jbd2: fix data-race and null-ptr-deref in jbd2_journal_dirty_metadata()

Since handle->h_transaction may be a NULL pointer, so we should change it to call is_handle_aborted(handle) first before dereferencing it.

And the following data-race was reported in my fuzzer:

==================================================================

BUG: KCSAN: data-race in jbd2_journal_dirty_metadata / jbd2_journal_dirty_metadata

write to 0xffff888011024104 of 4 bytes by task 10881 on cpu 1:

 jbd2_journal_dirty_metadata+0x2a5/0x770 fs/jbd2/transaction.c:1556
 __ext4_handle_dirty_metadata+0xe7/0x4b0 fs/ext4/ext4_jbd2.c:358
 ext4_do_update_inode fs/ext4/inode.c:5220 [inline]
 ext4_mark_iloc_dirty+0x32c/0xd50 fs/ext4/inode.c:5869
 __ext4_mark_inode_dirty+0xe1/0x450 fs/ext4/inode.c:6074
 ext4_dirty_inode+0x98/0xc0 fs/ext4/inode.c:6103

....

read to 0xffff888011024104 of 4 bytes by task 10880 on cpu 0:

 jbd2_journal_dirty_metadata+0xf2/0x770 fs/jbd2/transaction.c:1512
 __ext4_handle_dirty_metadata+0xe7/0x4b0 fs/ext4/ext4_jbd2.c:358
 ext4_do_update_inode fs/ext4/inode.c:5220 [inline]
 ext4_mark_iloc_dirty+0x32c/0xd50 fs/ext4/inode.c:5869
 __ext4_mark_inode_dirty+0xe1/0x450 fs/ext4/inode.c:6074
 ext4_dirty_inode+0x98/0xc0 fs/ext4/inode.c:6103

....

value changed: 0x00000000 -> 0x00000001
==================================================================

According to this crash report, there is a read/write data-race in jh->b_modified.

This is because the b_state_lock is locked too late.

For some reason, jbd2_journal_dirty_metadata() has been written in a way that it does not lock b_state_lock before checking jh->b_transaction.

However, This makes the code that checks jh->b_transaction messy and causes a data-race in jh->b_* variables. Since locking b_state_lock earlier is not enough to significantly affect performance and most of the functions defined in transaction.c lock b_state_lock before reading/writing jh->b_*.

Thereforce, I think it would be appropriate to modify jbd2_journal_dirty_metadata() to lock b_state_lock earlier as well.

Reported-by [email protected]

Closes https://syzkaller.appspot.com/bug?extid=de24c3fe3c4091051710

Fixes 6e06ae88edae ("jbd2: speedup jbd2_journal_dirty_metadata()")

Signed-off-by Jeongjun Park <[email protected]>

---
 fs/jbd2/transaction.c | 49 +++++++++++++++----------------------------
 1 file changed, 17 insertions(+), 32 deletions(-)

fs/jbd2/transaction.c +17 -33

index cbc4785462f5..7e6dbf37396f 100644
@@ -1496,41 +1496,25 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
 	jbd2_debug(5, "journal_head %p\n", jh);
 	JBUFFER_TRACE(jh, "entry");
 
-	/*
-	 * This and the following assertions are unreliable since we may see jh
-	 * in inconsistent state unless we grab bh_state lock. But this is
-	 * crucial to catch bugs so let's do a reliable check until the
-	 * lockless handling is fully proven.
-	 */
-	if (data_race(jh->b_transaction != transaction &&
-	    jh->b_next_transaction != transaction)) {
-		spin_lock(&jh->b_state_lock);
-		J_ASSERT_JH(jh, jh->b_transaction == transaction ||
-				jh->b_next_transaction == transaction);
-		spin_unlock(&jh->b_state_lock);
-	}
+	spin_lock(&jh->b_state_lock);
+
+	J_ASSERT_JH(jh, jh->b_transaction == transaction ||
+			jh->b_next_transaction == transaction);
+
 	if (jh->b_modified == 1) {
 		/* If it's in our transaction it must be in BJ_Metadata list. */
-		if (data_race(jh->b_transaction == transaction &&
-		    jh->b_jlist != BJ_Metadata)) {
-			spin_lock(&jh->b_state_lock);
-			if (jh->b_transaction == transaction &&
-			    jh->b_jlist != BJ_Metadata)
-				pr_err("JBD2: assertion failure: h_type=%u "
-				       "h_line_no=%u block_no=%llu jlist=%u\n",
-				       handle->h_type, handle->h_line_no,
-				       (unsigned long long) bh->b_blocknr,
-				       jh->b_jlist);
-			J_ASSERT_JH(jh, jh->b_transaction != transaction ||
-					jh->b_jlist == BJ_Metadata);
-			spin_unlock(&jh->b_state_lock);
-		}
-		goto out;
+		if (jh->b_transaction == transaction &&
+			jh->b_jlist != BJ_Metadata)
+			pr_err("JBD2: assertion failure: h_type=%u "
+			       "h_line_no=%u block_no=%llu jlist=%u\n",
+			       handle->h_type, handle->h_line_no,
+			       (unsigned long long) bh->b_blocknr,
+			       jh->b_jlist);
+		J_ASSERT_JH(jh, jh->b_transaction != transaction ||
+				jh->b_jlist == BJ_Metadata);
+		goto out_unlock_bh;
 	}
 
