linux-kernel - Re: [PATCH] netfilter: nf_flow_table_offload: Correct memcpy size for flow_overload

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Date:   Fri, 6 Dec 2019 16:58:30 -0600
From:   Justin Forbes <jmforbes@...uxtx.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] netfilter: nf_flow_table_offload: Correct memcpy size for flow_overload_mangle

On Tue, Dec 3, 2019 at 2:50 PM Laura Abbott <labbott@...hat.com> wrote:
>
> On 12/3/19 12:01 PM, Marcelo Ricardo Leitner wrote:
> > On Tue, Dec 03, 2019 at 11:03:45AM -0500, Laura Abbott wrote:
> >> The sizes for memcpy in flow_offload_mangle don't match
> >> the source variables, leading to overflow errors on some
> >> build configurations:
> >>
> >> In function 'memcpy',
> >>      inlined from 'flow_offload_mangle' at net/netfilter/nf_flow_table_offload.c:112:2,
> >>      inlined from 'flow_offload_port_dnat' at net/netfilter/nf_flow_table_offload.c:373:2,
> >>      inlined from 'nf_flow_rule_route_ipv4' at net/netfilter/nf_flow_table_offload.c:424:3:
> >> ./include/linux/string.h:376:4: error: call to '__read_overflow2' declared with attribute error: detected read beyond size of object passed as 2nd parameter
> >>    376 |    __read_overflow2();
> >>        |    ^~~~~~~~~~~~~~~~~~
> >> make[2]: *** [scripts/Makefile.build:266: net/netfilter/nf_flow_table_offload.o] Error 1
> >>
> >> Fix this by using the corresponding type.
> >>
> >> Fixes: c29f74e0df7a ("netfilter: nf_flow_table: hardware offload support")
> >> Signed-off-by: Laura Abbott <labbott@...hat.com>
> >> ---
> >> Seen on a Fedora powerpc little endian build with -O3 but it looks like
> >> it is correctly catching an error with doing a memcpy outside the source
> >> variable.
> >
> > Hi,
> >
> > It is right but the fix is not. In that call trace:
> >
> > flow_offload_port_dnat() {
> > ...
> >          u32 mask = ~htonl(0xffff);
> >          __be16 port;
> > ...
> >          flow_offload_mangle(entry, flow_offload_l4proto(flow), offset,
> >                                   (u8 *)&port, (u8 *)&mask);
> > }
> >
> > port should have a 32b storage as well, and aligned with the mask.
> >
> >> ---
> >>   net/netfilter/nf_flow_table_offload.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/net/netfilter/nf_flow_table_offload.c b/net/netfilter/nf_flow_table_offload.c
> >> index c54c9a6cc981..526f894d0bdb 100644
> >> --- a/net/netfilter/nf_flow_table_offload.c
> >> +++ b/net/netfilter/nf_flow_table_offload.c
> >> @@ -108,8 +108,8 @@ static void flow_offload_mangle(struct flow_action_entry *entry,
> >>      entry->id = FLOW_ACTION_MANGLE;
> >>      entry->mangle.htype = htype;
> >>      entry->mangle.offset = offset;
> >> -    memcpy(&entry->mangle.mask, mask, sizeof(u32));
> >> -    memcpy(&entry->mangle.val, value, sizeof(u32));
> >                                     ^^^^^         ^^^ which is &port in the call above
> >> +    memcpy(&entry->mangle.mask, mask, sizeof(u8));
> >> +    memcpy(&entry->mangle.val, value, sizeof(u8));
> >
> > This fix would cause it to copy only the first byte, which is not the
> > intention.
> >
>
> Thanks for the review. I took another look at fixing this and I
> think it might be better for the maintainer or someone who is more
> familiar with the code to fix this. I ended up down a rabbit
> hole trying to get the types to work and I wasn't confident about
> the casting.
>

Any update on this? It is definitely a problem on PPC LE.

Thanks,
Justin