[PATCH net 7/8] netfilter: nft_fib: fix stale stack leak via the OIFNAME register

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

From: Davide Ornaghi <d.ornaghi97@gmail.com>

For NFT_FIB_RESULT_OIFNAME the destination register is declared with
len = IFNAMSIZ (four 32-bit registers), but on the lookup-fail,
RTN_LOCAL and oif-mismatch paths nft_fib{4,6}_eval() only writes one
register via "*dest = 0". The remaining three registers are left as
whatever was on the stack in nft_do_chain()'s struct nft_regs, and a
downstream expression that loads the register span can leak that
uninitialised kernel stack to userspace.

The NFTA_FIB_F_PRESENT existence check has the same shape: it is only
meaningful for NFT_FIB_RESULT_OIF, yet it was accepted for any result type
while the eval stores a single byte via nft_reg_store8(), leaving the rest
of the declared span stale.

Fix both:

 - replace the bare "*dest = 0" in the eval with nft_fib_store_result(),
   which strscpy_pad()s the whole IFNAMSIZ for OIFNAME (and is already
   used on the other early-return path), and

 - restrict NFTA_FIB_F_PRESENT to NFT_FIB_RESULT_OIF and declare its
   destination as a single u8, so the marked span matches the one byte
   the eval writes.

Fixes: f6d0cbcf09c5 ("netfilter: nf_tables: add fib expression")
Suggested-by: Florian Westphal <fw@strlen.de>
Cc: stable@vger.kernel.org
Signed-off-by: Davide Ornaghi <d.ornaghi97@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/ipv4/netfilter/nft_fib_ipv4.c | 2 +-
 net/ipv6/netfilter/nft_fib_ipv6.c | 2 +-
 net/netfilter/nft_fib.c           | 6 ++++++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/netfilter/nft_fib_ipv4.c b/net/ipv4/netfilter/nft_fib_ipv4.c
index 9d0c6d75109b..177d738825b4 100644
--- a/net/ipv4/netfilter/nft_fib_ipv4.c
+++ b/net/ipv4/netfilter/nft_fib_ipv4.c
@@ -128,7 +128,7 @@ void nft_fib4_eval(const struct nft_expr *expr, struct nft_regs *regs,
 		fl4.saddr = get_saddr(iph->daddr);
 	}
 
-	*dest = 0;
+	nft_fib_store_result(dest, priv, NULL);
 
 	if (fib_lookup(nft_net(pkt), &fl4, &res, FIB_LOOKUP_IGNORE_LINKSTATE))
 		return;
diff --git a/net/ipv6/netfilter/nft_fib_ipv6.c b/net/ipv6/netfilter/nft_fib_ipv6.c
index 2dbe44715df3..b9ad7cac1417 100644
--- a/net/ipv6/netfilter/nft_fib_ipv6.c
+++ b/net/ipv6/netfilter/nft_fib_ipv6.c
@@ -239,7 +239,7 @@ void nft_fib6_eval(const struct nft_expr *expr, struct nft_regs *regs,
 
 	lookup_flags = nft_fib6_flowi_init(&fl6, priv, pkt, oif, iph);
 
-	*dest = 0;
+	nft_fib_store_result(dest, priv, NULL);
 	ret = nft_fib6_lookup(nft_net(pkt), &fl6, &res, lookup_flags);
 	if (ret || res.fib6_flags & (RTF_REJECT | RTF_ANYCAST | RTF_LOCAL))
 		return;
diff --git a/net/netfilter/nft_fib.c b/net/netfilter/nft_fib.c
index 327a5f33659c..a1632e308f18 100644
--- a/net/netfilter/nft_fib.c
+++ b/net/netfilter/nft_fib.c
@@ -107,6 +107,12 @@ int nft_fib_init(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		return -EINVAL;
 	}
 
+	if (priv->flags & NFTA_FIB_F_PRESENT) {
+		if (priv->result != NFT_FIB_RESULT_OIF)
+			return -EINVAL;
+		len = sizeof(u8);
+	}
+
 	err = nft_parse_register_store(ctx, tb[NFTA_FIB_DREG], &priv->dreg,
 				       NULL, NFT_DATA_VALUE, len);
 	if (err < 0)
-- 
2.47.3