[PATCH v2 1/4] iomap: don't resubmit an ioend after ->writeback_submit() failed

[Andrea Parri <parri.andrea@gmail.com>]

iomap_add_to_ioend() submits the pending ioend through
->writeback_submit() before allocating a new one for the current range.
When the submission fails the helper completes the ioend with an error,
but iomap_add_to_ioend() returns the error without clearing
wpc->wb_ctx.  iomap_writepages() then submits whatever wpc->wb_ctx
points to, so the already completed ioend is submitted a second time.

For XFS the second bio_endio() lands in xfs_end_bio(), which
list_add_tail()s the already linked ioend into ip->i_ioend_list.  This
corrupts the list and leaves a use-after-free/double-free window against
the ioend completion worker.  Reproduced with a fault-injected
->writeback_submit() failure on a reflinked XFS file with several
CoW writeback ranges in flight: the unfixed kernel hits repeated
"list_add double add" warnings from __list_add_valid_or_report(),
the fixed kernel fails writeback cleanly.

Clear wpc->wb_ctx when ->writeback_submit() fails.  The old
iomap_submit_ioend() cleared the context unconditionally; that clear was
lost when submission moved to iomap_ioend_writeback_submit().  The final
->writeback_submit() call in iomap_writepages() needs no equivalent fix:
it is the last thing the function does before returning, and every
caller allocates its iomap_writepage_ctx on the stack for a single call,
so wpc->wb_ctx is never read again afterwards.

Fixes: f4fa7981fa26 ("iomap: hide ioends from the generic writeback code")
Cc: <stable@vger.kernel.org> # v6.17
Reviewed-by: Brian Foster <bfoster@redhat.com>
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 fs/iomap/ioend.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/iomap/ioend.c b/fs/iomap/ioend.c
index 7bbbb417f9152..32ae292a84cbe 100644
--- a/fs/iomap/ioend.c
+++ b/fs/iomap/ioend.c
@@ -246,8 +246,16 @@ ssize_t iomap_add_to_ioend(struct iomap_writepage_ctx *wpc, struct folio *folio,
 new_ioend:
 		if (ioend) {
 			error = wpc->ops->writeback_submit(wpc, 0);
-			if (error)
+			if (error) {
+				/*
+				 * ->writeback_submit() completed the ioend
+				 * with an error, so drop the stale context;
+				 * iomap_writepages() would otherwise submit
+				 * it a second time.
+				 */
+				wpc->wb_ctx = NULL;
 				return error;
+			}
 		}
 		wpc->wb_ctx = ioend = iomap_alloc_ioend(wpc, pos, ioend_flags);
 	}
-- 
2.53.0