[PATCH v2 net] net: skbuff: propagate shared-frag marker in skb_zerocopy()

[Zizhen Liu <pointerliu@126.com>]

skb_zerocopy() moves frag descriptors from @from into @to and takes a
reference on each frag, but it does not copy skb_shinfo(from)->flags into
skb_shinfo(to)->flags.  A destination skb holding the same page-cache
backed frags as the source therefore reports skb_has_shared_frag() as
false.

That breaks the invariant relied on by later in-place writers.  ESP input
checks skb_has_shared_frag() before deciding whether an uncloned
nonlinear skb can skip skb_cow_data(); with the marker lost, an skb that
skb_zerocopy() built for nfnetlink_queue or for an openvswitch upcall is
decrypted in place, letting an unprivileged user write into the page
cache of a root-owned read-only file.

Propagate SKBFL_SHARED_FRAG when frag descriptors are copied, as done for
the other frag-transfer helpers.

Fixes: af2806f8f90a ("net: Export skb_zerocopy() to zerocopy from one skb to another")
Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Cc: stable@kernel.org
Signed-off-by: Zizhen Liu <pointerliu@126.com>
---

v2:
 - correct the first Fixes tag to af2806f8f90a, which introduced
   skb_zerocopy(); the function did not exist when cef401de7be8 was merged.

 net/core/skbuff.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 41beaf625..2bfabde31 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3942,6 +3942,8 @@ skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
 	}
 	skb_shinfo(to)->nr_frags = j;
 
+	skb_shinfo(to)->flags |= skb_shinfo(from)->flags & SKBFL_SHARED_FRAG;
+
 	if (i > 0 && from->unreadable)
 		to->unreadable = 1;
 
-- 
2.43.0