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Re: Bug#751417: linux-image-3.2.0-4-5kc-malta: no SIGKILL after prctl(PR

To: stable <stable@vger.kernel.org>, linux-mips@linux-mips.org, Ralf Baechle <ralf@linux-mips.org>
Subject: Re: Bug#751417: linux-image-3.2.0-4-5kc-malta: no SIGKILL after prctl(PR_SET_SECCOMP, 1, ...) on MIPS
From: Ben Hutchings <ben@decadent.org.uk>
Date: Thu, 12 Jun 2014 21:21:41 +0100
Cc: 751417@bugs.debian.org, team@security.debian.org, Plamen Alexandrov <plamen@aomeda.com>, Markos Chandras <markos.chandras@imgtec.com>
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On Thu, 2014-06-12 at 20:36 +0100, Ben Hutchings wrote:
> Control: tag -1 security upstream patch moreinfo
> Control: severity -1 grave
> Control: found -1 3.14.5-1

Aurelien Jarno pointed out this appears to be fixed upstream in 3.15:

commit 137f7df8cead00688524c82360930845396b8a21
Author: Markos Chandras <markos.chandras@imgtec.com>
Date:   Wed Jan 22 14:40:00 2014 +0000

    MIPS: asm: thread_info: Add _TIF_SECCOMP flag

It looks like this can be cherry-picked cleanly onto stable branches for
3.13 and 3.14.  For 3.11 and 3.12, it will need trivial adjustment.

For branches older than 3.11, this needs to be cherry-picked first:

commit e7f3b48af7be9f8007a224663a5b91340626fed5
Author: Ralf Baechle <ralf@linux-mips.org>
Date:   Wed May 29 01:02:18 2013 +0200

    MIPS: Cleanup flags in syscall flags handlers.

Ben.

> On Thu, 2014-06-12 at 16:19 +0000, Plamen Alexandrov wrote:
> > Package: src:linux
> > Version: 3.2.51-1
> > Severity: normal
> > 
> > Under MIPS the system call prctl(PR_SET_SECCOMP, 1, ...) does not behave as 
> > expected.
> > According to the manual page, after calling it with 1 as a second argument, 
> > any consecutive system calls other than read(), write(), _exit() and 
> > sigreturn() should result in the delivery of SIGKILL. However, under MIPS 
> > any consecutive system call behaves as if prctl(PR_SET_SECCOMP, 1, ...) was 
> > never called.
> > 
> > Here is a simple example that can be used to reproduce the bug:
> > 
> > plamen@debian-mips:/tmp$ id
> > uid=1000(plamen) gid=1000(user) groups=1000(user)
> > plamen@debian-mips:/tmp$ cat prctl.c 
> > #include <unistd.h>
> > #include <sys/prctl.h>
> > #include <stdio.h>
> > 
> > int main(void)
> > {
> >     if (prctl(PR_SET_SECCOMP, 1, 0, 0, 0) != 0)
> >             return 0;
> >     uid_t uid = getuid();
> >     printf("%u\n", (unsigned)uid);
> >     return 0;
> > }
> > plamen@debian-mips:/tmp$ gcc prctl.c -o prctl
> > plamen@debian-mips:/tmp$ ./prctl 
> > 1000
> > 
> > There is no change if I replace
> >     if (prctl(PR_SET_SECCOMP, 1, 0, 0, 0) != 0)
> > with
> >     if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_STRICT, 0, 0, 0) != 0)
> > and I add #include <linux/seccomp.h>
> 
> Indeed, I see no check for seccomp on the MIPS syscall 'fast path'.  The
> seccomp check appears to be done on the 'slow path' which is used only
> if tracing or audit is also enabled for the task.  If I run the above
> program under strace, it is killed as expected.
> 
> Could you test whether the attached patches fix this?  (Instructions for
> rebuilding the Debian kernel package with patches can be found at
> <http://kernel-handbook.alioth.debian.org/ch-common-tasks.html#s-common-official>.
>   These patches apply to 'wheezy'.)
> 
> Ben.
> 

-- 
Ben Hutchings
The program is absolutely right; therefore, the computer must be wrong.

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