Messages in this thread Patch in this message | | | Date | Fri, 1 Jul 2022 02:58:00 +0300 | From | Vasily Averin <> | Subject | [PATCH v2] x86/fault: ignore RSVD flag in error code if P flag is 0 |
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Several older Intel CPUs have this or a similar erratum. For instance, the "Intel Xeon Processor 5400 Series Specification Update" [1] has
"AX74. 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. Intel has not observed this erratum with any commercially available software.
Workaround: Page-fault handlers should ignore the RSVD flag in the error code if the P flag is 0"
This problem has been observed several times on several nodes using Intel Xeon E5450 processors. These nodes were crashed after "Bad pagetable: 000c" messages like this:
Corrupted page table at address 7f62d5b48e68 PGD 80000002e92bf067 PUD 1c99c5067 PMD 195015067 PTE 7fffffffb78b680 Bad pagetable: 000c [#1] SMP
Error code here is 0xc, it have set RSVD flag (bit 3), however P flag (bit 0) is clear.
Let's follow the recommendations and ignore the RSVD flag in the cases described.
Link: [1] https://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/xeon-5400-spec-update.pdf Link: https://lore.kernel.org/all/aae9c7c6-989c-0261-470a-252537493b53@openvz.org Reported-by: Steve Sipes <steve.sipes@comandsolutions.com> Signed-off-by: Vasily Averin <vvs@openvz.org> --- v2: added original reporter improved patch description, added link to CPU spec update --- 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
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