[PATCH net] xsk: reject tx_metadata_len smaller than struct xsk_tx_metadata

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

xdp_umem_reg() validates tx_metadata_len for upper bound (<256) and
alignment (%8) but not a lower bound.  xsk_skb_metadata() computes
meta = buffer - pool->tx_metadata_len then unconditionally accesses
the full 24-byte struct xsk_tx_metadata, so any value less than
sizeof(struct xsk_tx_metadata) allows an out-of-bounds read.

KASAN reports this as:

  BUG: KASAN: vmalloc-out-of-bounds in xsk_skb_metadata+0x4b2/0x500
  Read of size 8 at addr ffffc90000f11000 by task exploit/148

  xsk_skb_metadata (net/xdp/xsk.c:837)
  xsk_build_skb (net/xdp/xsk.c)
  __xsk_generic_xmit (net/xdp/xsk.c)
  xsk_sendmsg (net/xdp/xsk.c)

Add a lower-bound check in xdp_umem_reg() to reject tx_metadata_len
values that cannot cover the full metadata struct.

Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
 net/xdp/xdp_umem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
index 58da2f4f4397..d16ad9d8f919 100644
--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -208,7 +208,8 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
 		return -EINVAL;
 
 	if (mr->flags & XDP_UMEM_TX_METADATA_LEN) {
-		if (mr->tx_metadata_len >= 256 || mr->tx_metadata_len % 8)
+		if (mr->tx_metadata_len < sizeof(struct xsk_tx_metadata) ||
+		    mr->tx_metadata_len >= 256 || mr->tx_metadata_len % 8)
 			return -EINVAL;
 		umem->tx_metadata_len = mr->tx_metadata_len;
 	}
-- 
2.53.0