[PATCH] net: gro: avoid parsing headers from shared fragments

[Kyle Zeng <kylebot@openai.com>]

When an skb has an empty linear head, skb_gro_reset_offset() can expose
the first page fragment directly to GRO header parsers. For a
SKBFL_SHARED_FRAG skb, those bytes may still be writable through a file
mapping. Headers validated during receive can therefore differ from the
headers later copied into the linear head and used during completion.

For example, changing an IPv6 destination-options length between the
short and full header fetches in ipv6_gro_pull_exthdrs() can make
ipv6_gro_complete() pass an out-of-bounds offset to udp6_gro_complete().
This is reachable with an ordinary TCP GSO skb containing spliced file
data, without an skb zerocopy destructor or a second GRO packet.

Keep the direct-fragment fast path disabled for skbs with externally
writable fragments. With an empty linear head, frag0_len remains zero,
so skb_gro_header_slow() pulls each required header into the linear head
before it is inspected. Subsequent pulls preserve the already validated
prefix. This covers both gro_receive_skb() and napi_gro_frags() while
leaving the existing linear-head and private-fragment fast paths alone.

Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
I have a reproducer for this issue and can share via DM

 net/core/gro.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/gro.c b/net/core/gro.c
index 1b033855a20d..d6cf2d497934 100644
--- a/net/core/gro.c
+++ b/net/core/gro.c
@@ -413,7 +413,8 @@ static inline void skb_gro_reset_offset(struct sk_buff *skb, u32 nhoff)
 	headlen = skb_headlen(skb);
 	NAPI_GRO_CB(skb)->frag0 = skb->data;
 	NAPI_GRO_CB(skb)->frag0_len = headlen;
-	if (headlen)
+	/* Shared fragments must be pulled into the linear head before parsing. */
+	if (headlen || skb_has_shared_frag(skb))
 		return;
 
 	pinfo = skb_shinfo(skb);

base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3
-- 
2.56.0