Re: [PATCH 1/4] signals/sigaltstack: If SS_AUTODISARM, bypass on_sig_stack
From: Stas Sergeev
Date: Sat May 14 2016 - 07:18:51 EST
14.05.2016 07:18, Andy Lutomirski ÐÐÑÐÑ:
On May 8, 2016 7:05 PM, "Stas Sergeev" <stsp@xxxxxxx> wrote:
09.05.2016 04:32, Andy Lutomirski ÐÐÑÐÑ:
On May 7, 2016 7:38 AM, "Stas Sergeev" <stsp@xxxxxxx> wrote:
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?
Stack corruption. Don't do that.
Only after your change, I have to admit. :)
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?
The intention is to make everything completely explicit. With
SS_AUTODISARM, the kernel knows directly whether you're on the signal
stack, and there should be no need to look at sp. If you set
SS_AUTODISARM and get a signal, the signal stack gets disarmed. If
you take a nested signal, it's delivered normally. When you return
all the way out, the signal stack is re-armed.
For DOSEMU, this means that no 16-bit register state can possibly
cause a signal to be delivered wrong, because the register state when
a signal is raised won't affect delivery, which seems like a good
thing to me.
Yes, but doesn't affect dosemu1 which doesn't use SS_AUTODISARM.
So IMHO the SS check should still be added, even if not for dosemu2.
If this behavior would be problematic for you, can you explain why?
Only theoretically: if someone sets SS_AUTODISARM inside a
sighandler. Since this doesn't give EPERM, I wouldn't deliberately
make it a broken scenario (esp if it wasn't before the particular change).
Ideally it would give EPERM, but we can't, so doesn't matter much.
I just wanted to warn about the possible regression.
I suppose we could return an error if you are on the sigstack when
setting SS_AUTODISARM, although I was hoping to avoid yet more special
cases.
Hmm.
How about extending the generic check then?
Currently it is roughly:
if (on_sig_stack(sp)) return -EPERM;
and we could do:
if (on_sig_stack(sp) || on_new_sas(new_sas, sp)) return -EPERM;
Looks like it will close the potential hole opened by your commit
without introducing the special case for SS_AUTODISARM.
What do you think?