[PATCH bpf v2 01/11] bpf: Fix bounds check for skb-backed dynptrs

[Emil Tsalapatis <emil@etsalapatis.com>]

The skb_pointer_if_linear() function checks whether a
memory region of length len starting at offset off into
the skb is in the linear area, and returns a pointer to
the region if so. The check currently subtracts between
skb_headlen and offset of the check, and since skb_headlen
is unsigned the subtraction can underflow. This causes the
bounds check to spuriously pass and generate an arbitrary
pointer of the form *(skb->data + off).

The only user of this helper is currently skb-backed BPF
dynptr code. Returning the wrong pointer leads to the
dynptr erroneously being backed with invalid memory.

Ensure the subtraction cannot underflow, and fail the check if
it would. Use u64 arithmetic to also prevent overflow when
calculating (skb_headlen(skb) - off) since off is unsigned.

Fixes: 6f5a630d7c57 ("bpf, net: Introduce skb_pointer_if_linear().")
Reported-by: Nicholas Carlini <nicholas@carlini.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 include/linux/skbuff.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 421f6fc45..c8e219030 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -4372,7 +4372,10 @@ skb_header_pointer_careful(const struct sk_buff *skb, int offset,
 static inline void * __must_check
 skb_pointer_if_linear(const struct sk_buff *skb, int offset, int len)
 {
-	if (likely(skb_headlen(skb) - offset >= len))
+	unsigned int uoffset = (unsigned int)offset;
+
+	if (likely(uoffset <= skb_headlen(skb) &&
+		   (unsigned int)len <= skb_headlen(skb) - uoffset))
 		return skb->data + offset;
 	return NULL;
 }
-- 
2.54.0