-	journal = transaction->t_journal;
-	spin_lock(&jh->b_state_lock);
-
 	if (is_handle_aborted(handle)) {
 		/*
 		 * Check journal aborting with @jh->b_state_lock locked,

@@ -1543,6 +1527,8 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
 		goto out_unlock_bh;
 	}
 
+	journal = transaction->t_journal;
+
 	if (jh->b_modified == 0) {
 		/*
 		 * This buffer's got modified and becoming part

@@ -1628,7 +1614,6 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
 	spin_unlock(&journal->j_list_lock);
 out_unlock_bh:
 	spin_unlock(&jh->b_state_lock);
-out:
 	JBUFFER_TRACE(jh, "exit");
 	return ret;
 }
--

Quoted 29 lines · Jeongjun Park

Since handle->h_transaction may be a NULL pointer, so we should change it to call is_handle_aborted(handle) first before dereferencing it.

And the following data-race was reported in my fuzzer:

==================================================================

BUG: KCSAN: data-race in jbd2_journal_dirty_metadata / jbd2_journal_dirty_metadata

write to 0xffff888011024104 of 4 bytes by task 10881 on cpu 1:

 jbd2_journal_dirty_metadata+0x2a5/0x770 fs/jbd2/transaction.c:1556
 __ext4_handle_dirty_metadata+0xe7/0x4b0 fs/ext4/ext4_jbd2.c:358
 ext4_do_update_inode fs/ext4/inode.c:5220 [inline]
 ext4_mark_iloc_dirty+0x32c/0xd50 fs/ext4/inode.c:5869
 __ext4_mark_inode_dirty+0xe1/0x450 fs/ext4/inode.c:6074
 ext4_dirty_inode+0x98/0xc0 fs/ext4/inode.c:6103

....

read to 0xffff888011024104 of 4 bytes by task 10880 on cpu 0:

 jbd2_journal_dirty_metadata+0xf2/0x770 fs/jbd2/transaction.c:1512
 __ext4_handle_dirty_metadata+0xe7/0x4b0 fs/ext4/ext4_jbd2.c:358
 ext4_do_update_inode fs/ext4/inode.c:5220 [inline]
 ext4_mark_iloc_dirty+0x32c/0xd50 fs/ext4/inode.c:5869
 __ext4_mark_inode_dirty+0xe1/0x450 fs/ext4/inode.c:6074
 ext4_dirty_inode+0x98/0xc0 fs/ext4/inode.c:6103

....

value changed: 0x00000000 -> 0x00000001
==================================================================

Quoted 7 lines

According to this crash report, there is a read/write data-race in jh->b_modified.

This is because the b_state_lock is locked too late.

For some reason, jbd2_journal_dirty_metadata() has been written in a way that it does not lock b_state_lock before checking jh->b_transaction.

Yes and that is deliberate because for some buffers like bitmaps the contention (and cacheline bouncing) on the b_state_lock can be actually quite significant. See commit 6e06ae88edae ("jbd2: speedup jbd2_journal_dirty_metadata()") for a bit more description.

Quoted 5 lines

However, This makes the code that checks jh->b_transaction messy and causes a data-race in jh->b_* variables. Since locking b_state_lock earlier is not enough to significantly affect performance and most of the functions defined in transaction.c lock b_state_lock before reading/writing jh->b_*.

