[PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a bad area

[Sean Christopherson]

Kernel addresses are always mapped with _PAGE_USER=0, i.e. PKRU
isn't enforced, and so we should never see X86_PF_PK set on a
kernel address fault.  WARN once to capture the issue in case we
somehow don't die, e.g. the access originated in userspace.

Remove a similar check and its comment from spurious_fault_check().
The intent of the comment (and later code[1]) was simply to document
that spurious faults due to protection keys should be impossible, but
that's irrelevant and a bit of a red herring since we should never
get a protection keys fault on a kernel address regardless of the
kernel's TLB flushing behavior.

[1] http://lists-archives.com/linux-kernel/28407455-x86-pkeys-new-page-fault-error-code-bit-pf_pk.html

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
---
There's no indication that this condition has ever been encountered.
I came across the code in spurious_fault_check() and was confused as
to why we would unconditionally treat a protection keys fault as
spurious when the comment explicitly stated that such a case should
be impossible.

Dave Hansen suggested adding a WARN_ON_ONCE in spurious_fault_check(),
but it seemed more appropriate to freak out on any protection keys
fault on a kernel address since that would imply a hardware issue or
kernel bug.  I omitted a Suggested-by since this isn't necessarily
what Dave had in mind.

 arch/x86/mm/fault.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 2aafa6ab6103..f19a55972136 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1040,12 +1040,6 @@ static int spurious_fault_check(unsigned long error_code, pte_t *pte)
 
 	if ((error_code & X86_PF_INSTR) && !pte_exec(*pte))
 		return 0;
-	/*
-	 * Note: We do not do lazy flushing on protection key
-	 * changes, so no spurious fault will ever set X86_PF_PK.
-	 */
-	if ((error_code & X86_PF_PK))
-		return 1;
 
 	return 1;
 }
@@ -1241,6 +1235,14 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 	 * protection error (error_code & 9) == 0.
 	 */
 	if (unlikely(fault_in_kernel_space(address))) {
+		/*
+		 * We should never encounter a protection keys fault on a
+		 * kernel address as kernel address are always mapped with
+		 * _PAGE_USER=0, i.e. PKRU isn't enforced.
+		 */
+		if (WARN_ON_ONCE(error_code & X86_PF_PK))
+			goto bad_kernel_address;
+
 		if (!(error_code & (X86_PF_RSVD | X86_PF_USER | X86_PF_PROT))) {
 			if (vmalloc_fault(address) >= 0)
 				return;
@@ -1253,6 +1255,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 		/* kprobes don't want to hook the spurious faults: */
 		if (kprobes_fault(regs))
 			return;
+
+bad_kernel_address:
 		/*
 		 * Don't take the mm semaphore here. If we fixup a prefetch
 		 * fault we could otherwise deadlock:
-- 
2.18.0