linux-kernel - [PATCH v3] mm/mlock: fix two bugs in user_shm

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Date:   Tue, 22 Mar 2022 16:09:18 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     <akpm@...ux-foundation.org>, <ebiederm@...ssion.com>,
        <hughd@...gle.com>
CC:     <legion@...nel.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <linmiaohe@...wei.com>
Subject: [PATCH v3] 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@...wei.com>
---
v2->v3:
  simplify the code and fix another bug per Eric. Thanks Eric for discussion.
  drop Acked-by tag as the code changes. Sorry Hugh.
v1->v2:
  correct Fixes tag and collect Acked-by tag.
  Thanks Hugh for review!
---
 mm/mlock.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 25934e7db3e1..37f969ec68fa 100644
--- 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;
 	}
-- 
2.23.0