[PATCH net v2 1/2] net: core: propagate unreadable flag in skb_zerocopy

[Mina Almasry <almasrymina@google.com>]

skb_zerocopy() fails to propagate the unreadable flag when copying
devmem fragments, causing target skbs to appear as readable memory.

This patch fixes the flag propagation. Additionally, it returns -EFAULT
if standard payload is mixed with unreadable devmem fragments during
extraction, and clamps unreadable skb lengths in openvswitch
queue_userspace_packet() to avert truncated invalid payloads.

Fixes: 65249feb6b3d ("net: add support for skbs with unreadable frags")
Cc: Pavel Begunkov <asml.silence@gmail.com>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Bobby Eshleman <bobbyeshleman@gmail.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Aaron Conole <aconole@redhat.com>
Cc: Eelco Chaudron <echaudro@redhat.com>
Cc: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

---
v2:
- Return -EFAULT when mixing unreadable and readable frags (Pavel).
- Clamp unreadable skb lengths for openvswitch queue drops (sashiko).
v1: https://lore.kernel.org/r/20260801125308.1342897-1-almasrymina@google.com

Openvswitch maintainers: PTAL at the openvswitch changes closely. They
are reported by sashiko as an also-need part of this fix:
https://sashiko.dev/#/patchset/20260706155219.23757-1-fw%40strlen.de
---
 net/core/skbuff.c          | 13 ++++++++++++-
 net/openvswitch/datapath.c |  2 ++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index ba3dbac80fb49..d21af68156950 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3870,7 +3870,8 @@ EXPORT_SYMBOL_GPL(skb_zerocopy_headlen);
  *	Return value:
  *	0: everything is OK
  *	-ENOMEM: couldn't orphan frags of @from due to lack of memory
- *	-EFAULT: skb_copy_bits() found some problem with skb geometry
+ *	-EFAULT: skb_copy_bits() found some problem with skb geometry, or readable head
+ *      payload would be mixed with unreadable frags.
  */
 int
 skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
@@ -3905,10 +3906,17 @@ skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
 		}
 	}
 
+	if (!skb_frags_readable(from) && j > 0 && len) {
+		put_page(virt_to_head_page(from->head));
+		return -EFAULT;
+	}
+
 	skb_len_add(to, len + plen);
 
 	if (unlikely(skb_orphan_frags(from, GFP_ATOMIC))) {
 		skb_tx_error(from);
+		if (j > 0)
+			put_page(virt_to_head_page(from->head));
 		return -ENOMEM;
 	}
 	skb_zerocopy_clone(to, from, GFP_ATOMIC);
@@ -3928,6 +3936,9 @@ skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
 	}
 	skb_shinfo(to)->nr_frags = j;
 
+	if (i > 0 && from->unreadable)
+		to->unreadable = 1;
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(skb_zerocopy);
diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index ae69b2cabab9e..7c663d7846174 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -480,6 +480,8 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 	}
 
 	skb_len = min(skb->len, cutlen);
+	if (!skb_frags_readable(skb))
+		skb_len = min_t(size_t, skb_len, skb_headlen(skb));
 	if (nla_attr_size(skb_len) > USHRT_MAX) {
 		err = -EFBIG;
 		goto out;

base-commit: dd057113ac7ba5bdd2aed3d9405305911152f911
-- 
2.55.0.679.g6767b8d81c-goog