Subject: Re: [PATCH v5 06/10] powerpc/signal64: Replace setup_sigcontext() w/ unsafe_setup_sigcontext()

From: Daniel Axtens <[email-masked]>
Date: Fri, 12 Feb 2021 16:41:31 +1100

Hi Chris,

> Previously setup_sigcontext() performed a costly KUAP switch on every
> uaccess operation. These repeated uaccess switches cause a significant
> drop in signal handling performance.
>
> Rewrite setup_sigcontext() to assume that a userspace write access window
> is open. Replace all uaccess functions with their 'unsafe' versions
> which avoid the repeated uaccess switches.

Just to clarify the commit message a bit: you're also changing the
callers of the old safe versions to first open the window, then call the
unsafe variants, then close the window again.

> Signed-off-by: Christopher M. Riedl <[email-masked]>
> ---
>  arch/powerpc/kernel/signal_64.c | 70 ++++++++++++++++++++-------------
>  1 file changed, 43 insertions(+), 27 deletions(-)
>
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index 8e1d804ce552..4248e4489ff1 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -101,9 +101,13 @@ static void prepare_setup_sigcontext(struct task_struct *tsk, int ctx_has_vsx_re
>   * Set up the sigcontext for the signal frame.
>   */
>  
> -static long setup_sigcontext(struct sigcontext __user *sc,
> -		struct task_struct *tsk, int signr, sigset_t *set,
> -		unsigned long handler, int ctx_has_vsx_region)
> +#define unsafe_setup_sigcontext(sc, tsk, signr, set, handler,		\
> +				ctx_has_vsx_region, e)			\
> +	unsafe_op_wrap(__unsafe_setup_sigcontext(sc, tsk, signr, set,	\
> +			handler, ctx_has_vsx_region), e)
> +static long notrace __unsafe_setup_sigcontext(struct sigcontext __user *sc,
> +					struct task_struct *tsk, int signr, sigset_t *set,
> +					unsigned long handler, int ctx_has_vsx_region)
>  {
>  	/* When CONFIG_ALTIVEC is set, we _always_ setup v_regs even if the
>  	 * process never used altivec yet (MSR_VEC is zero in pt_regs of
> @@ -118,20 +122,19 @@ static long setup_sigcontext(struct sigcontext __user *sc,
>  #endif
>  	struct pt_regs *regs = tsk->thread.regs;
>  	unsigned long msr = regs->msr;
> -	long err = 0;
>  	/* Force usr to alway see softe as 1 (interrupts enabled) */
>  	unsigned long softe = 0x1;
>  
>  	BUG_ON(tsk != current);
>  
>  #ifdef CONFIG_ALTIVEC
> -	err |= __put_user(v_regs, &sc->v_regs);
> +	unsafe_put_user(v_regs, &sc->v_regs, efault_out);
>  
>  	/* save altivec registers */
>  	if (tsk->thread.used_vr) {
>  		/* Copy 33 vec registers (vr0..31 and vscr) to the stack */
> -		err |= __copy_to_user(v_regs, &tsk->thread.vr_state,
> -				      33 * sizeof(vector128));
> +		unsafe_copy_to_user(v_regs, &tsk->thread.vr_state,
> +				    33 * sizeof(vector128), efault_out);
>  		/* set MSR_VEC in the MSR value in the frame to indicate that sc->v_reg)
>  		 * contains valid data.
>  		 */
> @@ -140,12 +143,12 @@ static long setup_sigcontext(struct sigcontext __user *sc,
>  	/* We always copy to/from vrsave, it's 0 if we don't have or don't
>  	 * use altivec.
>  	 */
> -	err |= __put_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33]);
> +	unsafe_put_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33], efault_out);
>  #else /* CONFIG_ALTIVEC */
> -	err |= __put_user(0, &sc->v_regs);
> +	unsafe_put_user(0, &sc->v_regs, efault_out);
>  #endif /* CONFIG_ALTIVEC */
>  	/* copy fpr regs and fpscr */
> -	err |= copy_fpr_to_user(&sc->fp_regs, tsk);
> +	unsafe_copy_fpr_to_user(&sc->fp_regs, tsk, efault_out);
>  
>  	/*
>  	 * Clear the MSR VSX bit to indicate there is no valid state attached
> @@ -160,24 +163,27 @@ static long setup_sigcontext(struct sigcontext __user *sc,
>  	 */
>  	if (tsk->thread.used_vsr && ctx_has_vsx_region) {
>  		v_regs += ELF_NVRREG;
> -		err |= copy_vsx_to_user(v_regs, tsk);
> +		unsafe_copy_vsx_to_user(v_regs, tsk, efault_out);
>  		/* set MSR_VSX in the MSR value in the frame to
>  		 * indicate that sc->vs_reg) contains valid data.
>  		 */
>  		msr |= MSR_VSX;
>  	}
>  #endif /* CONFIG_VSX */
> -	err |= __put_user(&sc->gp_regs, &sc->regs);
> +	unsafe_put_user(&sc->gp_regs, &sc->regs, efault_out);
>  	WARN_ON(!FULL_REGS(regs));
> -	err |= __copy_to_user(&sc->gp_regs, regs, GP_REGS_SIZE);
> -	err |= __put_user(msr, &sc->gp_regs[PT_MSR]);
> -	err |= __put_user(softe, &sc->gp_regs[PT_SOFTE]);
> -	err |= __put_user(signr, &sc->signal);
> -	err |= __put_user(handler, &sc->handler);
> +	unsafe_copy_to_user(&sc->gp_regs, regs, GP_REGS_SIZE, efault_out);
> +	unsafe_put_user(msr, &sc->gp_regs[PT_MSR], efault_out);
> +	unsafe_put_user(softe, &sc->gp_regs[PT_SOFTE], efault_out);
> +	unsafe_put_user(signr, &sc->signal, efault_out);
> +	unsafe_put_user(handler, &sc->handler, efault_out);
>  	if (set != NULL)
> -		err |=  __put_user(set->sig[0], &sc->oldmask);
> +		unsafe_put_user(set->sig[0], &sc->oldmask, efault_out);
>  
> -	return err;
> +	return 0;
> +
> +efault_out:
> +	return -EFAULT;
>  }
>  
>  #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> @@ -664,12 +670,15 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
>  
>  	if (old_ctx != NULL) {
>  		prepare_setup_sigcontext(current, ctx_has_vsx_region);
> -		if (!access_ok(old_ctx, ctx_size)
> -		    || setup_sigcontext(&old_ctx->uc_mcontext, current, 0, NULL, 0,
> -					ctx_has_vsx_region)
> -		    || __copy_to_user(&old_ctx->uc_sigmask,
> -				      &current->blocked, sizeof(sigset_t)))
> +		if (!user_write_access_begin(old_ctx, ctx_size))
>  			return -EFAULT;

