CVE-2025-39879: In the Linux kernel, the following vulnerability has been resolved:  ceph: always call ceph_shift_unused_folios_left()  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.