[PATCH net 6/8] netfilter: nft_exthdr: fix register tracking for F_PRESENT flag

[Pablo Neira Ayuso <pablo@netfilter.org>]

From: Florian Westphal <fw@strlen.de>

nft_exthdr_init() passes user-controlled priv->len to
nft_parse_register_store(), which marks that many bytes in the
register bitmap as initialized.  However, when NFT_EXTHDR_F_PRESENT
is set, the eval paths write only 1 byte (nft_reg_store8) or
4 bytes (*dest = 0 on TCP/DCCP error path).  When len > 4,
registers beyond the first are never written, retaining
uninitialized stack data from nft_regs.

Bail out if userspace requests too much data when F_PRESENT is set.

Reported-by: Ji'an Zhou <eilaimemedsnaimel@gmail.com>
Fixes: c078ca3b0c5b ("netfilter: nft_exthdr: Add support for existence check")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nft_exthdr.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/netfilter/nft_exthdr.c b/net/netfilter/nft_exthdr.c
index e6a07c0df207..d3fc7969f123 100644
--- a/net/netfilter/nft_exthdr.c
+++ b/net/netfilter/nft_exthdr.c
@@ -532,6 +532,9 @@ static int nft_exthdr_init(const struct nft_ctx *ctx,
 			return err;
 	}
 
+	if ((flags & NFT_EXTHDR_F_PRESENT) && len != 1)
+		return -EINVAL;
+
 	priv->type   = nla_get_u8(tb[NFTA_EXTHDR_TYPE]);
 	priv->offset = offset;
 	priv->len    = len;
-- 
2.47.3