Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack

From: Stas Sergeev
Date: Sat May 07 2016 - 10:40:44 EST

03.05.2016 20:31, Andy Lutomirski ÐÐÑÐÑ:
If a signal stack is set up with SS_AUTODISARM, then the kernel
inherently avoids incorrectly resetting the signal stack if signals
recurse: the signal stack will be reset on the first signal
delivery. This means that we don't need check the stack pointer
when delivering signals if SS_AUTODISARM is set.

This will make segmented x86 programs more robust: currently there's
a hole that could be triggered if ESP/RSP appears to point to the
signal stack but actually doesn't due to a nonzero SS base.

Signed-off-by: Stas Sergeev <stsp@xxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
Cc: Amanieu d'Antras <amanieu@xxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Heinrich Schuchardt <xypron.glpk@xxxxxx>
Cc: Jason Low <jason.low2@xxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
Cc: Paul Moore <pmoore@xxxxxxxxxx>
Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Richard Weinberger <richard@xxxxxx>
Cc: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
include/linux/sched.h | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2950c5cd3005..8f03a93348b9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2576,6 +2576,18 @@ static inline int kill_cad_pid(int sig, int priv)
static inline int on_sig_stack(unsigned long sp)
+ /*
+ * If the signal stack is AUTODISARM then, by construction, we
+ * can't be on the signal stack unless user code deliberately set
+ * SS_AUTODISARM when we were already on the it.
"on the it" -> "on it".

Anyway, I am a bit puzzled with this patch.
You say "unless user code deliberately set
SS_AUTODISARM when we were already on the it"
so what happens in case it actually does?

Without your patch: if user sets up the same sas - no stack switch.
if user sets up different sas - stack switch on nested signal.

With your patch: stack switch in any case, so if user
set up same sas - stack corruption by nested signal.

Or am I missing the intention?