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

[Jann Horn]

On Tue, Nov 24, 2020 at 7:22 PM Bae, Chang Seok
<chang.seok.bae@intel.com> wrote:
> > On Nov 20, 2020, at 15:04, Jann Horn <jannh@google.com> wrote:
> > On Thu, Nov 19, 2020 at 8:40 PM Chang S. Bae <chang.seok.bae@intel.com> wrote:
> >>
> >> 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).

Ah, I think I understand what you're trying to do. But wouldn't the
better approach be to ensure that the existing on_sig_stack() check is
also used if we just switched to the signal stack? Something like:

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index be0d7d4152ec..2f57842fb4d6 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -237,7 +237,7 @@ 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 onsigstack = on_sig_stack(sp);
        int ret;

        /* redzone */
@@ -246,8 +246,10 @@ 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) {
                        sp = current->sas_ss_sp + current->sas_ss_size;
+                       onsigstack = true;
+               }
        } else if (IS_ENABLED(CONFIG_X86_32) &&
                   !onsigstack &&
                   regs->ss != __USER_DS &&