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

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: be6e8dc0ba84 ("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>

diff --git a/fs/btrfs/export.c b/fs/btrfs/export.c
index d062ac521051b81c41d0510481177fbe161a9744..230d9326b685c4e12dc0fffd4a86ebba68a55bd6 100644 (file)
--- a/fs/btrfs/export.c
+++ b/fs/btrfs/export.c
@@ -23,7 +23,11 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
        int type;
 
        if (parent && (len < BTRFS_FID_SIZE_CONNECTABLE)) {
-               *max_len = BTRFS_FID_SIZE_CONNECTABLE;
+               if (btrfs_root_id(BTRFS_I(inode)->root) !=
+                   btrfs_root_id(BTRFS_I(parent)->root))
+                       *max_len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
+               else
+                       *max_len = BTRFS_FID_SIZE_CONNECTABLE;
                return FILEID_INVALID;
        } else if (len < BTRFS_FID_SIZE_NON_CONNECTABLE) {
                *max_len = BTRFS_FID_SIZE_NON_CONNECTABLE;
@@ -45,6 +49,8 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
                parent_root_id = btrfs_root_id(BTRFS_I(parent)->root);
 
                if (parent_root_id != fid->root_objectid) {
+                       if (*max_len < BTRFS_FID_SIZE_CONNECTABLE_ROOT)
+                               return FILEID_INVALID;
                        fid->parent_root_objectid = parent_root_id;
                        len = BTRFS_FID_SIZE_CONNECTABLE_ROOT;
                        type = FILEID_BTRFS_WITH_PARENT_ROOT;