On Wed, May 12 2021 at 18:48, Chang Seok Bae wrote:

On May 11, 2021, at 11:36, Borislav Petkov [off-list ref] wrote:

quoted

I clumsily tried to register a SIGSEGV handler with

       act.sa_sigaction = my_sigsegv;
       sigaction(SIGSEGV, &act, NULL);

but that doesn't fire - task gets killed. Maybe I'm doing it wrong.

Since the altstack is already overflowed, perhaps set the flag like this -- not
using it to get the handler:

	act.sa_sigaction = my_sigsegv;
+	act.sa_flags = SA_SIGINFO;
	sigaction(SIGSEGV, &act, NULL);

FWIW, I think this is just a workaround for this case; in practice, altstack is
rather a backup for normal stack corruption.

That's the intended usage, but it's not limited to that and there exists
creative (ab)use of sigaltstack beyond catching the overflow of the
regular stack.

Thanks,

        tglx