Re: [PATCH v3 1/2] KVM: x86: Check EFER validity on KVM_SET_SREGS*

[Yosry Ahmed]

On Mon, Jul 13, 2026 at 11:02 AM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> When handling userspace SREGS writes, check the validity of EFER (i.e.
> allowed bits) before writing the new value of EFER through the
> per-vendor set_efer callbacks. This prevents userspace from writing
> bogus values (e.g. EFER.SVME=1 with nested=0).
>
> Note: on KVM_SET_MSRS, KVM only checks EFER validity in terms of KVM
> caps, not guest caps, so it is possible to set EFER bits that are
> supported by KVM but not by the guest CPUID. Potentially allowing
> userspace to set msrs before CPUID.
>
> However, for KVM_SET_SREGS*, check the validity of the set bits against
> both KVM and guest caps. This is consistent with other validity checks
> (e.g. for CR4) that check validity against guest caps, which already
> imposes the need to set CPUID before SREGS.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Change-Id: I45701ec440e4fdd8f086eb70db0c0845fb0ed509

Forgot to drop these, sorry :/