Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig()

[Borislav Petkov]

On Tue, Jan 21, 2020 at 12:18:43PM -0800, Yu-cheng Yu wrote:
> When returning from a signal, there is user state in a userspace memory
> buffer and supervisor state in registers.  The in-kernel buffer has neither
> valid user or supervisor state.  To restore both, save supervisor fpregs

The correct formulation is "neither ... nor ... "

> first (and protect them across context switches), then restore it along

s/it/them/

> with user state.
> 
> This patch introduces saving and restoring of supervisor xstates for a

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

> sigreturn in the following steps:
> 
> - Preserve supervisor register values by saving the whole fpu xstates.
>   Saving the whole is necessary because doing XSAVES with a partial
>   requested-feature bitmap (RFBM) changes xcomp_bv.  Since user xstates are
>   to be restored from a user buffer, saved user xstates are discarded.
> 
> - Set TIF_NEED_FPU_LOAD, and do __fpu_invalidate_fpregs_state().
>   This prevents a context switch from corrupting the saved xstates,
>   and xstate is considered to be loaded again on return to userland.

s/considered/going to/

> - Under fpregs_lock(), restore user xstates (from the user buffer), and
>   then supervisor xstates (from previously saved content).
> 
> - When both parts of the restoration succeed, mark fpregs as valid.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/fpu/signal.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index e3781a4a52a8..0d3e06a772b0 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -327,14 +327,22 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  	}
>  
>  	/*
> -	 * The current state of the FPU registers does not matter. By setting
> -	 * TIF_NEED_FPU_LOAD unconditionally it is ensured that the our xstate
> -	 * is not modified on context switch and that the xstate is considered
> +	 * Supervisor xstates are not modified by user space input, and
> +	 * need to be saved and restored.  Save the whole because doing
> +	 * partial XSAVES changes xcomp_bv.
> +	 * By setting TIF_NEED_FPU_LOAD it is ensured that our xstate is
> +	 * not modified on context switch and that the xstate is considered

Reflow those comments to 80 cols. There's room to the right.

>  	 * to be loaded again on return to userland (overriding last_cpu avoids
>  	 * the optimisation).
>  	 */
> -	set_thread_flag(TIF_NEED_FPU_LOAD);
> +	fpregs_lock();
> +	if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> +		if (xfeatures_mask_supervisor())
> +			copy_xregs_to_kernel(&fpu->state.xsave);
> +		set_thread_flag(TIF_NEED_FPU_LOAD);

So the code sets TIF_NEED_FPU_LOAD unconditionally, why are you changing
this?

Why don't you simply do:

		set_thread_flag(TIF_NEED_FPU_LOAD);
		fpregs_lock();
		if (xfeatures_mask_supervisor())
			copy_xregs_to_kernel(&fpu->state.xsave);
		fpregs_unlock();


> +	}
>  	__fpu_invalidate_fpregs_state(fpu);
> +	fpregs_unlock();
>  
>  	if ((unsigned long)buf_fx % 64)
>  		fx_only = 1;
> @@ -360,6 +368,9 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  		ret = copy_user_to_fpregs_zeroing(buf_fx, xfeatures_user, fx_only);
>  		pagefault_enable();

<--- comment here why you're doing this. That function is an abomination
and needs commenting at least.

>  		if (!ret) {
> +			if (xfeatures_mask_supervisor())
> +				copy_kernel_to_xregs(&fpu->state.xsave,
> +						     xfeatures_mask_supervisor());
>  			fpregs_mark_activate();
>  			fpregs_unlock();
>  			return 0;
> @@ -389,7 +400,12 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>  		fpregs_lock();
>  		if (unlikely(init_bv))
>  			copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> -		ret = copy_kernel_to_xregs_err(&fpu->state.xsave, xfeatures_user);
> +		/*
> +		 * Restore previously saved supervisor xstates along with
> +		 * copied-in user xstates.
> +		 */
> +		ret = copy_kernel_to_xregs_err(&fpu->state.xsave,
> +					       xfeatures_user | xfeatures_mask_supervisor());
>  
>  	} else if (use_fxsr()) {
>  		ret = __copy_from_user(&fpu->state.fxsave, buf_fx, state_size);
> -- 
> 2.21.0
> 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette