[PATCH v3 2/5] x86/mm: check exec permissions on fault

[Nadav Amit <nadav.amit@gmail.com>]

From: Nadav Amit <namit@vmware.com>

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
spurious page-faults due to instruction fetch are a reason for an access
error.

It is assumed that error code bits of "instruction fetch" and "write" in
the hardware error code are mutual exclusive, and the change assumes so.
However, to be on the safe side, especially if hypervisors misbehave,
assert this is the case and warn otherwise.

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: x86@kernel.org
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 arch/x86/mm/fault.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index d0074c6ed31a..ad0ef0a6087a 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1107,10 +1107,28 @@ 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)) {
+		/*
+		 * CPUs are not expected to set the two error code bits
+		 * together, but to ensure that hypervisors do not misbehave,
+		 * run an additional sanity check.
+		 */
+		if ((error_code & (X86_PF_WRITE|X86_PF_INSTR)) ==
+					(X86_PF_WRITE|X86_PF_INSTR)) {
+			WARN_ON_ONCE(1);
+			return 1;
+		}
+
 		/* 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