netfs: Fix oops in write-retry from mis-resetting the subreq iterator

Fix the resetting of the subrequest iterator in netfs_retry_write_stream()
to use the iterator-reset function as the iterator may have been shortened
by a previous retry.  In such a case, the amount of data to be written by
the subrequest is not "subreq->len" but "subreq->len -
subreq->transferred".

Without this, KASAN may see an error in iov_iter_revert():

   BUG: KASAN: slab-out-of-bounds in iov_iter_revert lib/iov_iter.c:633 [inline]
   BUG: KASAN: slab-out-of-bounds in iov_iter_revert+0x443/0x5a0 lib/iov_iter.c:611
   Read of size 4 at addr ffff88802912a0b8 by task kworker/u32:7/1147

   CPU: 1 UID: 0 PID: 1147 Comm: kworker/u32:7 Not tainted 6.15.0-rc6-syzkaller-00052-g9f35e33144ae #0 PREEMPT(full)
   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
   Workqueue: events_unbound netfs_write_collection_worker
   Call Trace:
    <TASK>
    __dump_stack lib/dump_stack.c:94 [inline]
    dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
    print_address_description mm/kasan/report.c:408 [inline]
    print_report+0xc3/0x670 mm/kasan/report.c:521
    kasan_report+0xe0/0x110 mm/kasan/report.c:634
    iov_iter_revert lib/iov_iter.c:633 [inline]
    iov_iter_revert+0x443/0x5a0 lib/iov_iter.c:611
    netfs_retry_write_stream fs/netfs/write_retry.c:44 [inline]
    netfs_retry_writes+0x166d/0x1a50 fs/netfs/write_retry.c:231
    netfs_collect_write_results fs/netfs/write_collect.c:352 [inline]
    netfs_write_collection_worker+0x23fd/0x3830 fs/netfs/write_collect.c:374
    process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3238
    process_scheduled_works kernel/workqueue.c:3319 [inline]
    worker_thread+0x6c8/0xf10 kernel/workqueue.c:3400
    kthread+0x3c2/0x780 kernel/kthread.c:464
    ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:153
    ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
    </TASK>

Fixes: cd0277ed0c18 ("netfs: Use new folio_queue data type and iterator instead of xarray iter")
Reported-by: syzbot+25b83a6f2c702075fcbc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=25b83a6f2c702075fcbc
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250519090707.2848510-2-dhowells@redhat.com
Tested-by: syzbot+25b83a6f2c702075fcbc@syzkaller.appspotmail.com
cc: Paulo Alcantara <pc@manguebit.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/netfs/write_retry.c b/fs/netfs/write_retry.c
index 545d33079a77d0d5a242ec0e1a694dfac8e7eceb..9b1ca8b0f4dd6bdf9ff2cc3e4efa51834167b27f 100644 (file)
--- a/fs/netfs/write_retry.c
+++ b/fs/netfs/write_retry.c
@@ -39,9 +39,10 @@ static void netfs_retry_write_stream(struct netfs_io_request *wreq,
                        if (test_bit(NETFS_SREQ_FAILED, &subreq->flags))
                                break;
                        if (__test_and_clear_bit(NETFS_SREQ_NEED_RETRY, &subreq->flags)) {
-                               struct iov_iter source = subreq->io_iter;
+                               struct iov_iter source;
 
-                               iov_iter_revert(&source, subreq->len - source.count);
+                               netfs_reset_iter(subreq);
+                               source = subreq->io_iter;
                                netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
                                netfs_reissue_write(stream, subreq, &source);
                        }