On Thu, Mar 25, 2021 at 11:54 AM Borislav Petkov [off-list ref] wrote:

On Thu, Mar 25, 2021 at 11:13:12AM -0700, Andy Lutomirski wrote:

quoted

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...

quoted

      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.

Nope.  on_sig_stack() is a horrible kludge and won't work here.  We
could have something like __on_sig_stack() or sp_is_on_sig_stack() or
something, though.

quoted

+             if (show_unhandled_signals && printk_ratelimit()) {
+                     pr_info("%s[%d] overflowed sigaltstack",
+                             tsk->comm, task_pid_nr(tsk));
+             }

Why do you even wanna issue that? It looks like callers will propagate
an error value up and people don't look at dmesg all the time.

I figure that the people whose programs spontaneously crash should get
a hint why if they look at dmesg.  Maybe the message should say
"overflowed sigaltstack -- try noavx512"?

We really ought to have a SIGSIGFAIL signal that's sent, double-fault
style, when we fail to send a signal.