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

[Vasily Averin <vvs@openvz.org>]

On 7/1/22 03:42, H. Peter Anvin wrote:
> On June 29, 2022 10:58:36 PM PDT, Vasily Averin <vvs@openvz.org> wrote:
>> 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@openvz.org
>> Signed-off-by: Vasily Averin <vvs@openvz.org>
>> ---
>> 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);
> 
> Are there other bits we could/should mask.out in the case P = 0? The
> only bits that should be able to appear are ones that are independent
> of the PTE content.
In accordance with the "Intel® 64 and IA-32 Architectures Software Developer’s
Manual Volume 3A: System Programming Guide, Part 1" there are several other
similar bits:
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf

"4.7 PAGE-FAULT EXCEPTIONS
...
• HLAT (bit 7).
This flag is 1 if there is no translation for the linear address using HLAT 
paging because, in one of the paging structure entries used to translate that
address, either the P flag was 0 or a reserved bit was set. An error code will
set this flag only if it clears bit 0 or sets bit 3. This flag will not be set
by a page fault resulting from a violation of access rights, nor for one
encountered during ordinary paging, including the case in which there has been
a restart of HLAT paging.

• SGX flag (bit 15).
This flag is 1 if the exception is unrelated to paging and resulted from 
violation of SGX-specific access-control requirements. Because such a violation
can occur only if there is no ordinary page fault, this flag is set only if
the P flag (bit 0) is 1 and the RSVD flag (bit 3) and the PK flag (bit 5)
are both 0."

However, only the RSVD flag has errata in real processors.
So I don't think any other bits should be masked in some way.

Thank you,
	Vasily Averin