mm: fix hugetlb bug due to user_shm_unlock call

2.6.30's commit 8a0bdec194c21c8fdef840989d0d7b742bb5d4bc removed
user_shm_lock() calls in hugetlb_file_setup() but left the
user_shm_unlock call in shm_destroy().

In detail:
Assume that can_do_hugetlb_shm() returns true and hence user_shm_lock()
is not called in hugetlb_file_setup(). However, user_shm_unlock() is
called in any case in shm_destroy() and in the following
atomic_dec_and_lock(&up->__count) in free_uid() is executed and if
up->__count gets zero, also cleanup_user_struct() is scheduled.

Note that sched_destroy_user() is empty if CONFIG_USER_SCHED is not set.
However, the ref counter up->__count gets unexpectedly non-positive and
the corresponding structs are freed even though there are live
references to them, resulting in a kernel oops after a lots of
shmget(SHM_HUGETLB)/shmctl(IPC_RMID) cycles and CONFIG_USER_SCHED set.

Hugh changed Stefan's suggested patch: can_do_hugetlb_shm() at the
time of shm_destroy() may give a different answer from at the time
of hugetlb_file_setup().  And fixed newseg()'s no_id error path,
which has missed user_shm_unlock() ever since it came in 2.6.9.

Reported-by: Stefan Huber <shuber2@gmail.com>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Tested-by: Stefan Huber <shuber2@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/ipc/shm.c b/ipc/shm.c
index 15dd238e5338..1bc4701ef4f0 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -174,7 +174,7 @@ static void shm_destroy(struct ipc_namespace *ns, struct shmid_kernel *shp)
         shm_unlock(shp);
         if (!is_file_hugepages(shp->shm_file))
                 shmem_lock(shp->shm_file, 0, shp->mlock_user);
-        else
+        else if (shp->mlock_user)
                 user_shm_unlock(shp->shm_file->f_path.dentry->d_inode->i_size,
                                                 shp->mlock_user);
         fput (shp->shm_file);
@@ -369,8 +369,8 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
                 /* hugetlb_file_setup applies strict accounting */
                 if (shmflg & SHM_NORESERVE)
                         acctflag = VM_NORESERVE;
-                file = hugetlb_file_setup(name, size, acctflag);
-                shp->mlock_user = current_user();
+                file = hugetlb_file_setup(name, size, acctflag,
+                                                        &shp->mlock_user);
         } else {
                 /*
                  * Do not allow no accounting for OVERCOMMIT_NEVER, even
@@ -410,6 +410,8 @@ static int newseg(struct ipc_namespace *ns, struct ipc_params *params)
         return error;
 
 no_id:
+        if (shp->mlock_user)    /* shmflg & SHM_HUGETLB case */
+                user_shm_unlock(size, shp->mlock_user);
         fput(file);
 no_file:
         security_shm_free(shp);