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Date:   Tue,  8 Dec 2020 23:13:51 +0100 (CET)
From:   Michal Kubecek <mkubecek@...e.cz>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH net] 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@...kaller.appspotmail.com
Signed-off-by: Michal Kubecek <mkubecek@...e.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

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