linux-kernel - Re: [PATCH 08/29] x86/fpu: Remove fpu->initialized usage in __fpu__restore

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Date:   Thu, 6 Dec 2018 21:07:22 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 08/29] x86/fpu: Remove fpu->initialized usage in
 __fpu__restore_sig()

On Wed, Nov 28, 2018 at 11:20:14PM +0100, Sebastian Andrzej Siewior wrote:
> This is a preparation for the removal of the ->initialized member in the
> fpu struct.
> __fpu__restore_sig() is deactivating the FPU via fpu__drop() and then
> setting manually ->initialized followed by fpu__restore(). The result is
> that it is possible to manipulate fpu->state and the state of registers
> won't be saved/restored on a context switch which would overwrite
> fpu->state.
> 
> Don't access the fpu->state while the content is read from user space
> and examined / sanitized. Use a temporary kmalloc() buffer for the
> preparation of the FPU registers and once the state is considered okay,
> load it. Should something go wrong, return with an error and without
> altering the original FPU registers.
> 
> The removal of "fpu__initialize()" is a nop because fpu->initialized is
> already set for the user task.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  arch/x86/include/asm/fpu/signal.h |  2 +-
>  arch/x86/kernel/fpu/regset.c      |  5 ++--
>  arch/x86/kernel/fpu/signal.c      | 41 ++++++++++++-------------------
>  3 files changed, 19 insertions(+), 29 deletions(-)

...

> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index d99a8ee9e185e..9c35598697b94 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -207,11 +207,11 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
>  }
>  
>  static inline void
> -sanitize_restored_xstate(struct task_struct *tsk,
> +sanitize_restored_xstate(union fpregs_state *state,
>  			 struct user_i387_ia32_struct *ia32_env,
>  			 u64 xfeatures, int fx_only)
>  {
> -	struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
> +	struct xregs_state *xsave = &state->xsave;
>  	struct xstate_header *header = &xsave->header;
>  
>  	if (use_xsave()) {
> @@ -238,7 +238,7 @@ sanitize_restored_xstate(struct task_struct *tsk,
>  		 */
>  		xsave->i387.mxcsr &= mxcsr_feature_mask;
>  
> -		convert_to_fxsr(tsk, ia32_env);
> +		convert_to_fxsr(&state->fxsave, ia32_env);
>  	}
>  }
>  
> @@ -284,8 +284,6 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  	if (!access_ok(VERIFY_READ, buf, size))
>  		return -EACCES;
>  
> -	fpu__initialize(fpu);
> -
>  	if (!static_cpu_has(X86_FEATURE_FPU))
>  		return fpregs_soft_set(current, NULL,
>  				       0, sizeof(struct user_i387_ia32_struct),
> @@ -314,41 +312,34 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  		 * thread's fpu state, reconstruct fxstate from the fsave
>  		 * header. Validate and sanitize the copied state.
>  		 */
> +		union fpregs_state *state;
> +		void *tmp;
>  		struct user_i387_ia32_struct env;
>  		int err = 0;

Sort those in reverse xmas order pls.

Otherwise looks ok.

-- 
Regards/Gruss,
    Boris.

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