[PATCH] btrfs: send: reject extents for non-regular inodes

[BUG]
A corrupted subvolume tree can leave an EXTENT_DATA item attached to an
inode whose mode is not S_IFREG or S_IFLNK. During send, such an item can
be treated as file data and crash through a NULL address_space operation:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor instruction fetch in kernel mode
  #PF: error_code(0x0010) - not-present page
  Call Trace:
    <TASK>
    read_pages+0x80b/0xb30 mm/readahead.c:173
    page_cache_ra_unbounded+0x40d/0x890 mm/readahead.c:302
    do_page_cache_ra mm/readahead.c:332 [inline]
    page_cache_ra_order+0xa16/0xcd0 mm/readahead.c:535
    page_cache_sync_ra+0x5ce/0x9d0 mm/readahead.c:626
    page_cache_sync_readahead include/linux/pagemap.h:1379 [inline]
    put_file_data fs/btrfs/send.c:5224 [inline]
    send_write fs/btrfs/send.c:5291 [inline]
    send_extent_data+0x16b2/0x29b0 fs/btrfs/send.c:5715
    send_write_or_clone fs/btrfs/send.c:6135 [inline]
    process_extent+0x5d4/0x17b0 fs/btrfs/send.c:6504
    changed_extent fs/btrfs/send.c:7079 [inline]
    changed_cb+0x22f9/0x3cd0 fs/btrfs/send.c:7245
    full_send_tree fs/btrfs/send.c:7318 [inline]
    send_subvol fs/btrfs/send.c:7910 [inline]
    btrfs_ioctl_send+0x46a9/0x57f0 fs/btrfs/send.c:8248
    ...

[CAUSE]
process_extent() skips extent items for symlinks but assumes every other
inode with an extent item is a regular file. For a corrupted non-regular
inode, btrfs_iget() does not install the regular file address_space
operations. The readahead fallback can then call a NULL read_folio
callback before the existing validation in btrfs_get_extent() can run.

[FIX]
Reject extent items for inode types other than regular files and symlinks
at the common send extent-processing boundary. Symlink handling is left
unchanged because send emits symlink data from read_symlink(). This covers
full, incremental and new-generation sends without adding a check to the
regular I/O path.

Signed-off-by: ZhengYuan Huang <[email protected]>
---
 fs/btrfs/send.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index dca3570168c7..f88623bbc491 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -6417,6 +6417,13 @@ static int process_extent(struct send_ctx *sctx,
 
 	if (S_ISLNK(sctx->cur_inode_mode))
 		return 0;
+	if (unlikely(!S_ISREG(sctx->cur_inode_mode))) {
+		btrfs_crit(sctx->send_root->fs_info,
+			   "send: extent for non-regular inode %llu root %llu mode 0%llo",
+			   key->objectid, btrfs_root_id(sctx->send_root),
+			   sctx->cur_inode_mode & S_IFMT);
+		return -EUCLEAN;
+	}
 
 	if (sctx->parent_root && !sctx->cur_inode_new) {
 		ret = is_extent_unchanged(sctx, path, key);
-- 
2.43.0