Messages in this thread | | | Subject | Re: [PATCH] fs: btrfs: fix possible use-after-free bug in error handling code of btrfs_get_root_ref() | From | Jia-Ju Bai <> | Date | Fri, 25 Mar 2022 16:04:17 +0800 |
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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
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