Re: [syzbot] [bcachefs?] KASAN: slab-use-after-free Read in bch2_reconstruct_alloc

[Lizhi Xu]

On Fri, 25 Oct 2024 14:49:48 +1030, Qu Wenruo wrote:
> > use the input logical can't find the extent root, so add sanity check for
> > extent root before search slot.
> >
> > #syz test
> >
> > diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> > index f8e1d5b2c512..87eaf5dd2d5d 100644
> > --- a/fs/btrfs/backref.c
> > +++ b/fs/btrfs/backref.c
> > @@ -2213,6 +2213,9 @@ int extent_from_logical(struct btrfs_fs_info *fs_info, u64 logical,
> >       key.objectid = logical;
> >       key.offset = (u64)-1;
> >
> > +     if (!extent_root)
> > +             return -ENOENT;
> 
> Considering we have a lot of such btrfs_search_slot() without checking
> if the csum/extent root is NULL, can we move the check into
> btrfs_search_slot()?
Yes, judging in btrfs_search_slot can fix the current issue while also
avoiding similar problems.

#syz test

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 0cc919d15b14..9c05cab473f5 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2010,7 +2010,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		      const struct btrfs_key *key, struct btrfs_path *p,
 		      int ins_len, int cow)
 {
-	struct btrfs_fs_info *fs_info = root->fs_info;
+	struct btrfs_fs_info *fs_info;
 	struct extent_buffer *b;
 	int slot;
 	int ret;
@@ -2023,6 +2023,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 	int min_write_lock_level;
 	int prev_cmp;
 
+	if (!root)
+		return -EINVAL;
+
+	fs_info = root->fs_info;
 	might_sleep();
 
 	lowest_level = p->lowest_level;