Subject: CVE-2024-43855: md: fix deadlock between mddev_suspend and flush bio

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

md: fix deadlock between mddev_suspend and flush bio

Deadlock occurs when mddev is being suspended while some flush bio is in
progress. It is a complex issue.

T1. the first flush is at the ending stage, it clears 'mddev->flush_bio'
    and tries to submit data, but is blocked because mddev is suspended
    by T4.
T2. the second flush sets 'mddev->flush_bio', and attempts to queue
    md_submit_flush_data(), which is already running (T1) and won't
    execute again if on the same CPU as T1.
T3. the third flush inc active_io and tries to flush, but is blocked because
    'mddev->flush_bio' is not NULL (set by T2).
T4. mddev_suspend() is called and waits for active_io dec to 0 which is inc
    by T3.

  T1		T2		T3		T4
  (flush 1)	(flush 2)	(third 3)	(suspend)
  md_submit_flush_data
   mddev->flush_bio = NULL;
   .
   .	 	md_flush_request
   .	  	 mddev->flush_bio = bio
   .	  	 queue submit_flushes
   .		 .
   .		 .		md_handle_request
   .		 .		 active_io + 1
   .		 .		 md_flush_request
   .		 .		  wait !mddev->flush_bio
   .		 .
   .		 .				mddev_suspend
   .		 .				 wait !active_io
   .		 .
   .		 submit_flushes
   .		 queue_work md_submit_flush_data
   .		 //md_submit_flush_data is already running (T1)
   .
   md_handle_request
    wait resume

The root issue is non-atomic inc/dec of active_io during flush process.
active_io is dec before md_submit_flush_data is queued, and inc soon
after md_submit_flush_data() run.
  md_flush_request
    active_io + 1
    submit_flushes
      active_io - 1
      md_submit_flush_data
        md_handle_request
        active_io + 1
          make_request
        active_io - 1

If active_io is dec after md_handle_request() instead of within
submit_flushes(), make_request() can be called directly intead of
md_handle_request() in md_submit_flush_data(), and active_io will
only inc and dec once in the whole flush process. Deadlock will be
fixed.

Additionally, the only difference between fixing the issue and before is
that there is no return error handling of make_request(). But after
previous patch cleaned md_write_start(), make_requst() only return error
in raid5_make_request() by dm-raid, see commit 41425f96d7aa ("dm-raid456,
md/raid456: fix a deadlock for dm-raid456 while io concurrent with
reshape)". Since dm always splits data and flush operation into two
separate io, io size of flush submitted by dm always is 0, make_request()
will not be called in md_submit_flush_data(). To prevent future
modifications from introducing issues, add WARN_ON to ensure
make_request() no error is returned in this context.

The Linux kernel CVE team has assigned CVE-2024-43855 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.1.75 with commit f9f2d957a8ea and fixed in 6.1.103 with commit 322260708131
	Issue introduced in 6.6.14 with commit 530cec617f5a and fixed in 6.6.44 with commit 2d0738a8322b
	Issue introduced in 6.8 with commit fa2bbff7b0b4 and fixed in 6.10.3 with commit ca963eefbc33
	Issue introduced in 6.8 with commit fa2bbff7b0b4 and fixed in 6.11 with commit 611d5cbc0b35
	Issue introduced in 6.7.2 with commit c4c2345214b6

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-43855
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	drivers/md/md.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/32226070813140234b6c507084738e8e8385c5c6
	https://git.kernel.org/stable/c/2d0738a8322bf4e5bfe693d16b3111928a9ccfbf
	https://git.kernel.org/stable/c/ca963eefbc3331222b6121baa696d49ba2008811
	https://git.kernel.org/stable/c/611d5cbc0b35a752e657a83eebadf40d814d006b