[PATCH 5.17 0063/1126] mm/mlock: fix two bugs in user_shm_lock()

[Greg Kroah-Hartman]

From: Miaohe Lin <linmiaohe@xxxxxxxxxx>

commit e97824ff663ce3509fe040431c713182c2f058b1 upstream.

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@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
v1: https://lkml.kernel.org/r/20220310132417.41189-1-linmiaohe@xxxxxxxxxx
v2: https://lkml.kernel.org/r/20220314064039.62972-1-linmiaohe@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20220322080918.59861-1-linmiaohe@xxxxxxxxxx
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/mlock.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -827,13 +827,12 @@ int user_shm_lock(size_t size, struct uc
 
 	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;
 	}