On 25/06/13 23:13, James Hogan wrote: > On 25 June 2013 22:40, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: >> Meanwhile, unprivileged users can make a MIPS kernel go BUG. >> >> How much of a problem is this? Obviously less of a problem with MIPS >> than it would be with some other CPU types, but I'd imagine it's still >> awkward in some environments. >> >> If this _is_ considered a problem, can we think of some nasty little >> hack which at least makes the effects less damaging, which we can also >> put into -stable kernels? > > The first rfc patch I sent sort of satisfies that by passing 127 if > sig==128, or slightly better would be passing 126 if sig>=127 (so that > SIFSIGNALED returns true). Effectively #ifdef'ing it on _NSIG>127 as > this patch does may be preferable too. > > That's probably the minimum change necessary to evade the BUG_ON > without removing it. The wait status code will still be wrong, but it > wasn't exactly right before so it's no worse. > > IMO changing the ABI by reducing _NSIG to 127 or 126 isn't appropriate > for stable. How does this look for a nasty/stable fix? >From 94d734526d61f5c74fd2df1c3ecb677495fc7a23 Mon Sep 17 00:00:00 2001 From: James Hogan <james.hogan@xxxxxxxxxx> Date: Wed, 26 Jun 2013 11:48:11 +0100 Subject: [PATCH 1/1] kernel/signal.c: fix BUG_ON with SIG128 (MIPS) MIPS has 128 signals, the highest of which has the number 128 (they start from 1). The following command causes get_signal_to_deliver() to pass this signal number straight through to do_group_exit() as the exit code: strace sleep 10 & sleep 1 && kill -128 `pidof sleep` However do_group_exit() checks for the core dump bit (0x80) in the exit code which matches in this particular case and the kernel panics: BUG_ON(exit_code & 0x80); /* core dumps don't get here */ As a quick fix, mask out higher bits in the signal number. This effectively matches the exit code from other code paths but avoids the BUG_ON. Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx> Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: David Daney <david.daney@xxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Cc: David Howells <dhowells@xxxxxxxxxx> Cc: Dave Jones <davej@xxxxxxxxxx> Cc: linux-mips@xxxxxxxxxxxxxx Cc: stable@xxxxxxxxxxxxxxx --- kernel/signal.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/kernel/signal.c b/kernel/signal.c index 113411b..9ea8f4f 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2366,8 +2366,14 @@ relock: /* * Death signals, no core dump. + * + * Some architectures (MIPS) have 128 signals which doesn't play + * nicely with the exit code since there are only 7 bits to + * store the terminating signal number. Mask out higher bits to + * avoid overflowing into the core dump bit and triggering + * BUG_ON in do_group_exit. */ - do_group_exit(info->si_signo); + do_group_exit(info->si_signo & 0x7f); /* NOTREACHED */ } spin_unlock_irq(&sighand->siglock); -- 1.8.1.2 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html