Re: [PATCH] netfilter: nf_flow_table_offload: Correct memcpy size for flow_overload_mangle

[Laura Abbott]

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@xxxxxxxxxx>
> > ---
> > 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.

Thanks,
Laura
 
> >   }
> >   
> >   static inline struct flow_action_entry *
> > --
> > 2.21.0