mm/mlock: fix two bugs in user_shm_lock()

user_shm_lock forgets to set allowed to 0 when get_ucounts fails. So the
later user_shm_unlock might do the extra dec_rlimit_ucounts. Also in the
RLIM_INFINITY case, user_shm_lock will success regardless of the value of
memlock where memblock == LONG_MAX && !capable(CAP_IPC_LOCK) should fail.
Fix all of these by changing the code to leave lock_limit at ULONG_MAX aka
RLIM_INFINITY, leave "allowed" initialized to 0 and remove the special case
of RLIM_INFINITY as nothing can be greater than ULONG_MAX.

Credit goes to Eric W. Biederman for proposing simplifying the code and
thus catching the later bug.

Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: stable@vger.kernel.org
v1: https://lkml.kernel.org/r/20220310132417.41189-1-linmiaohe@huawei.com
v2: https://lkml.kernel.org/r/20220314064039.62972-1-linmiaohe@huawei.com
Link: https://lkml.kernel.org/r/20220322080918.59861-1-linmiaohe@huawei.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/mm/mlock.c b/mm/mlock.c
index 25934e7..37f969e 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -827,13 +827,12 @@ int user_shm_lock(size_t size, struct ucounts *ucounts)
 
        locked = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;
        lock_limit = rlimit(RLIMIT_MEMLOCK);
-       if (lock_limit == RLIM_INFINITY)
-               allowed = 1;
-       lock_limit >>= PAGE_SHIFT;
+       if (lock_limit != RLIM_INFINITY)
+               lock_limit >>= PAGE_SHIFT;
        spin_lock(&shmlock_user_lock);
        memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
 
-       if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
+       if ((memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) {
                dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked);
                goto out;
        }