[PATCH] ocfs2: fix deadlock in ocfs2_get_system_file_inode

Kevin Paul Reddy Janagari
2025-04-24T16:30:12Z

commit: 7bf1823e010e8db2fb649c790bd1b449a75f52d8 upstream

syzbot has found a possible deadlock in ocfs2_get_system_file_inode [1].

The scenario is depicted here,

CPU0 CPU1
lock(&ocfs2_file_ip_alloc_sem_key);
lock(&osb->system_file_mutex);
lock(&ocfs2_file_ip_alloc_sem_key);
lock(&osb->system_file_mutex);

The function calls which could lead to this are:

CPU0
ocfs2_mknod - lock(&ocfs2_file_ip_alloc_sem_key);
.
.
.
ocfs2_get_system_file_inode - lock(&osb->system_file_mutex);

CPU1 -
ocfs2_fill_super - lock(&osb->system_file_mutex);
.
.
.
ocfs2_read_virt_blocks - lock(&ocfs2_file_ip_alloc_sem_key);

This issue can be resolved by making the down_read -> down_read_try
in the ocfs2_read_virt_blocks.

[1] https://syzkaller.appspot.com/bug?extid=e0055ea09f1f5e6fabdd

Link: https://lkml.kernel.org/r/20240924093257.7181-1-pvmohammedanees2003@gmail.com
Signed-off-by: Kevin Paul Reddy Janagari <kevinpaul468@gmail.com>
---
fs/ocfs2/extent_map.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/extent_map.c b/fs/ocfs2/extent_map.c
index 70a768b623cf..f7672472fa82 100644
--- a/fs/ocfs2/extent_map.c
+++ b/fs/ocfs2/extent_map.c
@@ -973,7 +973,13 @@ int ocfs2_read_virt_blocks(struct inode *inode, u64 v_block, int nr,
}

while (done < nr) {
- down_read(&OCFS2_I(inode)->ip_alloc_sem);
+ if (!down_read_trylock(&OCFS2_I(inode)->ip_alloc_sem)) {
+ rc = -EAGAIN;
+ mlog(ML_ERROR,
+ "Inode #%llu ip_alloc_sem is temporarily unavailable\n",
+ (unsigned long long)OCFS2_I(inode)->ip_blkno);
+ break;
+ }
rc = ocfs2_extent_map_get_blocks(inode, v_block + done,
&p_block, &p_count, NULL);
up_read(&OCFS2_I(inode)->ip_alloc_sem);
--
2.39.5