Re: [PATCH v2 3/4] x86/signal: Prevent an alternate stack overflow before a signal delivery

On Nov 20, 2020, at 15:04, Jann Horn [off-list ref] wrote:

On Thu, Nov 19, 2020 at 8:40 PM Chang S. Bae [off-list ref] wrote:

quoted

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index ee6f1ceaa7a2..cee41d684dc2 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c

@@ -251,8 +251,13 @@ 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)
+               if (sas_ss_flags(sp) == 0) {
+                       /* If the altstack might overflow, die with SIGSEGV: */
+                       if (!altstack_size_ok(current))
+                               return (void __user *)-1L;
+
                       sp = current->sas_ss_sp + current->sas_ss_size;
+               }

A couple lines further down, we have this (since commit 14fc9fbc700d):

       /*
        * 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)))
               return (void __user *)-1L;

Is that not working?

onsigstack is set at the beginning here. If a signal hits under normal stack,
this flag is not set. Then it will miss the overflow.

The added check allows to detect the sigaltstack overflow (always).

Thanks,
Chang