linux-kernel - [PATCH 5.15.y] ocfs2: fix deadlock in ocfs2_get_system_file

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Message-ID: <20250630083542.10121-1-pranav.tyagi03@gmail.com>
Date: Mon, 30 Jun 2025 14:05:42 +0530
From: Pranav Tyagi <pranav.tyagi03@...il.com>
To: ocfs2-devel@....oracle.com,
	linux-kernel@...r.kernel.org
Cc: mark@...heh.com,
	jlbec@...lplan.org,
	joseph.qi@...ux.alibaba.com,
	pvmohammedanees2003@...il.com,
	akpm@...ux-foundation.org,
	gregkh@...uxfoundation.org,
	sashal@...nel.org,
	skhan@...uxfoundation.org,
	stable@...r.kernel.org,
	linux-kernel-mentees@...ts.linux.dev,
	syzbot+e0055ea09f1f5e6fabdd@...kaller.appspotmail.com,
	Junxiao Bi <junxiao.bi@...cle.com>,
	Changwei Ge <gechangwei@...e.cn>,
	Gang He <ghe@...e.com>,
	Jun Piao <piaojun@...wei.com>,
	Pranav Tyagi <pranav.tyagi03@...il.com>
Subject: [PATCH 5.15.y] ocfs2: fix deadlock in ocfs2_get_system_file_inode

From: Mohammed Anees <pvmohammedanees2003@...il.com>

[ Upstream commit 7bf1823e010e8db2fb649c790bd1b449a75f52d8 ]

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

[ Backport to 5.15: context cleanly applied with no semantic changes.
Build-tested. ]

Link: https://lkml.kernel.org/r/20240924093257.7181-1-pvmohammedanees2003@gmail.com
Signed-off-by: Mohammed Anees <pvmohammedanees2003@...il.com>
Reviewed-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
Reported-by: <syzbot+e0055ea09f1f5e6fabdd@...kaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=e0055ea09f1f5e6fabdd
Tested-by: syzbot+e0055ea09f1f5e6fabdd@...kaller.appspotmail.com
Cc: Mark Fasheh <mark@...heh.com>
Cc: Joel Becker <jlbec@...lplan.org>
Cc: Junxiao Bi <junxiao.bi@...cle.com>
Cc: Changwei Ge <gechangwei@...e.cn>
Cc: Gang He <ghe@...e.com>
Cc: Jun Piao <piaojun@...wei.com>
Cc:  <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.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);
-- 
2.49.0