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

On 22/09/2020 02:51, Andy Lutomirski wrote:

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On Mon, Sep 21, 2020 at 9:15 AM Pavel Begunkov [off-list ref] wrote:

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On 21/09/2020 19:10, Pavel Begunkov wrote:

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On 20/09/2020 01:22, Andy Lutomirski wrote:

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On Sep 19, 2020, at 2:16 PM, Arnd Bergmann [off-list ref] wrote:

On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski [off-list ref] wrote:

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On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig [off-list ref] wrote:
On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:

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Said that, why not provide a variant that would take an explicit
"is it compat" argument and use it there?  And have the normal
one pass in_compat_syscall() to that...

That would help to not introduce a regression with this series yes.
But it wouldn't fix existing bugs when io_uring is used to access
read or write methods that use in_compat_syscall().  One example that
I recently ran into is drivers/scsi/sg.c.

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.

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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;
+

        /*
         * 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;
+