Re: [PATCH] MIPS: Fix strnlen_user access check

[Thomas Bogendoerfer]

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.

> 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.

> ISTM that it is very unlikely that the length passed to strnlen_user()
> is long enough to take potential buffer beyond the end of user
> address space.

see above.

Thomas.

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