btrfs: avoid potential out-of-bounds in btrfs_encode_fh()

[ Upstream commit dff4f9ff ]

The function btrfs_encode_fh() does not properly account for the three
cases it handles.

Before writing to the file handle (fh), the function only returns to the
user BTRFS_FID_SIZE_NON_CONNECTABLE (5 dwords, 20 bytes) or
BTRFS_FID_SIZE_CONNECTABLE (8 dwords, 32 bytes).

However, when a parent exists and the root ID of the parent and the
inode are different, the function writes BTRFS_FID_SIZE_CONNECTABLE_ROOT
(10 dwords, 40 bytes).

If *max_len is not large enough, this write goes out of bounds because
BTRFS_FID_SIZE_CONNECTABLE_ROOT is greater than
BTRFS_FID_SIZE_CONNECTABLE originally returned.

This results in an 8-byte out-of-bounds write at
fid->parent_root_objectid = parent_root_id.

A previous attempt to fix this issue was made but was lost.

https://lore.kernel.org/all/4CADAEEC020000780001B32C@vpn.id2.novell.com/



Although this issue does not seem to be easily triggerable, it is a
potential memory corruption bug that should be fixed. This patch
resolves the issue by ensuring the function returns the appropriate size
for all three cases and validates that *max_len is large enough before
writing any data.

Fixes: be6e8dc0 ("NFS support for btrfs - v3")
CC: stable@vger.kernel.org # 3.0+
Signed-off-by: Anderson Nascimento <anderson@allelesecurity.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
[ replaced btrfs_root_id() calls with direct ->root->root_key.objectid access ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>