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

[Bae, Chang Seok]

On Mar 25, 2021, at 11:54, Borislav Petkov <bp@xxxxxxx> wrote:
> 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.

But if sigaltstack()’ed with the SS_AUTODISARM flag, both on_sig_stack() and
sas_ss_flags() return 0 [1]. Then, segfault always here. v5 had the exact
issue before [2].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/sched/signal.h#n576
[2] https://lore.kernel.org/lkml/CALCETrXuFrHUU-L=HMofTgEDZk9muPnVtK=EjsTHqQ01XhbRYg@xxxxxxxxxxxxxx/

Thanks,
Chang