linux-kernel - [PATCH][ACKed] really nasty sigreturn bug on arm

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

Message-ID: <20100917133439.GX31363@ZenIV.linux.org.uk>
Date:	Fri, 17 Sep 2010 14:34:39 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Russell King <rmk@....linux.org.uk>
Subject: [PATCH][ACKed] really nasty sigreturn bug on arm

If a signal hits us outside of a syscall and another gets delivered
when we are in sigreturn (e.g. because it had been in sa_mask for
the first one and got sent to us while we'd been in the first handler),
we have a chance of returning from the second handler to location one
insn prior to where we ought to return.  If r0 happens to contain -513
(-ERESTARTNOINTR), sigreturn will get confused into doing restart
syscall song and dance.

Incredible joy to debug, since it manifests as random, infrequent and
very hard to reproduce double execution of instructions in userland
code...

The fix is simple - mark it "don't bother with restarts" in wrapper,
i.e. set r8 to 0 in sys_sigreturn and sys_rt_sigreturn wrappers,
suppressing the syscall restart handling on return from these guys.
They can't legitimately return a restart-worthy error anyway.

Testcase:
	#include <unistd.h>
	#include <signal.h>
	#include <stdlib.h>
	#include <sys/time.h>
	#include <errno.h>

	void f(int n)
	{
		__asm__ __volatile__(
			"ldr r0, [%0]\n"
			"b 1f\n"
			"b 2f\n"
			"1:b .\n"
			"2:\n" : : "r"(&n));
	}

	void handler1(int sig) { }
	void handler2(int sig) { raise(1); }
	void handler3(int sig) { exit(0); }

	main()
	{
		struct sigaction s = {.sa_handler = handler2};
		struct itimerval t1 = { .it_value = {1} };
		struct itimerval t2 = { .it_value = {2} };

		signal(1, handler1);

		sigemptyset(&s.sa_mask);
		sigaddset(&s.sa_mask, 1);
		sigaction(SIGALRM, &s, NULL);

		signal(SIGVTALRM, handler3);

		setitimer(ITIMER_REAL, &t1, NULL);
		setitimer(ITIMER_VIRTUAL, &t2, NULL);

		f(-513); /* -ERESTARTNOINTR */

		write(1, "buggered\n", 9);
		return 1;
	}

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Acked-by: Russell King <rmk+kernel@....linux.org.uk>
Cc: stable@...nel.org
---
 arch/arm/kernel/entry-common.S |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -418,11 +418,13 @@ ENDPROC(sys_clone_wrapper)
 
 sys_sigreturn_wrapper:
 		add	r0, sp, #S_OFF
+		mov	why, #0		@ prevent syscall restart handling
 		b	sys_sigreturn
 ENDPROC(sys_sigreturn_wrapper)
 
 sys_rt_sigreturn_wrapper:
 		add	r0, sp, #S_OFF
+		mov	why, #0		@ prevent syscall restart handling
 		b	sys_rt_sigreturn
 ENDPROC(sys_rt_sigreturn_wrapper)
 
-- 
1.5.6.5


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/