From: Josef Bacik josef@toxicpanda.com
commit bf53d4687b8f3f6b752f091eb85f62369a515dfd upstream.
In try_to_merge_free_space we attempt to find entries to the left and right of the entry we are adding to see if they can be merged. We search for an entry past our current info (saved into right_info), and then if right_info exists and it has a rb_prev() we save the rb_prev() into left_info.
However there's a slight problem in the case that we have a right_info, but no entry previous to that entry. At that point we will search for an entry just before the info we're attempting to insert. This will simply find right_info again, and assign it to left_info, making them both the same pointer.
Now if right_info _can_ be merged with the range we're inserting, we'll add it to the info and free right_info. However further down we'll access left_info, which was right_info, and thus get a use-after-free.
Fix this by only searching for the left entry if we don't find a right entry at all.
The CVE referenced had a specially crafted file system that could trigger this use-after-free. However with the tree checker improvements we no longer trigger the conditions for the UAF. But the original conditions still apply, hence this fix.
Reference: CVE-2019-19448 Fixes: 963030817060 ("Btrfs: use hybrid extents+bitmap rb tree for free space") CC: stable@vger.kernel.org # 4.4+ Signed-off-by: Josef Bacik josef@toxicpanda.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/free-space-cache.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/fs/btrfs/free-space-cache.c +++ b/fs/btrfs/free-space-cache.c @@ -2281,7 +2281,7 @@ out: static bool try_merge_free_space(struct btrfs_free_space_ctl *ctl, struct btrfs_free_space *info, bool update_stat) { - struct btrfs_free_space *left_info; + struct btrfs_free_space *left_info = NULL; struct btrfs_free_space *right_info; bool merged = false; u64 offset = info->offset; @@ -2297,7 +2297,7 @@ static bool try_merge_free_space(struct if (right_info && rb_prev(&right_info->offset_index)) left_info = rb_entry(rb_prev(&right_info->offset_index), struct btrfs_free_space, offset_index); - else + else if (!right_info) left_info = tree_search_offset(ctl, offset - 1, 0, 0);
/* See try_merge_free_space() comment. */