[PATCH bpf v2 1/2] bpf, xdp: clean head/meta when expanding it

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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: syzbot+0e6ddb1ef80986bdfe64@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/00000000000067f65105edbd295d@google.com/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 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);
 	xdp->data_meta = meta;
 
 	return 0;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index defa5bb881f4..7b1871f2eccf 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2761,7 +2761,8 @@ 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
+ *		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.
-- 
2.47.1