Re: [PATCH] fs: btrfs: fix possible use-after-free bug in error handling code of btrfs_get_root_ref()

Jia-Ju Bai
2022-03-25T08:04:27Z

On 2022/3/25 2:19, David Sterba wrote:
> On Thu, Mar 24, 2022 at 06:44:54AM -0700, Jia-Ju Bai wrote:
>> In btrfs_get_root_ref(), when btrfs_insert_fs_root() fails,
>> btrfs_put_root() will be called to possibly free the memory area of
>> the variable root. However, this variable is then used again in error
>> handling code after "goto fail", when ret is not -EEXIST.
>>
>> To fix this possible bug, btrfs_put_root() is only called when ret is
>> -EEXIST for "goto again".
>>
>> Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
>> ---
>> fs/btrfs/disk-io.c | 5 +++--
>> 1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index b30309f187cf..126f244cdf88 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -1850,9 +1850,10 @@ static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info,
>>
>> ret = btrfs_insert_fs_root(fs_info, root);
>> if (ret) {
>> - btrfs_put_root(root);
>> - if (ret == -EEXIST)
>> + if (ret == -EEXIST) {
>> + btrfs_put_root(root);
> I think this fix is correct, though it's not that clear. If you look how
> the code changed, there was the unconditional put and then followed by a
> free:
>
> 8c38938c7bb0 ("btrfs: move the root freeing stuff into btrfs_put_root")
>
> Here it's putting twice where one will be the final free.
>
> And then the whole refcounting gets updated in
>
> 4785e24fa5d2 ("btrfs: don't take an extra root ref at allocation time")
>
> which could be removing the wrong put, I'm not yet sure.

Thanks for the reply!

I think the bug should be introduced by this commit:
bc44d7c4b2b1 ("btrfs: push btrfs_grab_fs_root into btrfs_get_fs_root")

This commit has a change:
ret = btrfs_insert_fs_root(fs_info, root);
if (ret) {
+      btrfs_put_fs_root(root);
if (ret == -EEXIST) {
btrfs_free_fs_root(root);
goto again;
}

I could add a Fixes tag of this commit in my V2 patch.
Is it okay?

Best wishes,
Jia-Ju Bai