[PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

Pavel Begunkov asml.silence at gmail.com
Tue Sep 22 02:30:09 EDT 2020

On 22/09/2020 03:58, Andy Lutomirski wrote:
> On Mon, Sep 21, 2020 at 5:24 PM Pavel Begunkov <asml.silence at gmail.com> wrote:
>>>>>>> Ah, so reading /dev/input/event* would suffer from the same issue,
>>>>>>> and that one would in fact be broken by your patch in the hypothetical
>>>>>>> case that someone tried to use io_uring to read /dev/input/event on x32...
>>>>>>> For reference, I checked the socket timestamp handling that has a
>>>>>>> number of corner cases with time32/time64 formats in compat mode,
>>>>>>> but none of those appear to be affected by the problem.
>>>>>>>> Aside from the potentially nasty use of per-task variables, one thing
>>>>>>>> I don't like about PF_FORCE_COMPAT is that it's one-way.  If we're
>>>>>>>> going to have a generic mechanism for this, shouldn't we allow a full
>>>>>>>> override of the syscall arch instead of just allowing forcing compat
>>>>>>>> so that a compat syscall can do a non-compat operation?
>>>>>>> The only reason it's needed here is that the caller is in a kernel
>>>>>>> thread rather than a system call. Are there any possible scenarios
>>>>>>> where one would actually need the opposite?
>>>>>> I can certainly imagine needing to force x32 mode from a kernel thread.
>>>>>> As for the other direction: what exactly are the desired bitness/arch semantics of io_uring?  Is the operation bitness chosen by the io_uring creation or by the io_uring_enter() bitness?
>>>>> It's rather the second one. Even though AFAIR it wasn't discussed
>>>>> specifically, that how it works now (_partially_).
>>>> Double checked -- I'm wrong, that's the former one. Most of it is based
>>>> on a flag that was set an creation.
>>> Could we get away with making io_uring_enter() return -EINVAL (or
>>> maybe -ENOTTY?) if you try to do it with bitness that doesn't match
>>> the io_uring?  And disable SQPOLL in compat mode?
>> Something like below. If PF_FORCE_COMPAT or any other solution
>> doesn't lend by the time, I'll take a look whether other io_uring's
>> syscalls need similar checks, etc.
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 0458f02d4ca8..aab20785fa9a 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -8671,6 +8671,10 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
>>         if (ctx->flags & IORING_SETUP_R_DISABLED)
>>                 goto out;
>> +       ret = -EINVAl;
>> +       if (ctx->compat != in_compat_syscall())
>> +               goto out;
>> +
> This seems entirely reasonable to me.  Sharing an io_uring ring
> between programs with different ABIs seems a bit nutty.
>>         /*
>>          * For SQ polling, the thread will do all submissions and completions.
>>          * Just return the requested submit count, and wake the thread if
>> @@ -9006,6 +9010,10 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
>>         if (ret)
>>                 goto err;
>> +       ret = -EINVAL;
>> +       if (ctx->compat)
>> +               goto err;
>> +
> I may be looking at a different kernel than you, but aren't you
> preventing creating an io_uring regardless of whether SQPOLL is
> requested?

I diffed a not-saved file on a sleepy head, thanks for noticing.
As you said, there should be an SQPOLL check.

if (ctx->compat && (p->flags & IORING_SETUP_SQPOLL))
	goto err;

Pavel Begunkov

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