Re: [PATCH v1 net] af_packet: Fix fortified memcpy() without flex array.

[Kees Cook <keescook@chromium.org>]

On Mon, Oct 09, 2023 at 08:31:52AM -0700, Kuniyuki Iwashima wrote:
> Sergei Trofimovich reported a regression [0] caused by commit a0ade8404c3b
> ("af_packet: Fix warning of fortified memcpy() in packet_getname().").
> 
> It introduced a flex array sll_addr_flex in struct sockaddr_ll as a
> union-ed member with sll_addr to work around the fortified memcpy() check.
> 
> However, a userspace program uses a struct that has struct sockaddr_ll in
> the middle, where a flex array is illegal to exist.
> 
>   include/linux/if_packet.h:24:17: error: flexible array member 'sockaddr_ll::<unnamed union>::<unnamed struct>::sll_addr_flex' not at end of 'struct packet_info_t'
>      24 |                 __DECLARE_FLEX_ARRAY(unsigned char, sll_addr_flex);
>         |                 ^~~~~~~~~~~~~~~~~~~~
> To fix the regression, let's go back to the first attempt [1] telling
> memcpy() the actual size of the array.
> 
> Reported-by: Sergei Trofimovich <slyich@gmail.com>
> Closes: https://github.com/NixOS/nixpkgs/pull/252587#issuecomment-1741733002 [0]

Eww. That's a buggy definition -- it could get overflowed.

But okay, we don't break userspace.

> Link: https://lore.kernel.org/netdev/20230720004410.87588-3-kuniyu@amazon.com/ [1]
> Fixes: a0ade8404c3b ("af_packet: Fix warning of fortified memcpy() in packet_getname().")
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> ---
>  include/uapi/linux/if_packet.h | 6 +-----
>  net/packet/af_packet.c         | 7 ++++++-
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/uapi/linux/if_packet.h b/include/uapi/linux/if_packet.h
> index 4d0ad22f83b5..9efc42382fdb 100644
> --- a/include/uapi/linux/if_packet.h
> +++ b/include/uapi/linux/if_packet.h
> @@ -18,11 +18,7 @@ struct sockaddr_ll {
>  	unsigned short	sll_hatype;
>  	unsigned char	sll_pkttype;
>  	unsigned char	sll_halen;
> -	union {
> -		unsigned char	sll_addr[8];
> -		/* Actual length is in sll_halen. */
> -		__DECLARE_FLEX_ARRAY(unsigned char, sll_addr_flex);
> -	};
> +	unsigned char	sll_addr[8];
>  };

Yup, we need to do at least this.

>  
>  /* Packet types */
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 8f97648d652f..a84e00b5904b 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -3607,7 +3607,12 @@ static int packet_getname(struct socket *sock, struct sockaddr *uaddr,
>  	if (dev) {
>  		sll->sll_hatype = dev->type;
>  		sll->sll_halen = dev->addr_len;
> -		memcpy(sll->sll_addr_flex, dev->dev_addr, dev->addr_len);
> +
> +		/* Let __fortify_memcpy_chk() know the actual buffer size. */
> +		memcpy(((struct sockaddr_storage *)sll)->__data +
> +		       offsetof(struct sockaddr_ll, sll_addr) -
> +		       offsetofend(struct sockaddr_ll, sll_family),
> +		       dev->dev_addr, dev->addr_len);
>  	} else {
>  		sll->sll_hatype = 0;	/* Bad: we have no ARPHRD_UNSPEC */
>  		sll->sll_halen = 0;

I still think this is a mistake. We're papering over so many lies to the
compiler. :P If "uaddr" is actually "struct sockaddr_storage", then we
should update the callers... and if "struct sockaddr_ll" doesn't have a
fixed size trailing array, we should make a new struct that is telling
the truth. ;)

Perhaps add this to the UAPI:

+struct sockaddr_ll_flex {
+       unsigned short  sll_family;
+       __be16          sll_protocol;
+       int             sll_ifindex;
+       unsigned short  sll_hatype;
+       unsigned char   sll_pkttype;
+       unsigned char   sll_halen;
+       unsigned char   sll_addr[] __counted_by(sll_halen);
+};

And update the memcpy():

-       DECLARE_SOCKADDR(struct sockaddr_ll *, sll, uaddr);
+       struct sockaddr_ll_flex * sll = (struct sockaddr_ll_flex *)uaddr;

?

-- 
Kees Cook