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, 12 insertions, 8 deletions

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 941c8425c10b..cb88dac8ccaa 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -935,26 +935,28 @@ static int can_do_hugetlb_shm(void)
         return capable(CAP_IPC_LOCK) || in_group_p(sysctl_hugetlb_shm_group);
 }
 
-struct file *hugetlb_file_setup(const char *name, size_t size, int acctflag)
+struct file *hugetlb_file_setup(const char *name, size_t size, int acctflag,
+                                                struct user_struct **user)
 {
         int error = -ENOMEM;
-        int unlock_shm = 0;
         struct file *file;
         struct inode *inode;
         struct dentry *dentry, *root;
         struct qstr quick_string;
-        struct user_struct *user = current_user();
 
+        *user = NULL;
         if (!hugetlbfs_vfsmount)
                 return ERR_PTR(-ENOENT);
 
         if (!can_do_hugetlb_shm()) {
-                if (user_shm_lock(size, user)) {
-                        unlock_shm = 1;
+                *user = current_user();
+                if (user_shm_lock(size, *user)) {
                         WARN_ONCE(1,
                           "Using mlock ulimits for SHM_HUGETLB deprecated\n");
-                } else
+                } else {
+                        *user = NULL;
                         return ERR_PTR(-EPERM);
+                }
         }
 
         root = hugetlbfs_vfsmount->mnt_root;
@@ -996,8 +998,10 @@ out_inode:
 out_dentry:
         dput(dentry);
 out_shm_unlock:
-        if (unlock_shm)
-                user_shm_unlock(size, user);
+        if (*user) {
+                user_shm_unlock(size, *user);
+                *user = NULL;
+        }
         return ERR_PTR(error);
 }