Re: [PATCH v8 5/6] x86/signal: Detect and prevent an alternate signal stack overflow

[Bae, Chang Seok]

On Apr 22, 2021, at 01:46, David Laight <David.Laight@xxxxxxxxxx> wrote:
> From: Chang S. Bae
>> Sent: 22 April 2021 05:49
>> 
>> The kernel pushes context on to the userspace stack to prepare for the
>> user's signal handler. When the user has supplied an alternate signal
>> stack, via sigaltstack(2), it is easy for the kernel to verify that the
>> stack size is sufficient for the current hardware context.
>> 
>> Check if writing the hardware context to the alternate stack will exceed
>> it's size. If yes, then instead of corrupting user-data and proceeding with
>> the original signal handler, an immediate SIGSEGV signal is delivered.
> 
> What happens if SIGSEGV is caught?

Boris pointed out the relevant notes before [1]. I think "unpredictable
results" is a somewhat vague statement but process termination is unavoidable
in this situation.

In the thread [1], a new signal number was discussed for the signal delivery
failure, but my takeaway is this SIGSEGV is still recognizable.

FWIW, Len summarized other possible approaches as well [2].

>> Refactor the stack pointer check code from on_sig_stack() and use the new
>> helper.
>> 
>> While the kernel allows new source code to discover and use a sufficient
>> alternate signal stack size, this check is still necessary to protect
>> binaries with insufficient alternate signal stack size from data
>> corruption.
> ...
>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>> index 3f6a0fcaa10c..ae60f838ebb9 100644
>> --- a/include/linux/sched/signal.h
>> +++ b/include/linux/sched/signal.h
>> @@ -537,6 +537,17 @@ static inline int kill_cad_pid(int sig, int priv)
>> #define SEND_SIG_NOINFO ((struct kernel_siginfo *) 0)
>> #define SEND_SIG_PRIV	((struct kernel_siginfo *) 1)
>> 
>> +static inline int __on_sig_stack(unsigned long sp)
>> +{
>> +#ifdef CONFIG_STACK_GROWSUP
>> +	return sp >= current->sas_ss_sp &&
>> +		sp - current->sas_ss_sp < current->sas_ss_size;
>> +#else
>> +	return sp > current->sas_ss_sp &&
>> +		sp - current->sas_ss_sp <= current->sas_ss_size;
>> +#endif
>> +}
>> +
> 
> Those don't look different enough.

The difference is on the SS_AUTODISARM flag check.  This refactoring was
suggested as on_sig_stack() brought confusion [3].

Thanks,
Chang

[1] https://lore.kernel.org/lkml/20210414120608.GE10709@xxxxxxx/
[2] https://lore.kernel.org/lkml/CAJvTdKnpWL8y4N_BrCiK7fU0UXERwuuM8o84LUpp7Watxd8STw@xxxxxxxxxxxxxx/
[3] https://lore.kernel.org/lkml/20210325212733.GC32296@xxxxxxx/