Re: [PATCH v7 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

[Borislav Petkov]

On Thu, Mar 25, 2021 at 11:13:12AM -0700, Andy Lutomirski wrote:
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index ea794a083c44..53781324a2d3 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -237,7 +237,8 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>  	unsigned long math_size = 0;
>  	unsigned long sp = regs->sp;
>  	unsigned long buf_fx = 0;
> -	int onsigstack = on_sig_stack(sp);
> +	bool already_onsigstack = on_sig_stack(sp);
> +	bool entering_altstack = false;
>  	int ret;
>  
>  	/* redzone */
> @@ -246,15 +247,25 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>  
>  	/* This is the X/Open sanctioned signal stack switching.  */
>  	if (ka->sa.sa_flags & SA_ONSTACK) {
> -		if (sas_ss_flags(sp) == 0)
> +		/*
> +		 * This checks already_onsigstack via sas_ss_flags().
> +		 * Sensible programs use SS_AUTODISARM, which disables
> +		 * that check, and programs that don't use
> +		 * SS_AUTODISARM get compatible but potentially
> +		 * bizarre behavior.
> +		 */
> +		if (sas_ss_flags(sp) == 0) {
>  			sp = current->sas_ss_sp + current->sas_ss_size;
> +			entering_altstack = true;
> +		}
>  	} else if (IS_ENABLED(CONFIG_X86_32) &&
> -		   !onsigstack &&
> +		   !already_onsigstack &&
>  		   regs->ss != __USER_DS &&
>  		   !(ka->sa.sa_flags & SA_RESTORER) &&
>  		   ka->sa.sa_restorer) {
>  		/* This is the legacy signal stack switching. */
>  		sp = (unsigned long) ka->sa.sa_restorer;
> +		entering_altstack = true;
>  	}

What a mess this whole signal handling is. I need a course in signal
handling to understand what's going on here...

>  
>  	sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32),
> @@ -267,8 +278,16 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
>  	 * If we are on the alternate signal stack and would overflow it, don't.
>  	 * Return an always-bogus address instead so we will die with SIGSEGV.
>  	 */
> -	if (onsigstack && !likely(on_sig_stack(sp)))
> +	if (unlikely(entering_altstack &&
> +		     (sp <= current->sas_ss_sp ||
> +		      sp - current->sas_ss_sp > current->sas_ss_size))) {

You could've simply done

	if (unlikely(entering_altstack && !on_sig_stack(sp)))

here.


> +		if (show_unhandled_signals && printk_ratelimit()) {
> +			pr_info("%s[%d] overflowed sigaltstack",
> +				tsk->comm, task_pid_nr(tsk));
> +		}

Why do you even wanna issue that? It looks like callers will propagate
an error value up and people don't look at dmesg all the time.

Btw, s/tsk/current/g

IOW, this builds:

---
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index a06cb107c0e8..c00e932b5f18 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -234,10 +234,11 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	     void __user **fpstate)
 {
 	/* Default to using normal stack */
+	bool already_onsigstack = on_sig_stack(regs->sp);
+	bool entering_altstack = false;
 	unsigned long math_size = 0;
 	unsigned long sp = regs->sp;
 	unsigned long buf_fx = 0;
-	int onsigstack = on_sig_stack(sp);
 	int ret;
 
 	/* redzone */
@@ -246,15 +247,24 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(sp) == 0)
+		/*
+		 * This checks already_onsigstack via sas_ss_flags(). Sensible
+		 * programs use SS_AUTODISARM, which disables that check, and
+		 * programs that don't use SS_AUTODISARM get compatible but
+		 * potentially bizarre behavior.
+		 */
+		if (sas_ss_flags(sp) == 0) {
 			sp = current->sas_ss_sp + current->sas_ss_size;
+			entering_altstack = true;
+		}
 	} else if (IS_ENABLED(CONFIG_X86_32) &&
-		   !onsigstack &&
+		   !already_onsigstack &&
 		   regs->ss != __USER_DS &&
 		   !(ka->sa.sa_flags & SA_RESTORER) &&
 		   ka->sa.sa_restorer) {
 		/* This is the legacy signal stack switching. */
 		sp = (unsigned long) ka->sa.sa_restorer;
+		entering_altstack = true;
 	}
 
 	sp = fpu__alloc_mathframe(sp, IS_ENABLED(CONFIG_X86_32),
@@ -267,8 +277,14 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	 * If we are on the alternate signal stack and would overflow it, don't.
 	 * Return an always-bogus address instead so we will die with SIGSEGV.
 	 */
-	if (onsigstack && !likely(on_sig_stack(sp)))
+	if (unlikely(entering_altstack && !on_sig_stack(sp))) {
+
+		if (show_unhandled_signals && printk_ratelimit())
+			pr_info("%s[%d] overflowed sigaltstack",
+				current->comm, task_pid_nr(current));
+
 		return (void __user *)-1L;
+	}
 
 	/* save i387 and extended state */
 	ret = copy_fpstate_to_sigframe(*fpstate, (void __user *)buf_fx, math_size);


-- 
Regards/Gruss,
    Boris.

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