[PATCH v2] 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 by setting the flag inside the loop immediately after
__skb_frag_ref() and nr_frags++, so every exit path that attaches a frag
unconditionally propagates SKBFL_SHARED_FRAG.

Fixes: 5f2b6a909574 ("xfrm: iptfs: add skb-fragment sharing code")
Signed-off-by: Chen YanJun <moomichen@tencent.com>

---
v2: move SKBFL_SHARED_FRAG assignment inside the loop, immediately after
    __skb_frag_ref()/nr_frags++, to also cover the early-return path
    that fires when the requested length is satisfied mid-frag (pointed
    out by maintainer review of v1).
---
 net/xfrm/xfrm_iptfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
index ad810d1f97c0..597aedeac26e 100644
--- a/net/xfrm/xfrm_iptfs.c
+++ b/net/xfrm/xfrm_iptfs.c
@@ -480,6 +480,7 @@ static int iptfs_skb_add_frags(struct sk_buff *skb,
 		}
 		__skb_frag_ref(tofrag);
 		shinfo->nr_frags++;
+		shinfo->flags |= SKBFL_SHARED_FRAG;
 
 		/* see if we are done */
 		fraglen = tofrag->len;
-- 
2.47.0