net: skbuff: propagate shared-frag marker through frag-transfer helpers

Three frag-transfer helpers (__pskb_copy_fclone(), skb_try_coalesce(), and skb_shift()) fail to propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()->flags when moving frags from source to destination. __pskb_copy_fclone() defers the rest of the shinfo metadata to skb_copy_header() after copying frag descriptors, but that helper only carries over gso_{size,segs,type} and never touches skb_shinfo()->flags; skb_try_coalesce() and skb_shift() move frag descriptors directly and leave flags untouched. As a result, the destination skb keeps a reference to the same externally-owned or page-cache-backed pages while reporting skb_has_shared_frag() as false.

The mismatch is harmful in any in-place writer that uses skb_has_shared_frag() to decide whether shared pages must be detoured through skb_cow_data(). ESP input is one such writer (esp4.c, esp6.c), and a single nft 'dup to <local>' rule -- or any other nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d skb in esp_input() with the marker stripped, letting an unprivileged user write into the page cache of a root-owned read-only file via authencesn-ESN stray writes.

Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors were actually moved from the source. skb_copy() and skb_copy_expand() share skb_copy_header() too but linearize all paged data into freshly allocated head storage and emerge with nr_frags == 0, so skb_has_shared_frag() returns false on its own; they need no change.

The same omission exists in skb_gro_receive() and skb_gro_receive_list(). The former moves the incoming skb's frag descriptors into the accumulator's last sub-skb via two paths (a direct frag-move loop and the head_frag + memcpy path); the latter chains the incoming skb whole onto p's frag_list. Downstream skb_segment() reads only skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's shinfo as the nskb -- both p and lp must carry the marker.

Fixes cef401de7be8 ("net: fix possible wrong checksum generation")

Fixes f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")

Reported-by William Bowling <[email protected]>

Reported-by Hyunwoo Kim <[email protected]>

Cc [email protected]

Signed-off-by Hyunwoo Kim <[email protected]>

---
Changes in v3:
- Include the skb_gro_receive() audit patch suggested by Sultan
- v2: https://lore.kernel.org/all/agToIEDI4TaTNLRb@v4bel/
Changes in v2:
- Also propagate SHARED_FRAG in skb_try_coalesce() and skb_shift()
- v1: https://lore.kernel.org/all/agRfuVOeMI5pbHhY@v4bel/

---
 net/core/gro.c    | 4 ++++
 net/core/skbuff.c | 5 +++++
 2 files changed, 9 insertions(+)

Quoted 48 lines · Hyunwoo Kim

Three frag-transfer helpers (__pskb_copy_fclone(), skb_try_coalesce(), and skb_shift()) fail to propagate the SKBFL_SHARED_FRAG bit in skb_shinfo()->flags when moving frags from source to destination. __pskb_copy_fclone() defers the rest of the shinfo metadata to skb_copy_header() after copying frag descriptors, but that helper only carries over gso_{size,segs,type} and never touches skb_shinfo()->flags; skb_try_coalesce() and skb_shift() move frag descriptors directly and leave flags untouched. As a result, the destination skb keeps a reference to the same externally-owned or page-cache-backed pages while reporting skb_has_shared_frag() as false.

The mismatch is harmful in any in-place writer that uses skb_has_shared_frag() to decide whether shared pages must be detoured through skb_cow_data(). ESP input is one such writer (esp4.c, esp6.c), and a single nft 'dup to <local>' rule -- or any other nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d skb in esp_input() with the marker stripped, letting an unprivileged user write into the page cache of a root-owned read-only file via authencesn-ESN stray writes.

Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors were actually moved from the source. skb_copy() and skb_copy_expand() share skb_copy_header() too but linearize all paged data into freshly allocated head storage and emerge with nr_frags == 0, so skb_has_shared_frag() returns false on its own; they need no change.

The same omission exists in skb_gro_receive() and skb_gro_receive_list(). The former moves the incoming skb's frag descriptors into the accumulator's last sub-skb via two paths (a direct frag-move loop and the head_frag + memcpy path); the latter chains the incoming skb whole onto p's frag_list. Downstream skb_segment() reads only skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's shinfo as the nskb -- both p and lp must carry the marker.

Fixes cef401de7be8 ("net: fix possible wrong checksum generation")

Fixes f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")

Reported-by William Bowling <[email protected]>

Reported-by Hyunwoo Kim <[email protected]>

Cc [email protected]

Signed-off-by Hyunwoo Kim <[email protected]>

---
Changes in v3:
- Include the skb_gro_receive() audit patch suggested by Sultan
- v2: https://lore.kernel.org/all/agToIEDI4TaTNLRb@v4bel/
Changes in v2:
- Also propagate SHARED_FRAG in skb_try_coalesce() and skb_shift()
- v1: https://lore.kernel.org/all/agRfuVOeMI5pbHhY@v4bel/

Quoted 4 lines

---
 net/core/gro.c    | 4 ++++
 net/core/skbuff.c | 5 +++++
 2 files changed, 9 insertions(+)

I think we should also be propagating SKBFL_SHARED_FRAG in tcp_clone_payload(). It's copying frags from skbs in sk_write_queue to a new skb in the same way as those functions you're fixing here.

-------- 8< --------

net/ipv4/tcp_output.c +1 -0

index f9d8755705f7..6e4bb411dc04 100644
@@ -2626,6 +2626,7 @@ static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
 			todo = min_t(int, skb_frag_size(fragfrom),
 				     probe_size - len);
 			len += todo;
+			skb_shinfo(to)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
 			if (lastfrag &&
 			    skb_frag_page(fragfrom) == skb_frag_page(lastfrag) &&
 			    skb_frag_off(fragfrom) == skb_frag_off(lastfrag) +

Signature · 2 lines

-- 
Sabrina