io_uring/cancel: validate opcode for IORING_ASYNC_CANCEL_OP

io_uring/cancel: validate opcode for IORING_ASYNC_CANCEL_OP

io_async_cancel_prep() reads the opcode selector from sqe->len and
stores it in cancel->opcode, which is an 8-bit field. Since sqe->len
is a 32-bit value, values larger than U8_MAX are implicitly truncated.

This can cause unintended opcode matches when the truncated value
corresponds to a valid io_uring opcode. For example, submitting a value
such as 0x10b will be truncated to 0x0b (IORING_OP_TIMEOUT), allowing a
cancel request to match operations it did not intend to target.
Validate the opcode value before assigning it to the 8-bit field and
reject values outside the valid io_uring opcode range.

Signed-off-by: Amir Mohammad Jahangirzad <a.jahangirzad@gmail.com>
Link: https://patch.msgid.link/20260331232113.615972-1-a.jahangirzad@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>