On Mar 25, 2021, at 11:54, 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.

But if sigaltstack()’ed with the SS_AUTODISARM flag, both on_sig_stack() and
sas_ss_flags() return 0 [1]. Then, segfault always here. v5 had the exact
issue before [2].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/sched/signal.h#n576
[2] https://lore.kernel.org/lkml/CALCETrXuFrHUU-L=HMofTgEDZk9muPnVtK=EjsTHqQ01XhbRYg@mail.gmail.com/ (local)

Thanks,
Chang