Re: [PATCH 1/1] io_uring: prevent reg-wait speculations

[Jens Axboe <axboe@kernel.dk>]

On 11/18/24 6:43 PM, Pavel Begunkov wrote:
> On 11/19/24 01:29, Jens Axboe wrote:
>> On 11/18/24 6:29 PM, Pavel Begunkov wrote:
>>> With *ENTER_EXT_ARG_REG instead of passing a user pointer with arguments
>>> for the waiting loop the user can specify an offset into a pre-mapped
>>> region of memory, in which case the
>>> [offset, offset + sizeof(io_uring_reg_wait)) will be intepreted as the
>>> argument.
>>>
>>> As we address a kernel array using a user given index, it'd be a subject
>>> to speculation type of exploits.
>>>
>>> Fixes: d617b3147d54c ("io_uring: restore back registered wait arguments")
>>> Fixes: aa00f67adc2c0 ("io_uring: add support for fixed wait regions")
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> ---
>>>   io_uring/io_uring.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>> index da8fd460977b..3a3e4fca1545 100644
>>> --- a/io_uring/io_uring.c
>>> +++ b/io_uring/io_uring.c
>>> @@ -3207,6 +3207,7 @@ static struct io_uring_reg_wait *io_get_ext_arg_reg(struct io_ring_ctx *ctx,
>>>                end > ctx->cq_wait_size))
>>>           return ERR_PTR(-EFAULT);
>>>   +    barrier_nospec();
>>>       return ctx->cq_wait_arg + offset;
>>
>> We need something better than that, barrier_nospec() is a big slow
>> hammer...
> 
> Right, more of a discussion opener. I wonder if Jann can help here
> (see the other reply). I don't like back and forth like that, but if
> nothing works there is an option of returning back to reg-wait array
> indexes. Trivial to change, but then we're committing to not expanding
> the structure or complicating things if we do.

Then I think it should've been marked as a discussion point, because we
definitely can't do this. Soliciting input is perfectly fine. And yeah,
was thinking the same thing, if this is an issue then we just go back to
indexing again. At least both the problem and solution is well known
there. The original aa00f67adc2c0 just needed an array_index_nospec()
and it would've been fine.

Not a huge deal in terms of timing, either way.

I suspect we can do something similar here, with just clamping the
indexing offset. But let's hear what Jann thinks.

-- 
Jens Axboe