Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> wrote:
> When evaluating byteorder expressions with size 2, a union with 32-bit and
> 16-bit members is used. Since the 16-bit members are aligned to 32-bit,
> the array accesses will be out-of-bounds.
>
> It may lead to a stack-out-of-bounds access like the one below:
Yes, this is broken.
> Using simple s32 and s16 pointers for each of these accesses fixes the
> problem.
I'm not sure this is correct. Its certainly less wrong of course.
> Fixes: 96518518cc41 ("netfilter: add nftables")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Tanguy DUBROCA (@SidewayRE) from @Synacktiv working with ZDI
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
> ---
> net/netfilter/nft_byteorder.c | 17 ++++++++++-------
> 1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/net/netfilter/nft_byteorder.c b/net/netfilter/nft_byteorder.c
> index 9a85e797ed58..aa16bd2e92e2 100644
> --- a/net/netfilter/nft_byteorder.c
> +++ b/net/netfilter/nft_byteorder.c
> @@ -30,11 +30,14 @@ void nft_byteorder_eval(const struct nft_expr *expr,
> const struct nft_byteorder *priv = nft_expr_priv(expr);
> u32 *src = ®s->data[priv->sreg];
> u32 *dst = ®s->data[priv->dreg];
> - union { u32 u32; u16 u16; } *s, *d;
> + u32 *s32, *d32;
> + u16 *s16, *d16;
> unsigned int i;
>
> - s = (void *)src;
> - d = (void *)dst;
> + s32 = (void *)src;
> + d32 = (void *)dst;
> + s16 = (void *)src;
> + d16 = (void *)dst;
>
> switch (priv->size) {
> case 8: {
> @@ -62,11 +65,11 @@ void nft_byteorder_eval(const struct nft_expr *expr,
> switch (priv->op) {
> case NFT_BYTEORDER_NTOH:
> for (i = 0; i < priv->len / 4; i++)
> - d[i].u32 = ntohl((__force __be32)s[i].u32);
> + d32[i] = ntohl((__force __be32)s32[i]);
> break;
> case NFT_BYTEORDER_HTON:
> for (i = 0; i < priv->len / 4; i++)
> - d[i].u32 = (__force __u32)htonl(s[i].u32);
> + d32[i] = (__force __u32)htonl(s32[i]);
> break;
Ack, this looks better, but I'd just use src[i] and dst[i] rather than
the weird union pointers the original has.
> @@ -74,11 +77,11 @@ void nft_byteorder_eval(const struct nft_expr *expr,
> switch (priv->op) {
> case NFT_BYTEORDER_NTOH:
> for (i = 0; i < priv->len / 2; i++)
> - d[i].u16 = ntohs((__force __be16)s[i].u16);
> + d16[i] = ntohs((__force __be16)s16[i]);
This on the other hand... I'd say this should mimic what the 64bit
case is doing and use nft_reg_store16() nft_reg_load16() helpers for
the register accesses.
something like:
for (i = 0; i < priv->len / 2; i++) {
v16 = nft_reg_load16(&src[i]);
nft_reg_store16(&dst[i], + ntohs((__force __be16)v16));
}
[ not even compile tested ]
Same for the htons case.
On a slightly related note, some of the nftables test cases create bogus
conversions, e.g.:
# src/nft --debug=netlink add rule ip6 t c 'ct mark set ip6 dscp << 2 |
# 0x10'
ip6 t c
[ payload load 2b @ network header + 0 => reg 1 ]
[ bitwise reg 1 = ( reg 1 & 0x0000c00f ) ^ 0x00000000 ]
[ bitwise reg 1 = ( reg 1 >> 0x00000006 ) ]
[ byteorder reg 1 = ntoh(reg 1, 2, 1) ] // NO-OP! should be reg 1, 2, 2) I presume?
I'd suggest to add a patch for nf-next that rejects such crap.
