ceph: fix num_ops off-by-one when crypto allocation fails

Overview

In the Linux kernel, the following vulnerability has been resolved:

ceph: fix num_ops off-by-one when crypto allocation fails

move_dirty_folio_in_page_array() may fail if the file is encrypted, the dirty folio is not the first in the batch, and it fails to allocate a bounce buffer to hold the ciphertext. When that happens, ceph_process_folio_batch() simply redirties the folio and flushes the current batch -- it can retry that folio in a future batch.

However, if this failed folio is not contiguous with the last folio that did make it into the batch, then ceph_process_folio_batch() has already incremented ceph_wbc->num_ops; because it doesn't follow through and add the discontiguous folio to the array, ceph_submit_write() -- which expects that ceph_wbc->num_ops accurately reflects the number of contiguous ranges (and therefore the required number of "write extent" ops) in the writeback -- will panic the kernel:

BUG_ON(ceph_wbc->op_idx + 1 != req->r_num_ops);

This issue can be reproduced on affected kernels by writing to fscrypt-enabled CephFS file(s) with a 4KiB-written/4KiB-skipped/repeat pattern (total filesize should not matter) and gradually increasing the system's memory pressure until a bounce buffer allocation fails.

Fix this crash by decrementing ceph_wbc->num_ops back to the correct value when move_dirty_folio_in_page_array() fails, but the folio already started counting a new (i.e. still-empty) extent.

The defect corrected by this patch has existed since 2022 (see first Fixes:), but another bug blocked multi-folio encrypted writeback until recently (see second Fixes:). The second commit made it into 6.18.16, 6.19.6, and 7.0-rc1, unmasking the panic in those versions. This patch therefore fixes a regression (panic) introduced by cac190c7674f.

Affected

• Linux >= d55207717ded95c8f2760a30e93319fa313186e6 < f65a96fe2467a9b184c6ae963d95a260a202c98e
• Linux >= d55207717ded95c8f2760a30e93319fa313186e6 < 5dd151799d826d3736ae60c2b22cbc9d96a2a613
• Linux >= d55207717ded95c8f2760a30e93319fa313186e6 < 6200f41d6fcf2ac7e24866431e381cbc914560e4
• Linux >= d55207717ded95c8f2760a30e93319fa313186e6 < ba12c1e578890f6337a415b7dedf476c6d455105
• Linux >= d55207717ded95c8f2760a30e93319fa313186e6 < a0d9555bf9eaeba34fe6b6bb86f442fe08ba3842
• Linux 6.6

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.