Re: [PATCH] MIPS: Fix strnlen_user access check

[Thomas Bogendoerfer]

On Tue, Apr 13, 2021 at 04:01:13PM +0000, David Laight wrote:
> 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.

strnlen_user just did the equivalent of access_ok(s, 0) and I copy&pasted
the wrong access_ok() statement :-( 

> Or was there another change that affected strndup_user() ?

no, just the change in strnlen_user.

Thomas.

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