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List:       git-commits-head
Subject:    gfs2: Fix use-after-free in gfs2_glock_shrink_scan
From:       Linux Kernel Mailing List <linux-kernel () vger ! kernel ! org>
Date:       2021-05-31 16:36:32
Message-ID: git-mailbomb-linux-master-1ab19c5de4c537ec0d9b21020395a5b5a6c059b2 () kernel ! org
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Commit:     1ab19c5de4c537ec0d9b21020395a5b5a6c059b2
Parent:     b7f55d928e75557295c1ac280c291b738905b6fb
Refname:    refs/heads/master
Web:        https://git.kernel.org/torvalds/c/1ab19c5de4c537ec0d9b21020395a5b5a6c059b2
Author:     Hillf Danton <hdanton@sina.com>
AuthorDate: Tue May 18 16:46:25 2021 +0800
Committer:  Andreas Gruenbacher <agruenba@redhat.com>
CommitDate: Mon May 31 12:03:28 2021 +0200

    gfs2: Fix use-after-free in gfs2_glock_shrink_scan
    
    The GLF_LRU flag is checked under lru_lock in gfs2_glock_remove_from_lru() to
    remove the glock from the lru list in __gfs2_glock_put().
    
    On the shrink scan path, the same flag is cleared under lru_lock but because
    of cond_resched_lock(&lru_lock) in gfs2_dispose_glock_lru(), progress on the
    put side can be made without deleting the glock from the lru list.
    
    Keep GLF_LRU across the race window opened by cond_resched_lock(&lru_lock) to
    ensure correct behavior on both sides - clear GLF_LRU after list_del under
    lru_lock.
    
    Reported-by: syzbot <syzbot+34ba7ddbf3021981a228@syzkaller.appspotmail.com>
    Signed-off-by: Hillf Danton <hdanton@sina.com>
    Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/gfs2/glock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 797949e784ccd..d9cb261f55b06 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1795,6 +1795,7 @@ __acquires(&lru_lock)
 	while(!list_empty(list)) {
 		gl = list_first_entry(list, struct gfs2_glock, gl_lru);
 		list_del_init(&gl->gl_lru);
+		clear_bit(GLF_LRU, &gl->gl_flags);
 		if (!spin_trylock(&gl->gl_lockref.lock)) {
 add_back_to_lru:
 			list_add(&gl->gl_lru, &lru_list);
@@ -1840,7 +1841,6 @@ static long gfs2_scan_glock_lru(int nr)
 		if (!test_bit(GLF_LOCK, &gl->gl_flags)) {
 			list_move(&gl->gl_lru, &dispose);
 			atomic_dec(&lru_count);
-			clear_bit(GLF_LRU, &gl->gl_flags);
 			freed++;
 			continue;
 		}
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