Re: [PATCH 04/10] x86/fpu: Document reasoning of FX-only fallback

[Alexander Mikhalitsyn]

Am Mo., 15. Juni 2026 um 21:39 Uhr schrieb Andrei Vagin <avagin@xxxxxxxxxx>:
>
> Add a comment to check_xstate_in_sigframe() to explain reasoning behind
> falling back to the FX-only state when signal frame metadata is
> inconsistent.
>
> The fallback is intended to preserve backward compatibility with legacy
> user-space processes that are not aware of XSAVE states and might only
> fill or copy just the legacy FP state.
>
> This fallback is dangerous as it can trigger silent corruptions of
> user-space state by resetting extended registers if the process was
> using them but the frame metadata was malformed.
>
> XSAVE was introduced 15 years ago, we may need to consider removing this
> fallback entirely or introducing a sysctl to enable/disable it.
>
> Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>

Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxxx>

> ---
>  arch/x86/kernel/fpu/signal.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 6a14b528ac7f..85021c5ea649 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -54,6 +54,14 @@ static inline bool check_xstate_in_sigframe(struct fxregs_state __user *buf_fx,
>         if (likely(magic2 == FP_XSTATE_MAGIC2))
>                 return true;
>  err_setfx:
> +       /*
> +        * The fallback to FX-only state is used to preserve backward
> +        * compatibility with user-space processes that are not aware of xsave
> +        * states.
> +        *
> +        * In all other cases, returning false (to trigger SIGSEGV) is
> +        * preferred to avoid silent user-space state corruption.
> +        */
>         trace_x86_fpu_xstate_check_failed(x86_task_fpu(current));
>
>         /* Set the parameters for fx only state */
> --
> 2.54.0.1189.g8c84645362-goog
>
>