ubifs: ubifs_releasepage: Remove ubifs_assert(0) to valid this process
CVE: CVE-2023-53584
Component: kernel
CWE: CWE-367
Severity: Low
CVSS v3: 5.6/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Release date: 2023-01-01T00:00:00+00:00

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

ubifs: ubifs_releasepage: Remove ubifs_assert(0) to valid this process

There are two states for ubifs writing pages:
1. Dirty, Private
2. Not Dirty, Not Private

The normal process cannot go to ubifs_releasepage() which means there
exists pages being private but not dirty. Reproducer[1] shows that it
could occur (which maybe related to [2]) with following process:

     PA                     PB                    PC
lock(page)[PA]
ubifs_write_end
  attach_page_private         // set Private
  __set_page_dirty_nobuffers  // set Dirty
unlock(page)

write_cache_pages[PA]
  lock(page)
  clear_page_dirty_for_io(page)	// clear Dirty
  ubifs_writepage

                        do_truncation[PB]
			  truncate_setsize
			    i_size_write(inode, newsize) // newsize = 0

    i_size = i_size_read(inode)	// i_size = 0
    end_index = i_size >> PAGE_SHIFT
    if (page->index > end_index)
      goto out // jump
out:
unlock(page)   // Private, Not Dirty

						generic_fadvise[PC]
						  lock(page)
						  invalidate_inode_page
						    try_to_release_page
						      ubifs_releasepage
						        ubifs_assert(c, 0)
		                                        // bad assertion!
						  unlock(page)
			  truncate_pagecache[PB]

Then we may get following assertion failed:
  UBIFS error (ubi0:0 pid 1683): ubifs_assert_failed [ubifs]:
  UBIFS assert failed: 0, in fs/ubifs/file.c:1513
  UBIFS warning (ubi0:0 pid 1683): ubifs_ro_mode [ubifs]:
  switched to read-only mode, error -22
  CPU: 2 PID: 1683 Comm: aa Not tainted 5.16.0-rc5-00184-g0bca5994cacc-dirty #308
  Call Trace:
    dump_stack+0x13/0x1b
    ubifs_ro_mode+0x54/0x60 [ubifs]
    ubifs_assert_failed+0x4b/0x80 [ubifs]
    ubifs_releasepage+0x67/0x1d0 [ubifs]
    try_to_release_page+0x57/0xe0
    invalidate_inode_page+0xfb/0x130
    __invalidate_mapping_pages+0xb9/0x280
    invalidate_mapping_pagevec+0x12/0x20
    generic_fadvise+0x303/0x3c0
    ksys_fadvise64_64+0x4c/0xb0

[1] https://bugzilla.kernel.org/show_bug.cgi?id=215373
[2] https://linux-mtd.infradead.narkive.com/NQoBeT1u/patch-rfc-ubifs-fix-assert-failed-in-ubifs-set-page-dirty

Statement: The race occurs across three concurrent operations: Process A writes and dirties a page with UBIFS private data, then clears the dirty flag during writeback. Process B truncates the file to zero size. Process A checks the new file size, finds the page is now beyond EOF, and unlocks it without finishing writeback—leaving it private but not dirty. Process C then calls fadvise which attempts to invalidate the page, calling ubifs_releasepage on a private, non-dirty page. This state violates the driver's assumption that private pages are always dirty, triggering an assertion that switches the filesystem to read-only mode. The race window requires precise timing between write, truncate, and fadvise operations, but can occur during normal concurrent filesystem use.

Remediation: