On Tue, Nov 24, 2020 at 7:22 PM Bae, Chang Seok
[off-list ref] wrote:

quoted

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).

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 &&