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Re: [PATCH] btrfs: fix a NULL pointer dereference

On 3/14/19 4:15 AM, Qu Wenruo wrote:

On 2019/3/14 下午4:03, Nikolay Borisov wrote:

On 14.03.19 г. 10:02 ч., Qu Wenruo wrote:

On 2019/3/14 下午3:54, Nikolay Borisov wrote:

On 14.03.19 г. 9:50 ч., Kangjie Lu wrote:
btrfs_lookup_block_group may fail and return NULL. The fix goes
to out when it fails to avoid NULL pointer dereference.
Actually no, in this case btrfs_lookup_block_group must never fail
because if we have an allocated eb then it must have been allocated from
a bg.
Yep, that's the normal case.

However I'm wondering if it's possible to get a bad eb which is cached.

Then we could hit such situation.

So I still believe being safe here still makes sense, especially who
knows future fuzzed image will be.
Then I'd rather have ASSERT(cache)
Isn't assert() a bad idea for production build without assert() support?

I also agree with that, in general, assert should not be used in

production runs. The first patch might be better.



Signed-off-by: Kangjie Lu <kjlu@xxxxxxx>
  fs/btrfs/extent-tree.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 994f0cc41799..b1e7985bcb9d 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7303,6 +7303,8 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
pin = 0;
  		cache = btrfs_lookup_block_group(fs_info, buf->start);
+		if (!cache)
+			goto out;
if (btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN)) {
  			pin_down_extent(fs_info, cache, buf->start,