Re: [PATCH bpf 1/2] bpf: support SKF_NET_OFF and SKF_LL_OFF on skb frags

[Stanislav Fomichev <stfomichev@gmail.com>]

On 04/03, Willem de Bruijn wrote:
> Stanislav Fomichev wrote:
> > On 04/03, Willem de Bruijn wrote:
> > > From: Willem de Bruijn <willemb@google.com>
> > > 
> > > Classic BPF socket filters with SKB_NET_OFF and SKB_LL_OFF fail to
> > > read when these offsets extend into frags.
> > > 
> > > This has been observed with iwlwifi and reproduced with tun with
> > > IFF_NAPI_FRAGS. The below straightforward socket filter on UDP port,
> > > applied to a RAW socket, will silently miss matching packets.
> > > 
> > >     const int offset_proto = offsetof(struct ip6_hdr, ip6_nxt);
> > >     const int offset_dport = sizeof(struct ip6_hdr) + offsetof(struct udphdr, dest);
> > >     struct sock_filter filter_code[] = {
> > >             BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_AD_OFF + SKF_AD_PKTTYPE),
> > >             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, PACKET_HOST, 0, 4),
> > >             BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_NET_OFF + offset_proto),
> > >             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 2),
> > >             BPF_STMT(BPF_LD  + BPF_H   + BPF_ABS, SKF_NET_OFF + offset_dport),
> > > 
> > > This is unexpected behavior. Socket filter programs should be
> > > consistent regardless of environment. Silent misses are
> > > particularly concerning as hard to detect.
> > > 
> > > Use skb_copy_bits for offsets outside linear, same as done for
> > > non-SKF_(LL|NET) offsets.
> > > 
> > > Offset is always positive after subtracting the reference threshold
> > > SKB_(LL|NET)_OFF, so is always >= skb_(mac|network)_offset. The sum of
> > > the two is an offset against skb->data, and may be negative, but it
> > > cannot point before skb->head, as skb_(mac|network)_offset would too.
> > > 
> > > This appears to go back to when frag support was introduced to
> > > sk_run_filter in linux-2.4.4, before the introduction of git.
> > > 
> > > The amount of code change and 8/16/32 bit duplication are unfortunate.
> > > But any attempt I made to be smarter saved very few LoC while
> > > complicating the code.
> > > 
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Link: https://lore.kernel.org/netdev/20250122200402.3461154-1-maze@google.com/
> > > Link: https://elixir.bootlin.com/linux/2.4.4/source/net/core/filter.c#L244
> > > Reported-by: Matt Moeller <moeller.matt@gmail.com>
> > > Co-developed-by: Maciej Żenczykowski <maze@google.com>
> > > Signed-off-by: Maciej Żenczykowski <maze@google.com>
> > > Signed-off-by: Willem de Bruijn <willemb@google.com>
> > > ---
> > >  include/linux/filter.h |  3 --
> > >  kernel/bpf/core.c      | 21 ------------
> > >  net/core/filter.c      | 75 +++++++++++++++++++++++-------------------
> > >  3 files changed, 42 insertions(+), 57 deletions(-)
> > > 
> > > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > > index f5cf4d35d83e..708ac7e0cd36 100644
> > > --- a/include/linux/filter.h
> > > +++ b/include/linux/filter.h
> > > @@ -1496,9 +1496,6 @@ static inline u16 bpf_anc_helper(const struct sock_filter *ftest)
> > >  	}
> > >  }
> > >  
> > > -void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb,
> > > -					   int k, unsigned int size);
