[PATCH net 1/2] net: add skb_header_pointer_careful() helper

[Eric Dumazet <edumazet@google.com>]

This variant of skb_header_pointer() should be used in contexts
where @offset argument is user-controlled and could be negative.

Negative offsets are supported, as long as the zone starts
between skb->head and skb->data.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/skbuff.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 86737076101d4a8452e90fe78adcdcfdefb79169..72679aa7af587cad524feba82432154cfb8afbd3 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -4301,6 +4301,18 @@ skb_header_pointer(const struct sk_buff *skb, int offset, int len, void *buffer)
 				    skb_headlen(skb), buffer);
 }
 
+/* Variant of skb_header_pointer() where @offset is user-controlled
+ * and potentially negative.
+ */
+static inline void * __must_check
+skb_header_pointer_careful(const struct sk_buff *skb, int offset,
+			   int len, void *buffer)
+{
+	if (unlikely(offset < 0 && -offset > skb_headlen(skb)))
+		return NULL;
+	return skb_header_pointer(skb, offset, len, buffer);
+}
+
 static inline void * __must_check
 skb_pointer_if_linear(const struct sk_buff *skb, int offset, int len)
 {
-- 
2.53.0.rc1.217.geba53bf80e-goog