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

[David Laight]

From: Arnd Bergmann
> Sent: 20 September 2020 21:49
> 
> On Sun, Sep 20, 2020 at 9:28 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > On Sun, Sep 20, 2020 at 12:23 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > On Sun, Sep 20, 2020 at 08:10:31PM +0100, Al Viro wrote:
> > > > IMO it's much saner to mark those and refuse to touch them from io_uring...
> > >
> > > Simpler solution is to remove io_uring from the 32-bit syscall list.
> > > If you're a 32-bit process, you don't get to use io_uring.  Would
> > > any real users actually care about that?
> >
> > We could go one step farther and declare that we're done adding *any*
> > new compat syscalls :)
> 
> Would you also stop adding system calls to native 32-bit systems then?
> 
> On memory constrained systems (less than 2GB a.t.m.), there is still a
> strong demand for running 32-bit user space, but all of the recent Arm
> cores (after Cortex-A55) dropped the ability to run 32-bit kernels, so
> that compat mode may eventually become the primary way to run
> Linux on cheap embedded systems.
> 
> I don't think there is any chance we can realistically take away io_uring
> from the 32-bit ABI any more now.

Can't it just run requests from 32bit apps in a kernel thread that has
the 'in_compat_syscall' flag set?
Not that i recall seeing the code where it saves the 'compat' nature
of any requests.

It is already completely f*cked if you try to pass the command ring
to a child process - it uses the wrong 'mm'.
I suspect there are some really horrid security holes in that area.

	David.

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