jbd2: skip reading super block if it has been verified

We got a NULL pointer dereference issue below while running generic/475
I/O failure pressure test.

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 0 P4D 0
 Oops: 0002 [#1] PREEMPT SMP PTI
 CPU: 1 PID: 15600 Comm: fsstress Not tainted 6.4.0-rc5-xfstests-00055-gd3ab1bca26b4 torvalds#190
 RIP: 0010:jbd2_journal_set_features+0x13d/0x430
 ...
 Call Trace:
  <TASK>
  ? __die+0x23/0x60
  ? page_fault_oops+0xa4/0x170
  ? exc_page_fault+0x67/0x170
  ? asm_exc_page_fault+0x26/0x30
  ? jbd2_journal_set_features+0x13d/0x430
  jbd2_journal_revoke+0x47/0x1e0
  __ext4_forget+0xc3/0x1b0
  ext4_free_blocks+0x214/0x2f0
  ext4_free_branches+0xeb/0x270
  ext4_ind_truncate+0x2bf/0x320
  ext4_truncate+0x1e4/0x490
  ext4_handle_inode_extension+0x1bd/0x2a0
  ? iomap_dio_complete+0xaf/0x1d0

The root cause is the journal super block had been failed to write out
due to I/O fault injection, it's uptodate bit was cleared by
end_buffer_write_sync() and didn't reset yet in jbd2_write_superblock().
And it raced by journal_get_superblock()->bh_read(), unfortunately, the
read IO is also failed, so the error handling in
journal_fail_superblock() unexpectedly clear the journal->j_sb_buffer,
finally lead to above NULL pointer dereference issue.

If the journal super block had been read and verified, there is no need
to call bh_read() read it again even if it has been failed to written
out. So the fix could be simply move buffer_verified(bh) in front of
bh_read().

Fixes: d9eafe0 ("jbd2: factor out journal initialization from journal_get_superblock()")
Reported-by: Theodore Ts'o <[email protected]>
Signed-off-by: Zhang Yi <[email protected]>

fs/jbd2/journal.c

@@ -1919,16 +1919,16 @@ static int journal_get_superblock(journal_t *journal)
 	bh = journal->j_sb_buffer;
 
 	J_ASSERT(bh != NULL);
+	if (buffer_verified(bh))
+		return 0;
+
 	err = bh_read(bh, 0);
 	if (err < 0) {
 		printk(KERN_ERR
 			"JBD2: IO error reading journal superblock\n");
 		goto out;
 	}
 
-	if (buffer_verified(bh))
-		return 0;
-
 	sb = journal->j_superblock;
 
 	err = -EINVAL;