[PATCH 5.9 22/49] ethtool: fix stack overflow in ethnl_parse_bitset()

[Greg Kroah-Hartman]

From: Michal Kubecek <mkubecek@xxxxxxx>

[ Upstream commit a770bf515613c6e12ae904c3593e26016de99448 ]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
Link: https://lore.kernel.org/r/3487ee3a98e14cd526f55b6caaa959d2dcbcad9f.1607465316.git.mkubecek@xxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ethtool/bitset.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/net/ethtool/bitset.c
+++ b/net/ethtool/bitset.c
@@ -630,6 +630,8 @@ int ethnl_parse_bitset(unsigned long *va
 			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)