CVE-2026-68150: fs/super: fix emergency thaw double-unlock of s_umount

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

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

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

fs/super: fix emergency thaw double-unlock of s_umount

do_thaw_all() iterates over all superblocks via __iterate_supers()
with SUPER_ITER_EXCL, which acquires s_umount exclusively before
calling the callback and releases it afterwards. However, the
callback do_thaw_all_callback() calls thaw_super_locked() which
unconditionally releases s_umount on every code path. This results
in a second unlock attempt in __iterate_supers() that corrupts the
rwsem state, triggering a DEBUG_RWSEMS warning:

[  182.601148] sysrq: Emergency Thaw of all frozen filesystems
[  182.601865] ------------[ cut here ]------------
[  182.602375] DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)): count = 0x0, magic = 0xffff99b1011e5870, owner = 0x0, curr 0xffff99b101b06c80, list not empty
[  182.603817] WARNING: kernel/locking/rwsem.c:1412 at up_write+0xa3/0x170, CPU#2: kworker/2:1/53
[  182.604578] Modules linked in:
[  182.604864] CPU: 2 UID: 0 PID: 53 Comm: kworker/2:1 Not tainted 7.2.0-rc4-00001-gbd3bd93ea98a-dirty #4 PREEMPT(lazy)
[  182.605711] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1kylin1 04/01/2014
[  182.606417] Workqueue: events do_thaw_all
[  182.606750] RIP: 0010:up_write+0xaf/0x170
[  182.607076] Code: 19 3a 92 48 0f 44 c2 48 8b 55 08 48 8b 55 00 4c 8b 45 08 48 8b 55 00 48 8d 3d ad 91 e0 01 48 8b 4d 20 50 48 c7 c6 f0 8c 26 92 <67> 48 0f b9 3a e8 d7 93 4e 00 58 eb 81 48 83 7f 18 00 48 c7 c2 8d
[  182.608563] RSP: 0018:ffffb670001d7e08 EFLAGS: 00010246
[  182.609007] RAX: ffffffff92349e8d RBX: 0000000000000000 RCX: ffff99b1011e5870
[  182.609595] RDX: 0000000000000000 RSI: ffffffff92268cf0 RDI: ffffffff92914d10
[  182.610283] RBP: ffff99b1011e5870 R08: 0000000000000000 R09: ffff99b101b06c80
[  182.610847] R10: ffff99b10139a808 R11: fefefefefefefeff R12: 0000000000000000
[  182.611414] R13: ffffffff90cf74d0 R14: 0000000000000000 R15: ffff99b1011e5800
[  182.612009] FS:  0000000000000000(0000) GS:ffff99b1eaaee000(0000) knlGS:0000000000000000
[  182.612670] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  182.613146] CR2: 00000000005c631c CR3: 00000000013ee000 CR4: 00000000000006f0
[  182.613722] Call Trace:
[  182.613946]  <TASK>
[  182.614130]  __iterate_supers+0x128/0x150
[  182.614463]  do_thaw_all+0x1b/0x30
[  182.614759]  process_scheduled_works+0xbb/0x3f0
[  182.615150]  ? __pfx_worker_thread+0x10/0x10
[  182.615499]  worker_thread+0x129/0x270
[  182.615816]  ? __pfx_worker_thread+0x10/0x10
[  182.616201]  kthread+0xe2/0x120
[  182.616469]  ? __pfx_kthread+0x10/0x10
[  182.616792]  ret_from_fork+0x15b/0x240
[  182.617115]  ? __pfx_kthread+0x10/0x10
[  182.617426]  ret_from_fork_asm+0x1a/0x30
[  182.617761]  </TASK>
[  182.617968] ---[ end trace 0000000000000000 ]---
[  182.618412] Emergency Thaw complete

Fix this by switching to SUPER_ITER_UNLOCKED and acquiring s_umount
in the callback via super_lock_excl() before calling
thaw_super_locked(). This matches the locking pattern expected by
thaw_super_locked() and eliminates the double unlock.

While at it, remove the dead 'return;' at the end of
do_thaw_all_callback().

The Linux kernel CVE team has assigned CVE-2026-68150 to this issue.


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

	Issue introduced in 6.16 with commit 2992476528aeecbaee17ba0a6396a817481205a3 and fixed in 6.18.42 with commit c78e38745ff1b0457c4551e7f75ea15842df1169
	Issue introduced in 6.16 with commit 2992476528aeecbaee17ba0a6396a817481205a3 and fixed in 7.1.6 with commit 64017df6e61a3ce7159cee284109b92009985361
	Issue introduced in 6.16 with commit 2992476528aeecbaee17ba0a6396a817481205a3 and fixed in 7.2-rc5 with commit 503d67fbaec6fdeaba391cb497675071db9d16ea

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-2026-68150
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:
	fs/super.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/c78e38745ff1b0457c4551e7f75ea15842df1169
	https://git.kernel.org/stable/c/64017df6e61a3ce7159cee284109b92009985361
	https://git.kernel.org/stable/c/503d67fbaec6fdeaba391cb497675071db9d16ea