Re: [PATCH] MIPS: Fix strnlen_user access check

[Jinyang He]

On 04/12/2021 11:02 AM, Tiezhu Yang wrote:

> On 04/11/2021 07:04 PM, Jinyang He wrote:
> > Commit 04324f44cb69 ("MIPS: Remove get_fs/set_fs") brought a problem for
> > strnlen_user(). Jump out when checking access_ok() with condition that
> > (s + strlen(s)) < __UA_LIMIT <= (s + n). The old __strnlen_user_asm()
> > just checked (ua_limit & s) without checking (ua_limit & (s + n)).
> > Therefore, find strlen form s to __UA_LIMIT - 1 in that condition.
> >
> > Signed-off-by: Jinyang He <hejinyang@xxxxxxxxxxx>
> > ---
> >   arch/mips/include/asm/uaccess.h | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/mips/include/asm/uaccess.h 
> > b/arch/mips/include/asm/uaccess.h
> > index 91bc7fb..85ba0c8 100644
> > --- a/arch/mips/include/asm/uaccess.h
> > +++ b/arch/mips/include/asm/uaccess.h
> > @@ -630,8 +630,15 @@ static inline long strnlen_user(const char 
> > __user *s, long n)
> >   {
> >       long res;
> >   -    if (!access_ok(s, n))
> > -        return -0;
> > +    if (unlikely(n <= 0))
> > +        return 0;
> > +
> > +    if (!access_ok(s, n)) {
> > +        if (!access_ok(s, 0))
> > +            return 0;
> > +
> > +        n = __UA_LIMIT - (unsigned long)s - 1;
> > +    }
> >         might_fault();
> >       __asm__ __volatile__(
>
> The following simple changes are OK to fix this issue?
>
> diff --git a/arch/mips/include/asm/uaccess.h 
> b/arch/mips/include/asm/uaccess.h
> index 91bc7fb..eafc99b 100644
> --- a/arch/mips/include/asm/uaccess.h
> +++ b/arch/mips/include/asm/uaccess.h
> @@ -630,8 +630,8 @@ static inline long strnlen_user(const char __user 
> *s, long n)
>  {
>         long res;
>
> -       if (!access_ok(s, n))
> -               return -0;
> +       if (!access_ok(s, 1))
> +               return 0;
>
>         might_fault();
>         __asm__ __volatile__(
>
> Thanks,
> Tiezhu
Thanks for your comment. That looks similar to other archs, but I don't
know how the access_ok() implementation in other archs.

Using access_ok(s, 0) is similar to the old strnlen_user(). Using
access_ok(s, 1) may have a problem in this extreme case,
s = __UA_LIMIT - 1, *s = 0, and we hope it returns 1. But it returns 0 by
!access_ok(s, 1). Of course, it is so extrme.

More importantly, I want to set up a maximum for strnlen_user_asm. And do
not access the part of beyond __ua_limit. As follow shows,

                    +-----------+
                    |    ...    |
                    +-----------+  <---- s + n
                    |     0     |
                    +-----------+
                    |     s     |
                    +-----------+
                    |     r     |
                    +-----------+
                    |     e     |
                    +-----------+
                    |     h     |
                    +-----------+
                    |     t     |
                    +-----------+
                    |     o     |
                    +-----------+  <---- __UA_LIMIT
                    |     r     |
                    +-----------+
                    |     t     |
                    +-----------+
                    |     s     |
                    +-----------+  <---- s
                    |    ...    |
                    +-----------+

It is dangerous to access "others", for user, only "str" is safe.

I don't know whether it would be happend, I just limited it by change `n`.
Should do other things if meet __UA_LIMIT - 1?

Thanks,
Jinyang