linux-cve-announce - CVE-2024-56693: brd: defer automatic disk creation until module initialization succeeds

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Message-ID: <2024122835-CVE-2024-56693-b3c6@gregkh>
Date: Sat, 28 Dec 2024 10:45:45 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-56693: brd: defer automatic disk creation until module initialization succeeds

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

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

brd: defer automatic disk creation until module initialization succeeds

My colleague Wupeng found the following problems during fault injection:

BUG: unable to handle page fault for address: fffffbfff809d073
PGD 6e648067 P4D 123ec8067 PUD 123ec4067 PMD 100e38067 PTE 0
Oops: Oops: 0000 [#1] PREEMPT SMP KASAN NOPTI
CPU: 5 UID: 0 PID: 755 Comm: modprobe Not tainted 6.12.0-rc3+ #17
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.1-2.fc37 04/01/2014
RIP: 0010:__asan_load8+0x4c/0xa0
...
Call Trace:
 <TASK>
 blkdev_put_whole+0x41/0x70
 bdev_release+0x1a3/0x250
 blkdev_release+0x11/0x20
 __fput+0x1d7/0x4a0
 task_work_run+0xfc/0x180
 syscall_exit_to_user_mode+0x1de/0x1f0
 do_syscall_64+0x6b/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

loop_init() is calling loop_add() after __register_blkdev() succeeds and
is ignoring disk_add() failure from loop_add(), for loop_add() failure
is not fatal and successfully created disks are already visible to
bdev_open().

brd_init() is currently calling brd_alloc() before __register_blkdev()
succeeds and is releasing successfully created disks when brd_init()
returns an error. This can cause UAF for the latter two case:

case 1:
    T1:
modprobe brd
  brd_init
    brd_alloc(0) // success
      add_disk
        disk_scan_partitions
          bdev_file_open_by_dev // alloc file
          fput // won't free until back to userspace
    brd_alloc(1) // failed since mem alloc error inject
  // error path for modprobe will release code segment
  // back to userspace
  __fput
    blkdev_release
      bdev_release
        blkdev_put_whole
          bdev->bd_disk->fops->release // fops is freed now, UAF!

case 2:
    T1:                            T2:
modprobe brd
  brd_init
    brd_alloc(0) // success
                                   open(/dev/ram0)
    brd_alloc(1) // fail
  // error path for modprobe

                                   close(/dev/ram0)
                                   ...
                                   /* UAF! */
                                   bdev->bd_disk->fops->release

Fix this problem by following what loop_init() does. Besides,
reintroduce brd_devices_mutex to help serialize modifications to
brd_list.

The Linux kernel CVE team has assigned CVE-2024-56693 to this issue.


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

	Issue introduced in 5.14 with commit 7f9b348cb5e94259acdcbafbcaed55d3bb515304 and fixed in 5.15.174 with commit 41219c147df8bbd6591f59af5d695fb6c9a1cbff
	Issue introduced in 5.14 with commit 7f9b348cb5e94259acdcbafbcaed55d3bb515304 and fixed in 6.1.120 with commit 259bf925583ec9e3781df778cadf00594095090d
	Issue introduced in 5.14 with commit 7f9b348cb5e94259acdcbafbcaed55d3bb515304 and fixed in 6.6.64 with commit 410896624db639500f24f46478b4bfa05c76bf56
	Issue introduced in 5.14 with commit 7f9b348cb5e94259acdcbafbcaed55d3bb515304 and fixed in 6.11.11 with commit c0c2744cd2939ec5999c51dbaf2af16886548b7b
	Issue introduced in 5.14 with commit 7f9b348cb5e94259acdcbafbcaed55d3bb515304 and fixed in 6.12.2 with commit 63dfd728b30f79495dacc886127695a379805152
	Issue introduced in 5.14 with commit 7f9b348cb5e94259acdcbafbcaed55d3bb515304 and fixed in 6.13-rc1 with commit 826cc42adf44930a633d11a5993676d85ddb0842

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-2024-56693
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/block/brd.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/41219c147df8bbd6591f59af5d695fb6c9a1cbff
	https://git.kernel.org/stable/c/259bf925583ec9e3781df778cadf00594095090d
	https://git.kernel.org/stable/c/410896624db639500f24f46478b4bfa05c76bf56
	https://git.kernel.org/stable/c/c0c2744cd2939ec5999c51dbaf2af16886548b7b
	https://git.kernel.org/stable/c/63dfd728b30f79495dacc886127695a379805152
	https://git.kernel.org/stable/c/826cc42adf44930a633d11a5993676d85ddb0842