[PATCH v2 3/5] x86/mm: check exec permissions on fault

[Nadav Amit]

From: Nadav Amit <namit@xxxxxxxxxx>

access_error() currently does not check for execution permission
violation. As a result, spurious page-faults due to execution permission
violation cause SIGSEGV.

It appears not to be an issue so far, but the next patches avoid TLB
flushes on permission promotion, which can lead to this scenario. nodejs
for instance crashes when TLB flush is avoided on permission promotion.

Add a check to prevent access_error() from returning mistakenly that
page-faults due to instruction fetch are not allowed. Intel SDM does not
indicate whether "instruction fetch" and "write" in the hardware error
code are mutual exclusive, so check both before returning whether the
access is allowed.

Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 arch/x86/mm/fault.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index b2eefdefc108..e776130473ce 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1100,10 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
 				       (error_code & X86_PF_INSTR), foreign))
 		return 1;
 
-	if (error_code & X86_PF_WRITE) {
+	if (error_code & (X86_PF_WRITE | X86_PF_INSTR)) {
 		/* write, present and write, not present: */
-		if (unlikely(!(vma->vm_flags & VM_WRITE)))
+		if ((error_code & X86_PF_WRITE) &&
+		    unlikely(!(vma->vm_flags & VM_WRITE)))
 			return 1;
+
+		/* exec, present and exec, not present: */
+		if ((error_code & X86_PF_INSTR) &&
+		    unlikely(!(vma->vm_flags & VM_EXEC)))
+			return 1;
+
 		return 0;
 	}
 
-- 
2.25.1