Well, the code is written with the expectation that b_transaction, b_jlist, and b_modified fields can be changing underneath. Since this was implemented 10 years ago, we didn't really bother with properly annotating this but the code as is is safe. I agree the data_race() annotation for the b_modified check was missed when adding data_race() annotations 4 years ago so that should be fixed.

Honza

Quoted 82 lines

Thereforce, I think it would be appropriate to modify jbd2_journal_dirty_metadata() to lock b_state_lock earlier as well.

Reported-by [email protected]

Closes https://syzkaller.appspot.com/bug?extid=de24c3fe3c4091051710

Fixes 6e06ae88edae ("jbd2: speedup jbd2_journal_dirty_metadata()")

Signed-off-by Jeongjun Park <[email protected]>

---
 fs/jbd2/transaction.c | 49 +++++++++++++++----------------------------
 1 file changed, 17 insertions(+), 32 deletions(-)

fs/jbd2/transaction.c +17 -31

index cbc4785462f5..7e6dbf37396f 100644
@@ -1496,41 +1496,25 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
      jbd2_debug(5, "journal_head %p\n", jh);
      JBUFFER_TRACE(jh, "entry");
 
-     /*
-      * This and the following assertions are unreliable since we may see jh
-      * in inconsistent state unless we grab bh_state lock. But this is
-      * crucial to catch bugs so let's do a reliable check until the
-      * lockless handling is fully proven.
-      */
-     if (data_race(jh->b_transaction != transaction &&
-         jh->b_next_transaction != transaction)) {
-             spin_lock(&jh->b_state_lock);
-             J_ASSERT_JH(jh, jh->b_transaction == transaction ||
-                             jh->b_next_transaction == transaction);
-             spin_unlock(&jh->b_state_lock);
-     }
+     spin_lock(&jh->b_state_lock);
+
+     J_ASSERT_JH(jh, jh->b_transaction == transaction ||
+                     jh->b_next_transaction == transaction);
+
      if (jh->b_modified == 1) {
              /* If it's in our transaction it must be in BJ_Metadata list. */
-             if (data_race(jh->b_transaction == transaction &&
-                 jh->b_jlist != BJ_Metadata)) {
-                     spin_lock(&jh->b_state_lock);
-                     if (jh->b_transaction == transaction &&
-                         jh->b_jlist != BJ_Metadata)
-                             pr_err("JBD2: assertion failure: h_type=%u "
-                                    "h_line_no=%u block_no=%llu jlist=%u\n",
-                                    handle->h_type, handle->h_line_no,
-                                    (unsigned long long) bh->b_blocknr,
-                                    jh->b_jlist);
-                     J_ASSERT_JH(jh, jh->b_transaction != transaction ||
-                                     jh->b_jlist == BJ_Metadata);
-                     spin_unlock(&jh->b_state_lock);
-             }
-             goto out;
+             if (jh->b_transaction == transaction &&
+                     jh->b_jlist != BJ_Metadata)
+                     pr_err("JBD2: assertion failure: h_type=%u "
+                            "h_line_no=%u block_no=%llu jlist=%u\n",
+                            handle->h_type, handle->h_line_no,
+                            (unsigned long long) bh->b_blocknr,
+                            jh->b_jlist);
+             J_ASSERT_JH(jh, jh->b_transaction != transaction ||
+                             jh->b_jlist == BJ_Metadata);
+             goto out_unlock_bh;
      }
 
-     journal = transaction->t_journal;
-     spin_lock(&jh->b_state_lock);
-
      if (is_handle_aborted(handle)) {
              /*
               * Check journal aborting with @jh->b_state_lock locked,

@@ -1543,6 +1527,8 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
              goto out_unlock_bh;
      }
 
+     journal = transaction->t_journal;
+
      if (jh->b_modified == 0) {
              /*
               * This buffer's got modified and becoming part

fs/jbd2/transaction.c int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)

Quoted 8 lines

fs/jbd2/transaction.c +0 -2

@@ -1628,7 +1614,6 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
      spin_unlock(&journal->j_list_lock);
 out_unlock_bh:
      spin_unlock(&jh->b_state_lock);
-out:
      JBUFFER_TRACE(jh, "exit");
      return ret;
 }
--

--
Jan Kara <[email protected]>
SUSE Labs, CR