cifs: Release folio lock on fscache read hit.

Overview

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

cifs: Release folio lock on fscache read hit.

Under the current code, when cifs_readpage_worker is called, the call contract is that the callee should unlock the page. This is documented in the read_folio section of Documentation/filesystems/vfs.rst as:

The filesystem should unlock the folio once the read has completed, whether it was successful or not.

Without this change, when fscache is in use and cache hit occurs during a read, the page lock is leaked, producing the following stack on subsequent reads (via mmap) to the page:

$ cat /proc/3890/task/12864/stack [<0>] folio_wait_bit_common+0x124/0x350 [<0>] filemap_read_folio+0xad/0xf0 [<0>] filemap_fault+0x8b1/0xab0 [<0>] __do_fault+0x39/0x150 [<0>] do_fault+0x25c/0x3e0 [<0>] __handle_mm_fault+0x6ca/0xc70 [<0>] handle_mm_fault+0xe9/0x350 [<0>] do_user_addr_fault+0x225/0x6c0 [<0>] exc_page_fault+0x84/0x1b0 [<0>] asm_exc_page_fault+0x27/0x30

This requires a reboot to resolve; it is a deadlock.

Note however that the call to cifs_readpage_from_fscache does mark the page clean, but does not free the folio lock. This happens in __cifs_readpage_from_fscache on success. Releasing the lock at that point however is not appropriate as cifs_readahead also calls cifs_readpage_from_fscache and does unconditionally release the lock after its return. This change therefore effectively makes cifs_readpage_worker work like cifs_readahead.

Affected

• Linux >= 0174ee9947bd0f24fee2794b35258960d108b7aa < 9e725386d4262ef23ae51993f04602bc535b5be2
• Linux >= 0174ee9947bd0f24fee2794b35258960d108b7aa < 7a9fb689c1a1dc373887621a3bfa3810df0abde4
• Linux >= 0174ee9947bd0f24fee2794b35258960d108b7aa < 69513dd669e243928f7450893190915a88f84a2b
• Linux 5.17

Remediation

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