[PATCH 5.6 020/166] xsk: Add missing check on user supplied headroom size

[Greg Kroah-Hartman]

From: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>

commit 99e3a236dd43d06c65af0a2ef9cb44306aef6e02 upstream.

Add a check that the headroom cannot be larger than the available
space in the chunk. In the current code, a malicious user can set the
headroom to a value larger than the chunk size minus the fixed XDP
headroom. That way packets with a length larger than the supported
size in the umem could get accepted and result in an out-of-bounds
write.

Fixes: c0c77d8fb787 ("xsk: add user memory registration support sockopt")
Reported-by: Bui Quang Minh <minhquangbui99@xxxxxxxxx>
Signed-off-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=207225
Link: https://lore.kernel.org/bpf/1586849715-23490-1-git-send-email-magnus.karlsson@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 net/xdp/xdp_umem.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -343,7 +343,7 @@ static int xdp_umem_reg(struct xdp_umem
 	u32 chunk_size = mr->chunk_size, headroom = mr->headroom;
 	unsigned int chunks, chunks_per_page;
 	u64 addr = mr->addr, size = mr->len;
-	int size_chk, err;
+	int err;
 
 	if (chunk_size < XDP_UMEM_MIN_CHUNK_SIZE || chunk_size > PAGE_SIZE) {
 		/* Strictly speaking we could support this, if:
@@ -382,8 +382,7 @@ static int xdp_umem_reg(struct xdp_umem
 			return -EINVAL;
 	}
 
-	size_chk = chunk_size - headroom - XDP_PACKET_HEADROOM;
-	if (size_chk < 0)
+	if (headroom >= chunk_size - XDP_PACKET_HEADROOM)
 		return -EINVAL;
 
 	umem->address = (unsigned long)addr;