[PATCH net] ethtool: fix stack overflow in ethnl_parse_bitset()

[Michal Kubecek]

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+9d39fa49d4df294aa...@syzkaller.appspotmail.com
Signed-off-by: Michal Kubecek <mkube...@suse.cz>
---
 net/ethtool/bitset.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ethtool/bitset.c b/net/ethtool/bitset.c
index 1fb3603d92ad..0515d6604b3b 100644
--- 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)
-- 
2.29.2