2023-09-14 12:18:07

by Yang Weijiang

Subject: [PATCH v6 08/25] x86/fpu/xstate: WARN if normal fpstate contains kernel dynamic xfeatures

fpu_kernel_dynamic_xfeatures now are __ONLY__ enabled by guest kernel and
used for guest fpstate, i.e., none for normal fpstate. The bits are added
when guest fpstate is allocated and fpstate->is_guest set to %true.

For normal fpstate, the bits should have been removed when init system FPU
settings, WARN_ONCE() if normal fpstate contains kernel dynamic xfeatures
before xsaves is executed.

Signed-off-by: Yang Weijiang <[email protected]>
---
arch/x86/kernel/fpu/xstate.h | 3 +++
1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index 9c6e3ca05c5c..c2b33a5db53d 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -186,6 +186,9 @@ static inline void os_xsave(struct fpstate *fpstate)
WARN_ON_FPU(!alternatives_patched);
xfd_validate_state(fpstate, mask, false);

+ WARN_ON_FPU(!fpstate->is_guest &&
+ (mask & fpu_kernel_dynamic_xfeatures));
+
XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);

/* We should never fault when copying to a kernel buffer: */
--
2.27.0

2023-10-31 17:46:57

by Maxim Levitsky

Subject: Re: [PATCH v6 08/25] x86/fpu/xstate: WARN if normal fpstate contains kernel dynamic xfeatures

On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> fpu_kernel_dynamic_xfeatures now are __ONLY__ enabled by guest kernel and
> used for guest fpstate, i.e., none for normal fpstate. The bits are added
> when guest fpstate is allocated and fpstate->is_guest set to %true.
>
> For normal fpstate, the bits should have been removed when init system FPU
> settings, WARN_ONCE() if normal fpstate contains kernel dynamic xfeatures
> before xsaves is executed.
>
> Signed-off-by: Yang Weijiang <[email protected]>
> ---
> arch/x86/kernel/fpu/xstate.h | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
> index 9c6e3ca05c5c..c2b33a5db53d 100644
> --- a/arch/x86/kernel/fpu/xstate.h
> +++ b/arch/x86/kernel/fpu/xstate.h
> @@ -186,6 +186,9 @@ static inline void os_xsave(struct fpstate *fpstate)
> WARN_ON_FPU(!alternatives_patched);
> xfd_validate_state(fpstate, mask, false);
>
> + WARN_ON_FPU(!fpstate->is_guest &&
> + (mask & fpu_kernel_dynamic_xfeatures));
> +
> XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);
>
> /* We should never fault when copying to a kernel buffer: */

I am not sure about this patch. It's true that now the kernel dynamic features
are for guest only, but in the future I can easily see a kernel dynamic feature
that will also be used in the kernel itself.

Maybe we can add a comment above this warning to say that _currently_ there are
no kernel dynamic features that are enabled for the host kernel.

Best regards,
Maxim Levitsky