Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault

[Andrew Cooper]

On 25/10/2021 11:59, Peter Zijlstra wrote:
> On Thu, Oct 21, 2021 at 05:21:10AM -0700, Nadav Amit wrote:
>> From: Nadav Amit <namit@xxxxxxxxxx>
>> Add a check to prevent access_error() from returning mistakenly that
>> page-faults due to instruction fetch are not allowed. Intel SDM does not
>> indicate whether "instruction fetch" and "write" in the hardware error
>> code are mutual exclusive, so check both before returning whether the
>> access is allowed.
> Dave, can we get that clarified? It seems a bit naf and leads to
> confusing code IMO.

There is no such thing as an instruction fetch (a read) causing a
modification to the mapping.  From this point of view, you'd never
expect to see them both set.

However, be aware that INSTR is only reported for NX || SMEP.  Without
either, instruction vs data accesses are distinguished internally (can
be demonstrated with SMAP) but not visible in the pagefault error code.

~Andrew