Subject: Re: [PATCH v5 07/10] powerpc/signal64: Replace restore_sigcontext() w/ unsafe_restore_sigcontext()

From: Daniel Axtens <[email-masked]>
Date: Sat, 13 Feb 2021 08:17:54 +1100

Hi Chris,

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

Much of the same comments apply here as to the last patch:
 - the commit message might be improved by adding that you are also
   changing the calling functions to open the uaccess window before
   calling into the new unsafe functions

 - I have checked that the safe to unsafe conversions look right.

 - I think you're opening too wide a window in user_read_access_begin,
   it seems to me that it could be reduced to just the
   uc_mcontext. (Again, not that it makes a difference with the current
   HW.)

Kind regards,
Daniel

> Signed-off-by: Christopher M. Riedl <[email-masked]>
> ---
>  arch/powerpc/kernel/signal_64.c | 68 ++++++++++++++++++++-------------
>  1 file changed, 41 insertions(+), 27 deletions(-)
>
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index 4248e4489ff1..d668f8af18fe 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -326,14 +326,14 @@ static long setup_tm_sigcontexts(struct sigcontext __user *sc,
>  /*
>   * Restore the sigcontext from the signal frame.
>   */
> -
> -static long restore_sigcontext(struct task_struct *tsk, sigset_t *set, int sig,
> -			      struct sigcontext __user *sc)
> +#define unsafe_restore_sigcontext(tsk, set, sig, sc, e) \
> +	unsafe_op_wrap(__unsafe_restore_sigcontext(tsk, set, sig, sc), e)
> +static long notrace __unsafe_restore_sigcontext(struct task_struct *tsk, sigset_t *set,
> +						int sig, struct sigcontext __user *sc)
>  {
>  #ifdef CONFIG_ALTIVEC
>  	elf_vrreg_t __user *v_regs;
>  #endif
> -	unsigned long err = 0;
>  	unsigned long save_r13 = 0;
>  	unsigned long msr;
>  	struct pt_regs *regs = tsk->thread.regs;
> @@ -348,27 +348,28 @@ static long restore_sigcontext(struct task_struct *tsk, sigset_t *set, int sig,
>  		save_r13 = regs->gpr[13];
>  
>  	/* copy the GPRs */
> -	err |= __copy_from_user(regs->gpr, sc->gp_regs, sizeof(regs->gpr));
> -	err |= __get_user(regs->nip, &sc->gp_regs[PT_NIP]);
> +	unsafe_copy_from_user(regs->gpr, sc->gp_regs, sizeof(regs->gpr),
> +			      efault_out);
> +	unsafe_get_user(regs->nip, &sc->gp_regs[PT_NIP], efault_out);
>  	/* get MSR separately, transfer the LE bit if doing signal return */
> -	err |= __get_user(msr, &sc->gp_regs[PT_MSR]);
> +	unsafe_get_user(msr, &sc->gp_regs[PT_MSR], efault_out);
>  	if (sig)
>  		regs->msr = (regs->msr & ~MSR_LE) | (msr & MSR_LE);
> -	err |= __get_user(regs->orig_gpr3, &sc->gp_regs[PT_ORIG_R3]);
> -	err |= __get_user(regs->ctr, &sc->gp_regs[PT_CTR]);
> -	err |= __get_user(regs->link, &sc->gp_regs[PT_LNK]);
> -	err |= __get_user(regs->xer, &sc->gp_regs[PT_XER]);
> -	err |= __get_user(regs->ccr, &sc->gp_regs[PT_CCR]);
> +	unsafe_get_user(regs->orig_gpr3, &sc->gp_regs[PT_ORIG_R3], efault_out);
> +	unsafe_get_user(regs->ctr, &sc->gp_regs[PT_CTR], efault_out);
> +	unsafe_get_user(regs->link, &sc->gp_regs[PT_LNK], efault_out);
> +	unsafe_get_user(regs->xer, &sc->gp_regs[PT_XER], efault_out);
> +	unsafe_get_user(regs->ccr, &sc->gp_regs[PT_CCR], efault_out);
>  	/* Don't allow userspace to set SOFTE */
>  	set_trap_norestart(regs);
> -	err |= __get_user(regs->dar, &sc->gp_regs[PT_DAR]);
> -	err |= __get_user(regs->dsisr, &sc->gp_regs[PT_DSISR]);
> -	err |= __get_user(regs->result, &sc->gp_regs[PT_RESULT]);
> +	unsafe_get_user(regs->dar, &sc->gp_regs[PT_DAR], efault_out);
