btrfs: fix extent map use-after-free when adding pages to compressed bio

At add_ra_bio_pages() we are accessing the extent map to calculate
'add_size' after we dropped our reference on the extent map, resulting
in a use-after-free. Fix this by computing 'add_size' before dropping our
extent map reference.

Reported-by: syzbot+853d80cba98ce1157ae6@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-btrfs/000000000000038144061c6d18f2@google.com/
Fixes: 6a4049102055 ("btrfs: subpage: make add_ra_bio_pages() compatible")
CC: stable@vger.kernel.org # 6.1+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index a149f3659b15f02f384401b937ec04183cc2e7c8..a8e2c461aff714fc075d509667caca23301405fc 100644 (file)
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -515,6 +515,7 @@ static noinline int add_ra_bio_pages(struct inode *inode,
                        put_page(page);
                        break;
                }
+               add_size = min(em->start + em->len, page_end + 1) - cur;
                free_extent_map(em);
 
                if (page->index == end_index) {
@@ -527,7 +528,6 @@ static noinline int add_ra_bio_pages(struct inode *inode,
                        }
                }
 
-               add_size = min(em->start + em->len, page_end + 1) - cur;
                ret = bio_add_page(orig_bio, page, add_size, offset_in_page(cur));
                if (ret != add_size) {
                        unlock_extent(tree, cur, page_end, NULL);