Re: [PATCH V2] x86/sev: Mark the code returning to user space as syscall gap

[Joerg Roedel]

Hi,

On Tue, Apr 12, 2022 at 08:49:08PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
> 
> When returning to user space, the %rsp is user controlled value.
> 
> If it is SNP-guest and the hypervisor decides to mess with the code-page
> for this path while a CPU is executing it.  This will cause a #VC on
> that CPU and that could hit in the syscall return path and mislead
> the #VC handler.
> 
> So make ip_within_syscall_gap() return true in this case.

With the SNP guest patches in tip-tree I think it actually becomes
possible that a #VC exception hits in these parts of the execution
stream. It requires good timing by the attacker, but it is not
impossible. Therefore:

Acked-by: Joerg Roedel <jroedel@xxxxxxx>

-- 
Jörg Rödel
jroedel@xxxxxxx

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