On Sun, Mar 30, 2025 at 8:27 PM Jiayuan Chen <[email protected]> wrote: > > The device allocates an skb, it additionally allocates a prepad size > (usually equal to NET_SKB_PAD or XDP_PACKET_HEADROOM) but leaves it > uninitialized. > > The bpf_xdp_adjust_head function moves skb->data forward, which allows > users to access data belonging to other programs, posing a security risk. > > Reported-by: [email protected] > Closes: https://lore.kernel.org/all/[email protected]/T/ > Signed-off-by: Jiayuan Chen <[email protected]> > --- > include/uapi/linux/bpf.h | 8 +++++--- > net/core/filter.c | 5 ++++- > tools/include/uapi/linux/bpf.h | 6 ++++-- > 3 files changed, 13 insertions(+), 6 deletions(-) > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h > index defa5bb881f4..be01a848cbbf 100644 > --- a/include/uapi/linux/bpf.h > +++ b/include/uapi/linux/bpf.h > @@ -2760,8 +2760,9 @@ union bpf_attr { > * > * long bpf_xdp_adjust_head(struct xdp_buff *xdp_md, int delta) > * Description > - * Adjust (move) *xdp_md*\ **->data** by *delta* bytes. Note that > - * it is possible to use a negative value for *delta*. This > helper > + * Adjust (move) *xdp_md*\ **->data** by *delta* bytes. Note that > + * it is possible to use a negative value for *delta*. If *delta* > + * is negative, the new header will be memset to zero. This > helper > * can be used to prepare the packet for pushing or popping > * headers. > * > @@ -2989,7 +2990,8 @@ union bpf_attr { > * long bpf_xdp_adjust_meta(struct xdp_buff *xdp_md, int delta) > * Description > * Adjust the address pointed by *xdp_md*\ **->data_meta** by > - * *delta* (which can be positive or negative). Note that this > + * *delta* (which can be positive or negative). If *delta* is > + * negative, the new meta will be memset to zero. Note that this > * operation modifies the address stored in *xdp_md*\ **->data**, > * so the latter must be loaded only after the helper has been > * called. > diff --git a/net/core/filter.c b/net/core/filter.c > index 46ae8eb7a03c..5f01d373b719 100644 > --- a/net/core/filter.c > +++ b/net/core/filter.c > @@ -3947,6 +3947,8 @@ BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, > int, offset) > if (metalen) > memmove(xdp->data_meta + offset, > xdp->data_meta, metalen); > + if (offset < 0) > + memset(data, 0, -offset); > xdp->data_meta += offset; > xdp->data = data; > > @@ -4239,7 +4241,8 @@ BPF_CALL_2(bpf_xdp_adjust_meta, struct xdp_buff *, xdp, > int, offset) > return -EINVAL; > if (unlikely(xdp_metalen_invalid(metalen))) > return -EACCES; > - > + if (offset < 0) > + memset(meta, 0, -offset);
Let's make everyone pay a performance penalty to silence KMSAN warning? I don't think it's a good trade off. Soft nack.
