RDMA/umad: Reject negative data_len in ib_umad_write

ib_umad_write computes data_len from user-controlled count and the
MAD header sizes. With a mismatched user MAD header size and RMPP
header length, data_len can become negative and reach ib_create_send_mad().
This can make the padding calculation exceed the segment size and trigger
an out-of-bounds memset in alloc_send_rmpp_list().

Add an explicit check to reject negative data_len before creating the
send buffer.

KASAN splat:
[  211.363464] BUG: KASAN: slab-out-of-bounds in ib_create_send_mad+0xa01/0x11b0
[  211.364077] Write of size 220 at addr ffff88800c3fa1f8 by task spray_thread/102
[  211.365867] ib_create_send_mad+0xa01/0x11b0
[  211.365887] ib_umad_write+0x853/0x1c80

Fixes: 2be8e3e ("IB/umad: Add P_Key index support")
Signed-off-by: YunJe Shin <ioerts@kookmin.ac.kr>
Link: https://patch.msgid.link/20260203100628.1215408-1-ioerts@kookmin.ac.kr
Signed-off-by: Leon Romanovsky <leon@kernel.org>

Author: YunJe Shin

drivers/infiniband/core/user_mad.c

@@ -514,7 +514,8 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 	struct rdma_ah_attr ah_attr;
 	struct ib_ah *ah;
 	__be64 *tid;
-	int ret, data_len, hdr_len, copy_offset, rmpp_active;
+	int ret, hdr_len, copy_offset, rmpp_active;
+	size_t data_len;
 	u8 base_version;
 
 	if (count < hdr_size(file) + IB_MGMT_RMPP_HDR)
@@ -588,7 +589,10 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 	}
 
 	base_version = ((struct ib_mad_hdr *)&packet->mad.data)->base_version;
-	data_len = count - hdr_size(file) - hdr_len;
+	if (check_sub_overflow(count, hdr_size(file) + hdr_len, &data_len)) {
+		ret = -EINVAL;
+		goto err_ah;
+	}
 	packet->msg = ib_create_send_mad(agent,
 					 be32_to_cpu(packet->mad.hdr.qpn),
 					 packet->mad.hdr.pkey_index, rmpp_active,