[PATCH] jbd2: don't advance j_fc_off before the buffer is recorded

jbd2_fc_get_buf() increments journal->j_fc_off before jbd2_journal_bmap()
and __getblk() have had a chance to fail.  When either does, the slot
j_fc_wbuf[fc_off] is never assigned, but j_fc_off already counts it.

ext4 then fails the fast commit and falls back, and the fallback path
reaches jbd2_fc_release_bufs() via ext4_fc_commit ->
jbd2_fc_end_commit_fallback -> __jbd2_fc_end_commit -> ext4_fc_cleanup.
That walks down from j_fc_off - 1 and put_bh()es every slot until it
sees NULL, so it also touches the slot that was never written.
j_fc_wbuf comes from a plain kmalloc() and is never zeroed, so a slot
used for the first time holds uninitialised heap data.

Advance j_fc_off only after the buffer head has been stored.  That
restores the invariant that j_fc_off covers exactly the filled slots,
which is what jbd2_fc_release_bufs() relies on: the live buffers of the
current fast commit sit above the NULLs the previous one released, so
stopping at the first NULL is correct.

Reproduced on a KASAN kernel with an ext4 fast_commit filesystem on a
loop device, truncating the backing file under the live mount so that
only the journal's fast-commit region falls past the end of the device.
__getblk() then fails for the first fast-commit block while the rest of
the journal is still addressable, so the journal is not aborted and the
fast commit reaches the fallback path.  j_fc_wbuf was poisoned to make
the read of the never-written slot visible:

  BUG: KASAN: wild-memory-access in jbd2_fc_release_bufs+0x6b/0xd0
  Write of size 4 at addr 5a5a5a5a5a5a5ab2 by task sync/961
   jbd2_fc_release_bufs+0x6b/0xd0
   ext4_fc_cleanup+0x97/0x830
   __jbd2_fc_end_commit+0x37/0xc0
   ext4_fc_commit+0x524/0x560
   ext4_sync_file+0x3c4/0x4b0
   __x64_sys_fsync+0x20/0x30

With this patch the same run leaves j_fc_off at 0 across repeated
__getblk() failures and completes cleanly.

Fixes: ff780b91efe9 ("jbd2: add fast commit machinery")
Reported-by: Yu Junzhe <[email protected]>
Closes: https://lore.kernel.org/linux-ext4/[email protected]/
Signed-off-by: Daejun Park <[email protected]>
---
 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 00f5a98f3d4f..55b4ce010228 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -843,7 +843,6 @@ int jbd2_fc_get_buf(journal_t *journal, struct buffer_head **bh_out)
 
 	fc_off = journal->j_fc_off;
 	blocknr = journal->j_fc_first + fc_off;
-	journal->j_fc_off++;
 	ret = jbd2_journal_bmap(journal, blocknr, &pblock);
 	if (ret)
 		return ret;
@@ -853,6 +852,7 @@ int jbd2_fc_get_buf(journal_t *journal, struct buffer_head **bh_out)
 		return -ENOMEM;
 
 	journal->j_fc_wbuf[fc_off] = bh;
+	journal->j_fc_off++;
 
 	*bh_out = bh;
 

base-commit: 9091c97be34083587a75db174aab51551d8e8543
-- 
2.43.0