Re: CVE-2024-36966: erofs: reliably distinguish block based and fscache mode

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi,

(+Cc Baokun Li)

On 2024/6/8 20:53, Greg Kroah-Hartman wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> erofs: reliably distinguish block based and fscache mode
> 
> When erofs_kill_sb() is called in block dev based mode, s_bdev may not
> have been initialised yet, and if CONFIG_EROFS_FS_ONDEMAND is enabled,
> it will be mistaken for fscache mode, and then attempt to free an anon_dev
> that has never been allocated, triggering the following warning:
> 
> ============================================
> ida_free called for id=0 which is not allocated.
> WARNING: CPU: 14 PID: 926 at lib/idr.c:525 ida_free+0x134/0x140
> Modules linked in:
> CPU: 14 PID: 926 Comm: mount Not tainted 6.9.0-rc3-dirty #630
> RIP: 0010:ida_free+0x134/0x140
> Call Trace:
>   <TASK>
>   erofs_kill_sb+0x81/0x90
>   deactivate_locked_super+0x35/0x80
>   get_tree_bdev+0x136/0x1e0
>   vfs_get_tree+0x2c/0xf0
>   do_new_mount+0x190/0x2f0
>   [...]
> ============================================
> 
> Now when erofs_kill_sb() is called, erofs_sb_info must have been
> initialised, so use sbi->fsid to distinguish between the two modes.
> 
> The Linux kernel CVE team has assigned CVE-2024-36966 to this issue.
> 
> 
> Affected and fixed versions
> ===========================
> 
> 	Fixed in 6.6.32 with commit f9b877a7ee31
> 	Fixed in 6.8.11 with commit dcdd49701e42
> 	Fixed in 6.9 with commit 7af2ae1b1531

For reference, this issue doesn't affect Linux kernel below 6.6.

This behavior ("s_bdev may not be initialized in erofs_kill_sb()")
is introduced due to commit aca740cecbe5 ("fs: open block device after
superblock creation").

In other words, previously .kill_sb() was called only after
fill_super failed and problematic erofs_kill_sb() called due to
setup_bdev_super() failure can only happen since Linux 6.6.

Thanks,
Gao Xiang

> 
> 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-2024-36966
> 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/erofs/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/f9b877a7ee312ec8ce17598a7ef85cb820d7c371
> 	https://git.kernel.org/stable/c/dcdd49701e429c55b3644fd70fc58d85745f8cfe
> 	https://git.kernel.org/stable/c/7af2ae1b1531feab5d38ec9c8f472dc6cceb4606
>