CVE-2026-72420: md/raid5: avoid R5_Overlap races while breaking stripe batches

CVE-2026-72420: md/raid5: avoid R5_Overlap races while breaking stripe batches

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

md/raid5: avoid R5_Overlap races while breaking stripe batches

KCSAN report a race in break_stripe_batch_list() vs. raid5_make_request()
on sh->dev[i].flags (plain word write vs. atomic bit op)..

and .. one possible scenario is:

CPU1                            CPU2
break_stripe_batch_list(sh1)
-> handle sh2
-> lock(sh2)
-> sh2->batch_head = NULL
-> unlock(sh2)
-> test_and_clear_bit(R5_Overlap, sh2->dev[i].flags)
-> wake_up_bit(sh2->dev[i].flags)
                                raid5_make_request()
                                -> add_all_stripe_bios(sh2)
                                -> lock(sh2)
                                -> stripe_bio_overlaps(sh2) returns true
				   batch_head is NULL, so new bio overlap
				   exist bio on sh2 -> true
                                -> set_bit(R5_Overlap, sh2->dev[i].flags)
                                -> unlock(sh2)
                                -> wait_on_bit(sh2->dev[i].flags)
-> sh2->dev[i].flags = sh1->dev[i].flags & ~R5_Overlap

No wait_up_bit(), CPU2 could be wait_on_bit() forever...

Fix by :
- Expand the protect zone.
- Use batch_head's device flag's snaphot when no held head_sh->stripe_lock.
- Move sh/head_sh->batch_head = NULL to the end of protected zone , and ,
  any concurrent add_all_stripe_bios() grabs sh->stripe_lock now either:
	- see batch_head != null, and , is rejected by stripe_bio_overlaps()
	  under the lock (no R5_Overlap wait ) , or ,
	- sees batch_head == NULL, only after dev[i].flags has already been
	  set and the prior R5_Overlap waiters worken.

KCSAN report:
================================================
  BUG: KCSAN: data-race in break_stripe_batch_list / raid5_make_request

  write (marked) to 0xffff8e89c8117548 of 8 bytes by task 4042 on cpu 0:
    raid5_make_request+0xea0/0x2930
    md_handle_request+0x4a2/0xa40
    md_submit_bio+0x109/0x1a0
    __submit_bio+0x2ec/0x390
    submit_bio_noacct_nocheck+0x457/0x710
    submit_bio_noacct+0x2a7/0xc20
    submit_bio+0x56/0x250
    blkdev_direct_IO+0x54c/0xda0
    blkdev_write_iter+0x38f/0x570
    aio_write+0x22b/0x490
    io_submit_one+0xa51/0xf70
    __x64_sys_io_submit+0xf7/0x220
    x64_sys_call+0x1907/0x1c60
    do_syscall_64+0x130/0x570
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

  read to 0xffff8e89c8117548 of 8 bytes by task 4010 on cpu 5:
    break_stripe_batch_list+0x249/0x480
    handle_stripe_clean_event+0x720/0x9b0
    handle_stripe+0x32fb/0x4500
    handle_active_stripes.isra.0+0x6e0/0xa50
    raid5d+0x7e0/0xba0
    md_thread+0x15a/0x2d0
    kthread+0x1e3/0x220
    ret_from_fork+0x37a/0x410
    ret_from_fork_asm+0x1a/0x30

  value changed: 0x0000000000000019 -> 0x0000000000000099 --> R5_Overlap

The Linux kernel CVE team has assigned CVE-2026-72420 to this issue.


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

	Issue introduced in 4.1 with commit fb642b92c267beeefd352af9bc461eac93a7552c and fixed in 6.12.97 with commit 8031b0d02bd221a5f9add4357e291fc2a527b83a
	Issue introduced in 4.1 with commit fb642b92c267beeefd352af9bc461eac93a7552c and fixed in 6.18.40 with commit 4d919c9b770996365806b6c8d701912d52baa306
	Issue introduced in 4.1 with commit fb642b92c267beeefd352af9bc461eac93a7552c and fixed in 7.1.5 with commit d684b72dfbd320623ccaab0779aa841190488e7c
	Issue introduced in 4.1 with commit fb642b92c267beeefd352af9bc461eac93a7552c and fixed in 7.2-rc1 with commit 55b77337bdd088c77461588e5ec094421b89911b

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-2026-72420
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/raid5.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/8031b0d02bd221a5f9add4357e291fc2a527b83a
	https://git.kernel.org/stable/c/4d919c9b770996365806b6c8d701912d52baa306
	https://git.kernel.org/stable/c/d684b72dfbd320623ccaab0779aa841190488e7c
	https://git.kernel.org/stable/c/55b77337bdd088c77461588e5ec094421b89911b