[PATCH] x86/fault: ignore RSVD flag in error code if P flag is 0

[Vasily Averin]

Some older Intel CPUs have errata:
"Not-Present Page Faults May Set the RSVD Flag in the Error Code

Problem:
An attempt to access a page that is not marked present causes a page
fault. Such a page fault delivers an error code in which both the
P flag (bit 0) and the RSVD flag (bit 3) are 0. Due to this erratum,
not-present page faults may deliver an error code in which the P flag
is 0 but the RSVD flag is 1.

Implication:
Software may erroneously infer that a page fault was due to a
reserved-bit violation when it was actually due to an attempt
to access a not-present page.

Workaround: Page-fault handlers should ignore the RSVD flag in the error
code if the P flag is 0."

This issues was observed on several nodes crashed with messages
httpd: Corrupted page table at address 7f62d5b48e68
PGD 80000002e92bf067 PUD 1c99c5067 PMD 195015067 PTE 7fffffffb78b680
Bad pagetable: 000c [#1] SMP

Let's follow the recommendation and will ignore the RSVD flag in the
error code if the P flag is 0

Link: https://lore.kernel.org/all/aae9c7c6-989c-0261-470a-252537493b53@xxxxxxxxxx
Signed-off-by: Vasily Averin <vvs@xxxxxxxxxx>
---
 arch/x86/mm/fault.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index fe10c6d76bac..ffc6d6bd2a22 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1481,6 +1481,15 @@ handle_page_fault(struct pt_regs *regs, unsigned long error_code,
 	if (unlikely(kmmio_fault(regs, address)))
 		return;
 
+	/*
+	 * Some older Intel CPUs have errata
+	 * "Not-Present Page Faults May Set the RSVD Flag in the Error Code"
+	 * It is recommended to ignore the RSVD flag (bit 3) in the error code
+	 * if the P flag (bit 0) is 0.
+	 */
+	if (unlikely((error_code & X86_PF_RSVD) && !(error_code & X86_PF_PROT)))
+		error_code &= ~X86_PF_RSVD;
+
 	/* Was the fault on kernel-controlled part of the address space? */
 	if (unlikely(fault_in_kernel_space(address))) {
 		do_kern_addr_fault(regs, error_code, address);
-- 
2.36.1