ethtool: fix stack overflow in ethnl_parse_bitset()

Syzbot reported a stack overflow in bitmap_from_arr32() called from
ethnl_parse_bitset() when bitset from netlink message is longer than
target bitmap length. While ethnl_compact_sanity_checks() makes sure that
trailing part is all zeros (i.e. the request does not try to touch bits
kernel does not recognize), we also need to cap change_bits to nbits so
that we don't try to write past the prepared bitmaps.

Fixes: 88db6d1e4f62 ("ethtool: add ethnl_parse_bitset() helper")
Reported-by: syzbot+9d39fa49d4df294aab93@syzkaller.appspotmail.com
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Link: https://lore.kernel.org/r/3487ee3a98e14cd526f55b6caaa959d2dcbcad9f.1607465316.git.mkubecek@suse.cz
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ethtool/bitset.c b/net/ethtool/bitset.c
index 1fb3603..0515d66 100644 (file)
--- a/net/ethtool/bitset.c
+++ b/net/ethtool/bitset.c
@@ -628,6 +628,8 @@ int ethnl_parse_bitset(unsigned long *val, unsigned long *mask,
                        return ret;
 
                change_bits = nla_get_u32(tb[ETHTOOL_A_BITSET_SIZE]);
+               if (change_bits > nbits)
+                       change_bits = nbits;
                bitmap_from_arr32(val, nla_data(tb[ETHTOOL_A_BITSET_VALUE]),
                                  change_bits);
                if (change_bits < nbits)