[PATCH net v2 0/2] Fix skb length accounting after XDP frag adjustment

[PATCH net v2 0/2] Fix skb length accounting after XDP frag adjustment

[Sun Jian]

Hi,

This series fixes skb length accounting after XDP fragment adjustment in
generic XDP and veth.

v1:
https://lore.kernel.org/bpf/20260727032535.13469-1-sun.jian.kdev@gmail.com/

Changes in v2:
- Move the veth fragment accounting before the linear tail adjustment, so
  __skb_put() observes a linear skb after a shrink removes all fragments.
- Fold the skb->len update into the xdp_buff_has_frags() branch, as suggested
  by Lorenzo Bianconi.
- Clarify why the old data_len contribution must be removed before adding the
  updated one, as requested by Maciej Fijalkowski.

Sun Jian (2):
  net: fix skb length accounting after generic XDP frag adjustment
  veth: fix skb length accounting after XDP frag adjustment

 drivers/net/veth.c | 23 +++++++++++++++--------
 net/core/dev.c     | 10 +++++++---
 2 files changed, 22 insertions(+), 11 deletions(-)

Range-diff against v1:
1:  b914fb40f348 ! 1:  2d716a1bd570 net: fix skb length accounting after generic XDP frag adjustment
    @@ Commit message
     
         Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
         Cc: stable@vger.kernel.org
    -    Link: https://lore.kernel.org/r/20260720141859.19FF41F000E9@smtp.kernel.org
         Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
         Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
     
    @@ net/core/dev.c: u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buf
      
      	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
     -	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
    -+	 * (e.g. bpf_xdp_adjust_tail), update skb length fields here.
    ++	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
    ++	 * from skb->len before updating data_len, then add the new one back.
      	 */
    +-	if (xdp_buff_has_frags(xdp))
     +	skb->len -= skb->data_len;
    - 	if (xdp_buff_has_frags(xdp))
    ++	if (xdp_buff_has_frags(xdp)) {
      		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
    - 	else
    +-	else
    ++		skb->len += skb->data_len;
    ++	} else {
      		skb->data_len = 0;
    -+	skb->len += skb->data_len;
    ++	}
      
      	/* check if XDP changed eth hdr such SKB needs update */
      	eth = (struct ethhdr *)xdp->data;
2:  59c79966c84a ! 2:  e5b9383bc46e veth: fix skb length accounting after XDP frag adjustment
    @@ Commit message
         Subtract the old data_len before replacing it and add the new data_len
         afterwards, keeping skb->len and skb->data_len synchronized.
     
    +    The fragment accounting must run before the linear tail adjustment:
    +    when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
    +    releases all fragments, and __skb_put() requires skb->data_len == 0
    +    by that point.
    +
         A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
         1024 bytes from its fragment area. Before the fix, all 10 runs produced
         corrupted payloads. After the fix, all 10 runs matched the expected
    @@ Commit message
     
         Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
         Cc: stable@vger.kernel.org
    -    Link: https://lore.kernel.org/r/20260720141859.19FF41F000E9@smtp.kernel.org
         Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
         Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
     
      ## drivers/net/veth.c ##
     @@ drivers/net/veth.c: static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
    - 		__skb_put(skb, off); /* positive on grow, negative on shrink */
      
    + 	skb_reset_mac_header(skb);
    + 
    +-	/* check if bpf_xdp_adjust_tail was used */
    +-	off = xdp->data_end - orig_data_end;
    +-	if (off != 0)
    +-		__skb_put(skb, off); /* positive on grow, negative on shrink */
    +-
      	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
     -	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
    -+	 * (e.g. bpf_xdp_adjust_tail), update skb length fields here.
    ++	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
    ++	 * from skb->len before updating data_len, then add the new one back.
    ++	 * This must precede the linear tail adjustment below: a changed
    ++	 * data_end implies that no fragments remain, and __skb_put() requires
    ++	 * a linear skb.
      	 */
    +-	if (xdp_buff_has_frags(xdp))
     +	skb->len -= skb->data_len;
    - 	if (xdp_buff_has_frags(xdp))
    ++	if (xdp_buff_has_frags(xdp)) {
      		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
    - 	else
    +-	else
    ++		skb->len += skb->data_len;
    ++	} else {
      		skb->data_len = 0;
    -+	skb->len += skb->data_len;
    ++	}
    ++
    ++	/* check if bpf_xdp_adjust_tail was used */
    ++	off = xdp->data_end - orig_data_end;
    ++	if (off != 0)
    ++		__skb_put(skb, off); /* positive on grow, negative on shrink */
      
      	skb->protocol = eth_type_trans(skb, rq->dev);
      

base-commit: 97ac08560d236ca17f6606d9e671118e5eae5721
-- 
2.43.0

[PATCH net v2 1/2] net: fix skb length accounting after generic XDP frag adjustment

[Sun Jian]

Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 net/core/dev.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 5933c5dab09e..5c37cf6c4aa1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5517,12 +5517,16 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
 	}
 
 	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
-	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
+	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
+	 * from skb->len before updating data_len, then add the new one back.
 	 */
-	if (xdp_buff_has_frags(xdp))
+	skb->len -= skb->data_len;
+	if (xdp_buff_has_frags(xdp)) {
 		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
-	else
+		skb->len += skb->data_len;
+	} else {
 		skb->data_len = 0;
+	}
 
 	/* check if XDP changed eth hdr such SKB needs update */
 	eth = (struct ethhdr *)xdp->data;
-- 
2.43.0

[PATCH net v2 2/2] veth: fix skb length accounting after XDP frag adjustment

[Sun Jian]

veth exposes non-linear skb fragments through an xdp_buff. If an XDP
program adjusts the fragment area, veth_xdp_rcv_skb() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

The fragment accounting must run before the linear tail adjustment:
when bpf_xdp_adjust_tail() shrinks the packet into the linear area it
releases all fragments, and __skb_put() requires skb->data_len == 0
by that point.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 drivers/net/veth.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 00e34afd858e..348391e87e14 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -865,18 +865,25 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 
 	skb_reset_mac_header(skb);
 
-	/* check if bpf_xdp_adjust_tail was used */
-	off = xdp->data_end - orig_data_end;
-	if (off != 0)
-		__skb_put(skb, off); /* positive on grow, negative on shrink */
-
 	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
-	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
+	 * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution
+	 * from skb->len before updating data_len, then add the new one back.
+	 * This must precede the linear tail adjustment below: a changed
+	 * data_end implies that no fragments remain, and __skb_put() requires
+	 * a linear skb.
 	 */
-	if (xdp_buff_has_frags(xdp))
+	skb->len -= skb->data_len;
+	if (xdp_buff_has_frags(xdp)) {
 		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
-	else
+		skb->len += skb->data_len;
+	} else {
 		skb->data_len = 0;
+	}
+
+	/* check if bpf_xdp_adjust_tail was used */
+	off = xdp->data_end - orig_data_end;
+	if (off != 0)
+		__skb_put(skb, off); /* positive on grow, negative on shrink */
 
 	skb->protocol = eth_type_trans(skb, rq->dev);
 
-- 
2.43.0