RE: [PATCH] MIPS: Fix strnlen_user access check

[David Laight]

From: Thomas Bogendoerfer
> Sent: 13 April 2021 16:19
> 
> On Tue, Apr 13, 2021 at 12:37:25PM +0000, David Laight wrote:
> > From: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
> > > Sent: 13 April 2021 12:15
> > ...
> > > > The __access_ok() is noted with `Ensure that the range [addr, addr+size)
> > > > is within the process's address space`. Does the range checked by
> > > > __access_ok() on MIPS is [addr, addr+size]. So if we want to use
> > > > access_ok(s, 1), should we modify __access_ok()? Or my misunderstanding?
> > >
> > > you are right, I'm going to apply
> > >
> > > https://patchwork.kernel.org/project/linux-mips/patch/20190209194718.1294-1-paul.burton@xxxxxxxx/
> > >
> > > to fix that.
> >
> > Isn't that still wrong?
> > If an application does:
> > 	write(fd, (void *)0xffff0000, 0);
> > it should return 0, not -1 and EFAULT/SIGSEGV.
> 
> WRITE(2)                   Linux Programmer's Manual                  WRITE(2)
> [...]
>        If  count  is  zero  and  fd refers to a regular file, then write() may
>        return a failure status if one of the errors below is detected.  If  no
>        errors  are  detected,  or  error detection is not performed, 0 will be
>        returned without causing any other effect.  If count  is  zero  and  fd
>        refers  to a file other than a regular file, the results are not speci-
>        fied.
> [...]
>        EFAULT buf is outside your accessible address space.
> 
> at least it's covered by the man page on my Linux system.

Something related definitely caused grief in the setsockopt() changes.

> > There is also the question about why this makes any difference
> > to the original problem of logging in via the graphical interface.
> 
> kernel/module.c:        mod->args = strndup_user(uargs, ~0UL >> 1);
> 
> and strndup_user does a strnlen_user.

That call is just gross.
Why did it work before the removal of set_fs() etc.

Or was there another change that affected strndup_user() ?

	David

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