RDMA/uverbs: Validate wqe_size before using it in ib_uverbs_post_send

RDMA/uverbs: Validate wqe_size before using it in ib_uverbs_post_send

ib_uverbs_post_send() uses cmd.wqe_size from userspace without any
validation before passing it to kmalloc() and using the allocated
buffer as struct ib_uverbs_send_wr.

If a user provides a small wqe_size value (e.g., 1), kmalloc() will
succeed, but subsequent accesses to user_wr->opcode, user_wr->num_sge,
and other fields will read beyond the allocated buffer, resulting in
an out-of-bounds read from kernel heap memory. This could potentially
leak sensitive kernel information to userspace.

Additionally, providing an excessively large wqe_size can trigger a
WARNING in the memory allocation path, as reported by syzkaller.

This is inconsistent with ib_uverbs_unmarshall_recv() which properly
validates that wqe_size >= sizeof(struct ib_uverbs_recv_wr) before
proceeding.

Add the same validation for ib_uverbs_post_send() to ensure wqe_size
is at least sizeof(struct ib_uverbs_send_wr).

Fixes: c3bea3d2dc53 ("RDMA/uverbs: Use the iterator for ib_uverbs_unmarshall_recv()")
Signed-off-by: Yi Liu <liuy22@mails.tsinghua.edu.cn>
Link: https://patch.msgid.link/20260122142900.2356276-2-liuy22@mails.tsinghua.edu.cn
Signed-off-by: Leon Romanovsky <leon@kernel.org>