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Re: Assert in x86_emulate_wrapper triggerable by HVM domain
- To: Manuel Andreas <manuel.andreas@xxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 16 Apr 2025 15:53:35 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 16 Apr 2025 13:53:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 16.04.2025 15:52, Jan Beulich wrote:
> On 15.04.2025 23:52, Manuel Andreas wrote:
>> my fuzzing infrastructure discovered that an assert in
>> x86_emulate_wrapper is able to be triggered by an HVM domain executing a
>> specially crafted repeating movs instruction.
>>
>> Specifically, if the emulation of the rep movs instruction triggers an
>> exception (e.g. by accessing invalid memory after some amount of
>> iterations), the emulation will be halted at that point.
>> However, the instruction manual requires that _some_ register state
>> (namely the updated value of rcx) shall be commited, whereas the
>> instruction pointer needs to be rolled back to point to the address of
>> the instruction itself. The assert checks for the latter. Problematic is
>> the fact that for these type of repeating instructions, Xen seems to
>> eventually just commit all register state when it encounters an exception:
>
> If my analysis is correct, none of this matters here; the core emulator
> is working correctly. Hence also why the in-tree fuzzer wouldn't have
> caught it. Would you please give the patch a try that I just sent, with
> Cc to you (sorry, the list archive didn't pick it up yet, hence no link)?
https://lists.xen.org/archives/html/xen-devel/2025-04/msg01283.html
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