> > > -
> > >  static inline int bpf_tell_extensions(void)
> > >  {
> > >  	return SKF_AD_MAX;
> > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > > index ba6b6118cf50..0e836b5ac9a0 100644
> > > --- a/kernel/bpf/core.c
> > > +++ b/kernel/bpf/core.c
> > > @@ -68,27 +68,6 @@
> > >  struct bpf_mem_alloc bpf_global_ma;
> > >  bool bpf_global_ma_set;
> > >  
> > > -/* No hurry in this branch
> > > - *
> > > - * Exported for the bpf jit load helper.
> > > - */
> > > -void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb, int k, unsigned int size)
> > > -{
> > > -	u8 *ptr = NULL;
> > > -
> > > -	if (k >= SKF_NET_OFF) {
> > > -		ptr = skb_network_header(skb) + k - SKF_NET_OFF;
> > > -	} else if (k >= SKF_LL_OFF) {
> > > -		if (unlikely(!skb_mac_header_was_set(skb)))
> > > -			return NULL;
> > > -		ptr = skb_mac_header(skb) + k - SKF_LL_OFF;
> > > -	}
> > > -	if (ptr >= skb->head && ptr + size <= skb_tail_pointer(skb))
> > > -		return ptr;
> > > -
> > > -	return NULL;
> > > -}
> > > -
> > >  /* tell bpf programs that include vmlinux.h kernel's PAGE_SIZE */
> > >  enum page_size_enum {
> > >  	__PAGE_SIZE = PAGE_SIZE
> > > diff --git a/net/core/filter.c b/net/core/filter.c
> > > index bc6828761a47..b232b70dd10d 100644
> > > --- a/net/core/filter.c
> > > +++ b/net/core/filter.c
> > > @@ -221,21 +221,24 @@ BPF_CALL_3(bpf_skb_get_nlattr_nest, struct sk_buff *, skb, u32, a, u32, x)
> > >  BPF_CALL_4(bpf_skb_load_helper_8, const struct sk_buff *, skb, const void *,
> > >  	   data, int, headlen, int, offset)
> > >  {
> > > -	u8 tmp, *ptr;
> > > +	u8 tmp;
> > >  	const int len = sizeof(tmp);
> > >  
> > > -	if (offset >= 0) {
> > > -		if (headlen - offset >= len)
> > > -			return *(u8 *)(data + offset);
> > > -		if (!skb_copy_bits(skb, offset, &tmp, sizeof(tmp)))
> > > -			return tmp;
> > > -	} else {
> > > -		ptr = bpf_internal_load_pointer_neg_helper(skb, offset, len);
> > > -		if (likely(ptr))
> > > -			return *(u8 *)ptr;
> > 
> > [..]
> > 
> > > +	if (offset < 0) {
> > > +		if (offset >= SKF_NET_OFF)
> > > +			offset += skb_network_offset(skb) - SKF_NET_OFF;
> > > +		else if (offset >= SKF_LL_OFF && skb_mac_header_was_set(skb))
> > > +			offset += skb_mac_offset(skb) - SKF_LL_OFF;
> > > +		else
> > > +			return -EFAULT;
> > >  	}
> > 
> > nit: we now repeat the same logic three times, maybe still worth it to put it
> > into a helper? bpf_resolve_classic_offset or something. 
> 
> I definitely tried this in various ways. But since the core logic is
> only four lines and there is an early return on error, no helper
> really simplifies anything. It just adds a layer of indirection and
> more code in the end.

More code, but at least it de-duplicates the logic of translating
SKF_XXX_OFF? Something like the following below, but yeah, a matter
of preference, up to you.

static int bpf_skb_resolve_offset(skb, offset) {
	if (offset >= 0)
		return offset;

	if (offset >= SKF_NET_OFF)
		offset += skb_network_offset(skb) - SKF_NET_OFF;
	else if (offset >= SKF_LL_OFF && skb_mac_header_was_set(skb))
		offset += skb_mac_offset(skb) - SKF_LL_OFF;

	return -1;
}

BPF_CALL_4(bpf_skb_load_helper_8, const struct sk_buff *, skb, const void *,
           data, int, headlen, int, offset)
{
	offset = bpf_skb_resolve_offset(skb, offset);
	if (offset < 0)
		return -EFAULT;

	...
}