[PATCH] FS: JFS: Fix null-ptr-deref Read in txBegin

[mirimmad]

From: Immad Mir <mirimmad17@xxxxxxxxx>

syzkaller reported the following issue:

BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:72 [inline]
BUG: KASAN: null-ptr-deref in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
BUG: KASAN: null-ptr-deref in txBegin+0x131/0x6c0 fs/jfs/jfs_txnmgr.c:366
Read of size 8 at addr 0000000000000040 by task syz-executor.0/5098

CPU: 0 PID: 5098 Comm: syz-executor.0 Not tainted 6.3.0-rc3-syzkaller-00005-g7d31677bb7b1 #0
Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x1e7/0x2d0 lib/dump_stack.c:106
 print_report+0xe6/0x540 mm/kasan/report.c:433
 kasan_report+0x176/0x1b0 mm/kasan/report.c:536
 kasan_check_range+0x283/0x290 mm/kasan/generic.c:187
 instrument_atomic_read include/linux/instrumented.h:72 [inline]
 _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
 txBegin+0x131/0x6c0 fs/jfs/jfs_txnmgr.c:366
 jfs_link+0x1ac/0x5e0 fs/jfs/namei.c:802
 vfs_link+0x4ed/0x680 fs/namei.c:4522
 do_linkat+0x5cc/0x9e0 fs/namei.c:4593
 __do_sys_linkat fs/namei.c:4621 [inline]
 __se_sys_linkat fs/namei.c:4618 [inline]
 __x64_sys_linkat+0xdd/0xf0 fs/namei.c:4618
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

The issue can be resolved by checking whethere "log"
for a given superblock exists in the jfs_link function
before beginning a transaction.

Tested with syzbot.
Reported-by: syzbot+f1faa20eec55e0c8644c@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://syzkaller.appspot.com/bug?id=be7e52c50c5182cc09a09ea6fc456446b2039de3

Signed-off-by: Immad Mir <mirimmad17@xxxxxxxxx>
---
 fs/jfs/namei.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/jfs/namei.c b/fs/jfs/namei.c
index b29d68b5e..cd43b68e2 100644
--- a/fs/jfs/namei.c
+++ b/fs/jfs/namei.c
@@ -799,6 +799,8 @@ static int jfs_link(struct dentry *old_dentry,
 	if (rc)
 		goto out;

+	if (!(JFS_SBI(ip->i_sb)->log))
+		goto out;
 	tid = txBegin(ip->i_sb, 0);

 	mutex_lock_nested(&JFS_IP(dir)->commit_mutex, COMMIT_MUTEX_PARENT);
--
2.40.0