emulate vsyscall

Kirill A. Shutemov
2025-07-01T09:59:06Z

emulate_vsyscall() expects to see X86_PF_INSTR in PFEC on a vsyscall
page fault, but the CPU does not report X86_PF_INSTR if neither
X86_FEATURE_NX nor X86_FEATURE_SMEP are enabled.

X86_FEATURE_NX should be enabled on nearly all 64-bit CPUs, except for
early P4 processors that did not support this feature.

Instead of explicitly checking for X86_PF_INSTR, compare the fault
address against RIP.

On machines with X86_FEATURE_NX enabled, issue a warning if RIP is equal
to fault address but X86_PF_INSTR is absent.

Originally-by: Dave Hansen <dave.hansen@intel.com>
Link: https://lore.kernel.org/all/bd81a98b-f8d4-4304-ac55-d4151a1a77ab@intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
arch/x86/entry/vsyscall/vsyscall_64.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index c9103a6fa06e..0b0e0283994f 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -124,7 +124,8 @@ bool emulate_vsyscall(unsigned long error_code,
if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
return false;

- if (!(error_code & X86_PF_INSTR)) {
+ /* Avoid emulation unless userspace was executing from vsyscall page: */
+ if (address != regs->ip) {
/* Failed vsyscall read */
if (vsyscall_mode == EMULATE)
return false;
@@ -136,13 +137,16 @@ bool emulate_vsyscall(unsigned long error_code,
return false;
}

+
+ /* X86_PF_INSTR is only set when NX is supported: */
+ if (cpu_feature_enabled(X86_FEATURE_NX))
+ WARN_ON_ONCE(!(error_code & X86_PF_INSTR));
+
/*
* No point in checking CS -- the only way to get here is a user mode
* trap to a high address, which means that we're in 64-bit user code.
*/

- WARN_ON_ONCE(address != regs->ip);
-
if (vsyscall_mode == NONE) {
warn_bad_vsyscall(KERN_INFO, regs,
"vsyscall attempted with vsyscall=none");
--
2.47.2