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/[email protected]

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.