I notice we get rid of access_ok, but that's fine because
user_write_access_begin includes access_ok since Linus decided that was
a good idea.

> +
> +		unsafe_setup_sigcontext(&old_ctx->uc_mcontext, current, 0, NULL,
> +					0, ctx_has_vsx_region, efault_out);
> +		unsafe_copy_to_user(&old_ctx->uc_sigmask, &current->blocked,
> +				    sizeof(sigset_t), efault_out);
> +
> +		user_write_access_end();
>  	}
>  	if (new_ctx == NULL)
>  		return 0;
> @@ -698,6 +707,10 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
>  	/* This returns like rt_sigreturn */
>  	set_thread_flag(TIF_RESTOREALL);
>  	return 0;
> +
> +efault_out:
> +	user_write_access_end();
> +	return -EFAULT;
>  }
>  
>  
> @@ -850,9 +863,12 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
>  	} else {
>  		err |= __put_user(0, &frame->uc.uc_link);
>  		prepare_setup_sigcontext(tsk, 1);
> -		err |= setup_sigcontext(&frame->uc.uc_mcontext, tsk, ksig->sig,
> -					NULL, (unsigned long)ksig->ka.sa.sa_handler,
> -					1);
> +		if (!user_write_access_begin(frame, sizeof(struct rt_sigframe)))
> +			return -EFAULT;

Here you're opening a window for all of `frame`...

> +		err |= __unsafe_setup_sigcontext(&frame->uc.uc_mcontext, tsk,
... but here you're only passing in frame->uc.uc_mcontext for writing.

ISTR that the underlying AMR switch is fully on / fully off so I don't
think it really matters, but in this case should you be calling
user_write_access_begin() with &frame->uc.uc_mcontext and the size of
that?

> +						ksig->sig, NULL,
> +						(unsigned long)ksig->ka.sa.sa_handler, 1);
> +		user_write_access_end();
>  	}
>  	err |= __copy_to_user(&frame->uc.uc_sigmask, set, sizeof(*set));
>  	if (err)

Apart from the size thing, everything looks good to me. I checked that
all the things you've changed from safe to unsafe pass the same
parameters, and they all looked good to me.

With those caveats,
 Reviewed-by: Daniel Axtens <[email-masked]>

Kind regards,
Daniel