[PATCH] jbd2: check bh2jh() before accessing it

From: Xianting Tian <xianting_tian@126.com>
To: tytso@mit.edu, jack@suse.com
Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] jbd2: check bh2jh() before accessing it
Date: Sat, 18 Jul 2020 09:28:03 -0400	[thread overview]
Message-ID: <1595078883-8647-1-git-send-email-xianting_tian@126.com> (raw)

We met a crash issue when testing nbd device on kernel 4.14.0-115,
the scenario of the issue is "nbd device disconnected before unmounting
ext4 filesystem".
The call trace of the crash as below:
[346961.426274] block nbd2: Connection timed out
[346961.426943] EXT4-fs warning (device nbd2): ext4_end_bio:323: I/O error 10 writing to inode 5768758
			(offset 155926528 size 8192 starting block 8998070)
[346961.426957] Aborting journal on device nbd2-8.
[346961.427027] EXT4-fs error (device nbd2) in __ext4_new_inode:927: Readonly filesystem
 ... ...
[346961.437288] Buffer I/O error on dev nbd2, logical block 13139968, lost sync page write
[346961.437878] JBD2: Error -5 detected when updating journal superblock for nbd2-8.
[346961.438478] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[346961.452495] RIP: 0010:jbd2_journal_grab_journal_head+0x1e/0x40 [jbd2]  <== crash code offset is 0x1e(30)
[346961.453457] RSP: 0018:ffffc9000ffbbca8 EFLAGS: 00010206
[346961.454414] RAX: 0000000000000000 RBX: ffff881dafe04960 RCX: ffff881aee5b0ac8
[346961.455378] RDX: ffff881df7768690 RSI: ffff88100a5e9800 RDI: ffff880a22593d40
[346961.456360] RBP: ffffc9000ffbbca8 R08: ffff881dafe04960 R09: 000000018040001c
[346961.457332] R10: 000000002fe92601 R11: ffff88202fe90700 R12: ffff88100a5e9800
[346961.458302] R13: 0000000000000000 R14: ffff880a22593d40 R15: ffff881dafe04960
[346961.459269] FS:  0000000000000000(0000) GS:ffff88103e5c0000(0000) knlGS:0000000000000000
[346961.460250] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[346961.461216] CR2: 0000000000000008 CR3: 0000000001c09004 CR4: 00000000007606e0
[346961.462201] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[346961.463164] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[346961.465047] Call Trace:
[346961.465981]  __jbd2_journal_insert_checkpoint+0x28/0x80 [jbd2]
[346961.466907]  jbd2_journal_commit_transaction+0x1185/0x1a20 [jbd2]
[346961.467862]  ? lock_timer_base+0x7d/0xa0
[346961.468794]  kjournald2+0xd2/0x260 [jbd2]
[346961.469717]  ? remove_wait_queue+0x60/0x60
[346961.470630]  kthread+0x109/0x140
[346961.471533]  ? commit_timeout+0x10/0x10 [jbd2]
[346961.472438]  ? kthread_park+0x60/0x60
[346961.473521]  ? do_syscall_64+0x182/0x1b0
[346961.474546]  ret_from_fork+0x25/0x30

Analysis of the crash code as below:
struct journal_head *jbd2_journal_grab_journal_head(struct buffer_head *bh)
{
        struct journal_head *jh = NULL;

        jbd_lock_bh_journal_head(bh);
        if (buffer_jbd(bh)) {
                jh = bh2jh(bh); <== jh is NULL (bh->b_private = NULL)
                jh->b_jcount++; <== crash here!!!
				    b_jcount offset in 'struct journal_head' is 0x8
        }
        jbd_unlock_bh_journal_head(bh);
        return jh;
}

crash> dis -l jbd2_journal_grab_journal_head
0xffffffffa00b6050 <jbd2_journal_grab_journal_head>:    nopl   0x0(%rax,%rax,1) [FTRACE NOP]
0xffffffffa00b6055 <jbd2_journal_grab_journal_head+5>:  push   %rbp
0xffffffffa00b6056 <jbd2_journal_grab_journal_head+6>:  mov    %rsp,%rbp
0xffffffffa00b6059 <jbd2_journal_grab_journal_head+9>:  lock btsl $0x18,(%rdi)
0xffffffffa00b605e <jbd2_journal_grab_journal_head+14>: jb     0xffffffffa00b6079 <jbd2_journal_grab_journal_head+41>
0xffffffffa00b6060 <jbd2_journal_grab_journal_head+16>: mov    (%rdi),%rax
0xffffffffa00b6063 <jbd2_journal_grab_journal_head+19>: test   $0x20000,%eax
0xffffffffa00b6068 <jbd2_journal_grab_journal_head+24>: je     0xffffffffa00b6087 <jbd2_journal_grab_journal_head+55>
0xffffffffa00b606a <jbd2_journal_grab_journal_head+26>: mov    0x40(%rdi),%rax  <== jh is NULL(b_private's offset in 'struct buffer_head' is 0x40)
0xffffffffa00b606e <jbd2_journal_grab_journal_head+30>: addl   $0x1,0x8(%rax)  <== "jh->b_jcount++" crash!!!

According to the logical in above code, buffer_head has an attached
journal_head("buffer_jbd(bh)" is true), but buffer_head doesn't record
it(bh->b_private is NULL).
So testing if "buffer_jbd(bh)" is true can't guarantee "bh->b_private"
is not NULL under the abnormal test case.

Signed-off-by: Xianting Tian <xianting_tian@126.com>
---
 fs/jbd2/journal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index e494443..cb661d4 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2535,7 +2535,7 @@ struct journal_head *jbd2_journal_grab_journal_head(struct buffer_head *bh)
 	struct journal_head *jh = NULL;
 
 	jbd_lock_bh_journal_head(bh);
-	if (buffer_jbd(bh)) {
+	if (buffer_jbd(bh) && bh2jh(bh)) {
 		jh = bh2jh(bh);
 		jh->b_jcount++;
 	}
-- 
1.8.3.1

