[PATCH] xfrm: iptfs: propagate SKBFL_SHARED_FRAG in iptfs_skb_add_frags()

[Chen YanJun <635381823cyj@gmail.com>]

From: Chen YanJun <moomichen@tencent.com>

When iptfs_skb_add_frags() copies frag references from the source
frag walk into a new SKB, it increments the page reference count via
__skb_frag_ref() but does not propagate SKBFL_SHARED_FRAG to the
destination SKB's skb_shinfo->flags.

If the source SKB carries shared frags (e.g. from a page-pool backed
receive path), the new inner SKB will appear to ESP as having privately
owned frags.  A subsequent esp_input() call for a nested transport-mode
SA then takes the no-COW fast path and decrypts in place, writing over
pages that are still referenced by the outer IPTFS SKB.  This causes
kernel-visible memory corruption and can trigger a panic.

All other frag-transfer helpers in the kernel (skb_try_coalesce,
skb_gro_receive, __pskb_copy_fclone, skb_shift, skb_segment) correctly
propagate SKBFL_SHARED_FRAG; align iptfs_skb_add_frags() with this
convention.

Fixes: 5f2b6a909574 ("xfrm: iptfs: add skb-fragment sharing code")
Signed-off-by: Chen YanJun <moomichen@tencent.com>
---
 net/xfrm/xfrm_iptfs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
index ad810d1f97c0..0e0dcf47a470 100644
--- a/net/xfrm/xfrm_iptfs.c
+++ b/net/xfrm/xfrm_iptfs.c
@@ -496,6 +496,10 @@ static int iptfs_skb_add_frags(struct sk_buff *skb,
 		walk->past += frag->len;	/* careful, use src bv_len */
 		walk->fragi++;
 	}
+
+	if (skb_shinfo(skb)->nr_frags)
+		skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
+
 	return len;
 }
 
-- 
2.47.0