Re: [patch V3 01/66] x86/fpu: x86/fpu: Preserve supervisor states in sanitize_restored_user_xstate()

From: Borislav Petkov
Date: Sat Jun 19 2021 - 04:37:21 EST


On Fri, Jun 18, 2021 at 04:18:24PM +0200, Thomas Gleixner wrote:
> Subject: Re: [patch V3 01/66] x86/fpu: x86/fpu: Preserve supervisor states in

Prefix repeated.

> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> sanitize_restored_user_xstate() preserves the supervisor states only
> when the fx_only argument is zero, which allows unpriviledged user space
> to put supervisor states back into init state.

Yikes.

> Preserve them unconditionally.
>
> Fixes: 5d6b6a6f9b5c ("x86/fpu/xstate: Update sanitize_restored_xstate() for supervisor xstates")
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> arch/x86/kernel/fpu/signal.c | 26 ++++++++------------------
> 1 file changed, 8 insertions(+), 18 deletions(-)
>
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -221,28 +221,18 @@ sanitize_restored_user_xstate(union fpre
>
> if (use_xsave()) {
> /*
> - * Note: we don't need to zero the reserved bits in the
> - * xstate_header here because we either didn't copy them at all,
> - * or we checked earlier that they aren't set.
> + * Clear all features bit which are not set in

feature bits

> + * user_xfeatures and clear all extended features
> + * for fx_only mode.
> */
> + u64 mask = fx_only ? XFEATURE_MASK_FPSSE : user_xfeatures;
>
> /*
> - * 'user_xfeatures' might have bits clear which are
> - * set in header->xfeatures. This represents features that
> - * were in init state prior to a signal delivery, and need
> - * to be reset back to the init state. Clear any user
> - * feature bits which are set in the kernel buffer to get
> - * them back to the init state.
> - *
> - * Supervisor state is unchanged by input from userspace.
> - * Ensure supervisor state bits stay set and supervisor
> - * state is not modified.
> + * Supervisor state has to be preserved. The sigframe
> + * restore can only modify user features, i.e. @mask
> + * cannot contain them.
> */
> - if (fx_only)
> - header->xfeatures = XFEATURE_MASK_FPSSE;
> - else
> - header->xfeatures &= user_xfeatures |
> - xfeatures_mask_supervisor();
> + header->xfeatures &= mask | xfeatures_mask_supervisor();

With those addressed:

Reviewed-by: Borislav Petkov <bp@xxxxxxx>

Thx.

--
Regards/Gruss,
Boris.

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