> +	unsafe_get_user(regs->dsisr, &sc->gp_regs[PT_DSISR], efault_out);
> +	unsafe_get_user(regs->result, &sc->gp_regs[PT_RESULT], efault_out);
>  
>  	if (!sig)
>  		regs->gpr[13] = save_r13;
>  	if (set != NULL)
> -		err |=  __get_user(set->sig[0], &sc->oldmask);
> +		unsafe_get_user(set->sig[0], &sc->oldmask, efault_out);
>  
>  	/*
>  	 * Force reload of FP/VEC.
> @@ -378,29 +379,28 @@ static long restore_sigcontext(struct task_struct *tsk, sigset_t *set, int sig,
>  	regs->msr &= ~(MSR_FP | MSR_FE0 | MSR_FE1 | MSR_VEC | MSR_VSX);
>  
>  #ifdef CONFIG_ALTIVEC
> -	err |= __get_user(v_regs, &sc->v_regs);
> -	if (err)
> -		return err;
> +	unsafe_get_user(v_regs, &sc->v_regs, efault_out);
>  	if (v_regs && !access_ok(v_regs, 34 * sizeof(vector128)))
>  		return -EFAULT;
>  	/* Copy 33 vec registers (vr0..31 and vscr) from the stack */
>  	if (v_regs != NULL && (msr & MSR_VEC) != 0) {
> -		err |= __copy_from_user(&tsk->thread.vr_state, v_regs,
> -					33 * sizeof(vector128));
> +		unsafe_copy_from_user(&tsk->thread.vr_state, v_regs,
> +				      33 * sizeof(vector128), efault_out);
>  		tsk->thread.used_vr = true;
>  	} else if (tsk->thread.used_vr) {
>  		memset(&tsk->thread.vr_state, 0, 33 * sizeof(vector128));
>  	}
>  	/* Always get VRSAVE back */
>  	if (v_regs != NULL)
> -		err |= __get_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33]);
> +		unsafe_get_user(tsk->thread.vrsave, (u32 __user *)&v_regs[33],
> +				efault_out);
>  	else
>  		tsk->thread.vrsave = 0;
>  	if (cpu_has_feature(CPU_FTR_ALTIVEC))
>  		mtspr(SPRN_VRSAVE, tsk->thread.vrsave);
>  #endif /* CONFIG_ALTIVEC */
>  	/* restore floating point */
> -	err |= copy_fpr_from_user(tsk, &sc->fp_regs);
> +	unsafe_copy_fpr_from_user(tsk, &sc->fp_regs, efault_out);
>  #ifdef CONFIG_VSX
>  	/*
>  	 * Get additional VSX data. Update v_regs to point after the
> @@ -409,14 +409,17 @@ static long restore_sigcontext(struct task_struct *tsk, sigset_t *set, int sig,
>  	 */
>  	v_regs += ELF_NVRREG;
>  	if ((msr & MSR_VSX) != 0) {
> -		err |= copy_vsx_from_user(tsk, v_regs);
> +		unsafe_copy_vsx_from_user(tsk, v_regs, efault_out);
>  		tsk->thread.used_vsr = true;
>  	} else {
>  		for (i = 0; i < 32 ; i++)
>  			tsk->thread.fp_state.fpr[i][TS_VSRLOWOFFSET] = 0;
>  	}
>  #endif
> -	return err;
> +	return 0;
> +
> +efault_out:
> +	return -EFAULT;
>  }
>  
>  #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> @@ -701,8 +704,14 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
>  	if (__copy_from_user(&set, &new_ctx->uc_sigmask, sizeof(set)))
>  		do_exit(SIGSEGV);
>  	set_current_blocked(&set);
> -	if (restore_sigcontext(current, NULL, 0, &new_ctx->uc_mcontext))
> +
> +	if (!user_read_access_begin(new_ctx, ctx_size))
> +		return -EFAULT;
> +	if (__unsafe_restore_sigcontext(current, NULL, 0, &new_ctx->uc_mcontext)) {
> +		user_read_access_end();
>  		do_exit(SIGSEGV);
> +	}
> +	user_read_access_end();
>  
>  	/* This returns like rt_sigreturn */
>  	set_thread_flag(TIF_RESTOREALL);
> @@ -807,8 +816,13 @@ SYSCALL_DEFINE0(rt_sigreturn)
>  		 * causing a TM bad thing.
>  		 */
>  		current->thread.regs->msr &= ~MSR_TS_MASK;
> -		if (restore_sigcontext(current, NULL, 1, &uc->uc_mcontext))
> +		if (!user_read_access_begin(uc, sizeof(*uc)))
> +			return -EFAULT;
> +		if (__unsafe_restore_sigcontext(current, NULL, 1, &uc->uc_mcontext)) {
> +			user_read_access_end();
>  			goto badframe;
> +		}
> +		user_read_access_end();
>  	}
>  
>  	if (restore_altstack(&uc->uc_stack))
> -- 
> 2.26.1