On Wed, 2025-08-27 at 20:17 +0200, Max Kellermann wrote:
The function ceph_process_folio_batch() sets folio_batch entries to NULL, which is an illegal state. Before folio_batch_release() crashes due to this API violation, the function ceph_shift_unused_folios_left() is supposed to remove those NULLs from the array.
However, since commit ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method"), this shifting doesn't happen anymore because the "for" loop got moved to ceph_process_folio_batch(), and now the `i` variable that remains in ceph_writepages_start() doesn't get incremented anymore, making the shifting effectively unreachable much of the time.
Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write() method") added more preconditions for doing the shift, replacing the `i` check (with something that is still just as broken):
if ceph_process_folio_batch() fails, shifting never happens
if ceph_move_dirty_page_in_page_array() was never called (because ceph_process_folio_batch() has returned early for some of various reasons), shifting never happens
if `processed_in_fbatch` is zero (because ceph_process_folio_batch() has returned early for some of the reasons mentioned above or because ceph_move_dirty_page_in_page_array() has failed), shifting never happens
Since those two commits, any problem in ceph_process_folio_batch() could crash the kernel, e.g. this way:
BUG: kernel NULL pointer dereference, address: 0000000000000034 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: Oops: 0002 [#1] SMP NOPTI CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023 Workqueue: writeback wb_workfn (flush-ceph-1) RIP: 0010:folios_put_refs+0x85/0x140 Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 > RSP: 0018:ffffb880af8db778 EFLAGS: 00010207 RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003 RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0 RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0 R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000 FS: 0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0 PKRU: 55555554 Call Trace:
<TASK> ceph_writepages_start+0xeb9/0x1410
The crash can be reproduced easily by changing the ceph_check_page_before_write() return value to `-E2BIG`.
(Interestingly, the crash happens only if `huge_zero_folio` has already been allocated; without `huge_zero_folio`, is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL entries instead of dereferencing them. That makes reproducing the bug somewhat unreliable. See https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com for a discussion of this detail.)
My suggestion is to move the ceph_shift_unused_folios_left() to right after ceph_process_folio_batch() to ensure it always gets called to fix up the illegal folio_batch state.
Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method") Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/ Cc: stable@vger.kernel.org Signed-off-by: Max Kellermann max.kellermann@ionos.com
fs/ceph/addr.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c index 8b202d789e93..8bc66b45dade 100644 --- a/fs/ceph/addr.c +++ b/fs/ceph/addr.c @@ -1687,6 +1687,7 @@ static int ceph_writepages_start(struct address_space *mapping, process_folio_batch: rc = ceph_process_folio_batch(mapping, wbc, &ceph_wbc);
ceph_shift_unused_folios_left(&ceph_wbc.fbatch);@@ -1695,8 +1696,6 @@ static int ceph_writepages_start(struct address_space *mapping, goto release_folios; if (ceph_wbc.processed_in_fbatch) {
ceph_shift_unused_folios_left(&ceph_wbc.fbatch);if (folio_batch_count(&ceph_wbc.fbatch) == 0 && ceph_wbc.locked_pages < ceph_wbc.max_pages) { doutc(cl, "reached end fbatch, trying for more\n");
Let us try to reproduce the issue and to test the patch.
Thanks, Slava.