Re: [Xen-devel] [PATCH 1/7] x86/hvm: Correctly identify implicit supervisor accesses

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/02/17 14:03, Andrew Cooper wrote:
> All actions which refer to the active ldt/gdt/idt or task register
> (e.g. loading a new segment selector) are known as implicit supervisor
> accesses, even when the access originates from user code.

It turns out that this has a bugfix in it which I hadn't realised.

I have added:

"Right away, this fixes a bug during userspace emulation where a
pagewalk for a system table was (incorrectly) performed as a user
access, causing an access violation in the common case, as system tables
reside on supervisor mappings."

~Andrew

>
> The distinction is necessary in the pagewalk when SMAP is enabled.  Refer to
> Intel SDM Vol 3 "Access Rights" for the exact details.
>
> Introduce a new pagewalk input, and make use of the new system segment
> references in hvmemul_{read,write}().  While modifying those areas, move the
> calculation of the appropriate pagewalk input before its first use.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>



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