[PATCH net v2] seg6: validate SRH length before reading fixed fields

[Nuoqi Gui]

seg6_validate_srh() reads fixed SRH fields such as srh->type and
srh->hdrlen before checking that the supplied length covers the fixed
struct ipv6_sr_hdr fields.

The BPF SEG6 encap path reaches this with a BPF program-supplied pointer
and length: bpf_lwt_push_encap() and the SEG6 local BPF END_B6 and
END_B6_ENCAP actions call bpf_push_seg6_encap(), which forwards the
length to seg6_validate_srh() with no minimum-size guard.  A 2-byte SEG6
encap header can therefore make the validator read srh->type at offset 2
beyond the caller-supplied buffer.

Reject lengths shorter than the fixed SRH at the top of
seg6_validate_srh(), before any field is read.  This fixes the BPF helper
path and keeps the common validator robust.

Fixes: fe94cc290f53 ("bpf: Add IPv6 Segment Routing helpers")
Signed-off-by: Nuoqi Gui <gnq25@xxxxxxxxxxxxxxxxxxxxx>
---
Changes in v2:
- Narrowed the commit message to the BPF encap callers that can supply a
  too-short SRH length.
- Dropped the unnecessary cast in the minimum SRH length check.
- Link to v1: https://patch.msgid.link/20260620-f01-17-seg6-srh-len-v1-1-36cbb29c12f1@xxxxxxxxxxxxxxxxxxxxx

To: Andrea Mayer <andrea.mayer@xxxxxxxxxxx>
To: "David S. Miller" <davem@xxxxxxxxxxxxx>
To: Eric Dumazet <edumazet@xxxxxxxxxx>
To: Jakub Kicinski <kuba@xxxxxxxxxx>
To: Paolo Abeni <pabeni@xxxxxxxxxx>
To: Simon Horman <horms@xxxxxxxxxx>
To: Mathieu Xhonneux <m.xhonneux@xxxxxxxxx>
To: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
To: David Lebrun <dlebrun@xxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: bpf@xxxxxxxxxxxxxxx
---
 net/ipv6/seg6.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
index 1c3ad25700c4c..62a7eb7792026 100644
--- a/net/ipv6/seg6.c
+++ b/net/ipv6/seg6.c
@@ -29,6 +29,9 @@ bool seg6_validate_srh(struct ipv6_sr_hdr *srh, int len, bool reduced)
 	int max_last_entry;
 	int trailing;
 
+	if (len < sizeof(*srh))
+		return false;
+
 	if (srh->type != IPV6_SRCRT_TYPE_4)
 		return false;
 

---
base-commit: 96e7f9122aae0ed000ee321f324b812a447906d9
change-id: 20260619-f01-17-seg6-srh-len-a85f35427e0b

Best regards,
--  
Nuoqi Gui <gnq25@xxxxxxxxxxxxxxxxxxxxx>