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netfilter: nft_fib: fix stale stack leak via the OIFNAME register

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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: f6d0cbcf09 ("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>
This commit is contained in:
Davide Ornaghi 2026-06-10 12:39:12 +02:00 • committed by Pablo Neira Ayuso
commit ab185e0c4f
3 changed files with 8 additions and 2 deletions
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2
net/ipv4/netfilter/nft_fib_ipv4.c
View file

@ -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;

2
net/ipv6/netfilter/nft_fib_ipv6.c
View file

@ -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;

6
net/netfilter/nft_fib.c
View file

@ -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)