[2.6.20.17 review 22/58] Fix user struct leakage with locked IPC shem segment

Willy Tarreau
2007-08-22T08:58:33Z

[-- Attachment #1: 0022-Fix-user-struct-leakage-with-locked-IPC-shem-segment.patch --]
[-- Type: text/plain, Size: 1604 bytes --]

When user locks an ipc shmem segmant with SHM_LOCK ctl and the segment is
already locked the shmem_lock() function returns 0. After this the
subsequent code leaks the existing user struct:

== ipc/shm.c: sys_shmctl() ==
...
err = shmem_lock(shp->shm_file, 1, user);
if (!err) {
shp->shm_perm.mode |= SHM_LOCKED;
shp->mlock_user = user;
}
...
==

Other results of this are:
1. the new shp->mlock_user is not get-ed and will point to freed
memory when the task dies.
2. the RLIMIT_MEMLOCK is screwed on both user structs.

The exploit looks like this:

==
id = shmget(...);
setresuid(uid, 0, 0);
shmctl(id, SHM_LOCK, NULL);
setresuid(uid + 1, 0, 0);
shmctl(id, SHM_LOCK, NULL);
==

My solution is to return 0 to the userspace and do not change the
segment's user.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
ipc/shm.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index f8e10a2..10b7a2c 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -652,7 +652,7 @@ asmlinkage long sys_shmctl (int shmid, int cmd, struct shmid_ds __user *buf)
struct user_struct * user = current->user;
if (!is_file_hugepages(shp->shm_file)) {
err = shmem_lock(shp->shm_file, 1, user);
- if (!err) {
+ if (!err && !(shp->shm_perm.mode & SHM_LOCKED)){
shp->shm_perm.mode |= SHM_LOCKED;
shp->mlock_user = user;
}
--
1.5.2.5

--