Message-ID: <2025012132-CVE-2025-21658-78d9@gregkh>
Date: Tue, 21 Jan 2025 13:18:37 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2025-21658: btrfs: avoid NULL pointer dereference if no valid extent tree

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

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

btrfs: avoid NULL pointer dereference if no valid extent tree

[BUG]
Syzbot reported a crash with the following call trace:

  BTRFS info (device loop0): scrub: started on devid 1
  BUG: kernel NULL pointer dereference, address: 0000000000000208
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 106e70067 P4D 106e70067 PUD 107143067 PMD 0
  Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 1 UID: 0 PID: 689 Comm: repro Kdump: loaded Tainted: G           O       6.13.0-rc4-custom+ #206
  Tainted: [O]=OOT_MODULE
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
  RIP: 0010:find_first_extent_item+0x26/0x1f0 [btrfs]
  Call Trace:
   <TASK>
   scrub_find_fill_first_stripe+0x13d/0x3b0 [btrfs]
   scrub_simple_mirror+0x175/0x260 [btrfs]
   scrub_stripe+0x5d4/0x6c0 [btrfs]
   scrub_chunk+0xbb/0x170 [btrfs]
   scrub_enumerate_chunks+0x2f4/0x5f0 [btrfs]
   btrfs_scrub_dev+0x240/0x600 [btrfs]
   btrfs_ioctl+0x1dc8/0x2fa0 [btrfs]
   ? do_sys_openat2+0xa5/0xf0
   __x64_sys_ioctl+0x97/0xc0
   do_syscall_64+0x4f/0x120
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
   </TASK>

[CAUSE]
The reproducer is using a corrupted image where extent tree root is
corrupted, thus forcing to use "rescue=all,ro" mount option to mount the
image.

Then it triggered a scrub, but since scrub relies on extent tree to find
where the data/metadata extents are, scrub_find_fill_first_stripe()
relies on an non-empty extent root.

But unfortunately scrub_find_fill_first_stripe() doesn't really expect
an NULL pointer for extent root, it use extent_root to grab fs_info and
triggered a NULL pointer dereference.

[FIX]
Add an extra check for a valid extent root at the beginning of
scrub_find_fill_first_stripe().

The new error path is introduced by 42437a6386ff ("btrfs: introduce
mount option rescue=ignorebadroots"), but that's pretty old, and later
commit b979547513ff ("btrfs: scrub: introduce helper to find and fill
sector info for a scrub_stripe") changed how we do scrub.

So for kernels older than 6.6, the fix will need manual backport.

The Linux kernel CVE team has assigned CVE-2025-21658 to this issue.


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

	Issue introduced in 5.11 with commit 42437a6386ffeaaf200731e73d723ea491f3fe7d and fixed in 6.6.72 with commit 24b85a8b0310e0144da9ab30be42e87e6476638a
	Issue introduced in 5.11 with commit 42437a6386ffeaaf200731e73d723ea491f3fe7d and fixed in 6.12.10 with commit aee5f69f3e6cd82bfefaca1b70b40b6cd8f3f784
	Issue introduced in 5.11 with commit 42437a6386ffeaaf200731e73d723ea491f3fe7d and fixed in 6.13 with commit 6aecd91a5c5b68939cf4169e32bc49f3cd2dd329

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-2025-21658
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/btrfs/scrub.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/24b85a8b0310e0144da9ab30be42e87e6476638a
	https://git.kernel.org/stable/c/aee5f69f3e6cd82bfefaca1b70b40b6cd8f3f784
	https://git.kernel.org/stable/c/6aecd91a5c5b68939cf4169e32bc49f3cd2dd329

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