fs/coredump: prevent fsuid=0 dumps into user-controlled directories
authorJann Horn <jann@thejh.net>
Tue, 22 Mar 2016 21:25:36 +0000 (14:25 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 22 Mar 2016 22:36:02 +0000 (15:36 -0700)
commit378c6520e7d29280f400ef2ceaf155c86f05a71a
treed16654900c79dd882ece48eaaeda3afcffd10e5a
parent1333ab03150478df8d6f5673a91df1e50dc6ab97
fs/coredump: prevent fsuid=0 dumps into user-controlled directories

This commit fixes the following security hole affecting systems where
all of the following conditions are fulfilled:

 - The fs.suid_dumpable sysctl is set to 2.
 - The kernel.core_pattern sysctl's value starts with "/". (Systems
   where kernel.core_pattern starts with "|/" are not affected.)
 - Unprivileged user namespace creation is permitted. (This is
   true on Linux >=3.8, but some distributions disallow it by
   default using a distro patch.)

Under these conditions, if a program executes under secure exec rules,
causing it to run with the SUID_DUMP_ROOT flag, then unshares its user
namespace, changes its root directory and crashes, the coredump will be
written using fsuid=0 and a path derived from kernel.core_pattern - but
this path is interpreted relative to the root directory of the process,
allowing the attacker to control where a coredump will be written with
root privileges.

To fix the security issue, always interpret core_pattern for dumps that
are written under SUID_DUMP_ROOT relative to the root directory of init.

Signed-off-by: Jann Horn <jann@thejh.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
arch/um/drivers/mconsole_kern.c
fs/coredump.c
fs/fhandle.c
fs/open.c
include/linux/fs.h
kernel/sysctl_binary.c