AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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

iomap: avoid potential null folio->mapping deref during error reporting

When a buffered read fails, iomap_finish_folio_read() reports the error
with fserror_report_io(folio->mapping->host, ...). This is called after
ifs->read_bytes_pending has been decremented by the bytes attempted to
be read.

For a folio split across multiple read completions, the folio is only
guaranteed to stay locked while read_bytes_pending > 0. Once
iomap_finish_folio_read() decrements read_bytes_pending, another
in-flight read can complete and end the read on the folio, which unlocks
it. This allows truncate logic to run and detach the folio (set
folio->mapping to NULL). The error reporting path then can dereference a
NULL folio->mapping. As reported by Sam Sun, this is the race that can
occur:

CPU0: failed completion      CPU1: final completion     CPU2: truncate
-----------------------      ----------------------     --------------
read_bytes_pending -= len
finished = false
/* preempted before
   fserror_report_io() */
			     read_bytes_pending -= len
			     finished = true
			     folio_end_read()
							truncate clears
							folio->mapping
fserror_report_io(
  folio->mapping->host, ...)
	      ^ NULL deref

Fix this by reporting the error first before decrementing
ifs->read_bytes_pending.

https://git.kernel.org/stable/c/1ad453817a4077230d1ba88eb0868f05f824449a

https://git.kernel.org/stable/c/2eea7f44b9c8b42fd7d3a1a87c06a7cd1b99c327

[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","programFiles":["fs/iomap/buffered-io.c"],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","versions":[{"version":"a9d573ee88af980f14fdadb5c12bbf6a195fb3f1","lessThan":"1ad453817a4077230d1ba88eb0868f05f824449a","versionType":"git","status":"affected"},{"version":"a9d573ee88af980f14fdadb5c12bbf6a195fb3f1","lessThan":"2eea7f44b9c8b42fd7d3a1a87c06a7cd1b99c327","versionType":"git","status":"affected"}]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","programFiles":["fs/iomap/buffered-io.c"],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","versions":[{"version":"7.0","status":"affected"},{"version":"0","lessThan":"7.0","versionType":"semver","status":"unaffected"},{"version":"7.0.13","lessThanOrEqual":"7.0.*","versionType":"semver","status":"unaffected"},{"version":"7.1","lessThanOrEqual":"*","versionType":"original_commit_for_fix","status":"unaffected"}]}]