linux-kernel - Re: [PATCH] MIPS: Fix strnlen

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Date:   Mon, 12 Apr 2021 16:27:30 +0200
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Jinyang He <hejinyang@...ngson.cn>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: Fix strnlen_user access check

On Mon, Apr 12, 2021 at 11:02:19AM +0800, 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@...ngson.cn>
> > ---
> >   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__(

that's the fix I'd like to apply. Could someone send it as a formal
patch ? Thanks.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]