Re: [Patch] sysctl: forbid too long numbers

[Amerigo Wang]

Andrew Morton wrote:
> Also, fixing this is a non-backward-compatible change which could break
> existing userspace.  Although the chances of this seem fairly small.
>
> Or are they?  One could imagine a script which was tested and developed
> on a 64-bit system, which writes a >4G number into a pseudo file.  That
> script happens to work on 32-bit systems (it might not work _well_, but
> it'll work).  With this change, the write will fail on the 32-bit
> system and the entire application could bale out or something.
>
> I'm not saying that this is a reason to avoid making the change, but
> it's all a worry and needs consideration.
>
>   

Ah, I didn't consider this situation...
Hmm... but only taking the lower 32-bits really looks strange.

> The other worrisome thing about this change is that there may well be
> existing userspace which does
>
> 	echo 42foo > /proc/whatever
>
> and the conversion to strict_strtoul() will cause that script to
> newly fail.
>
> And the chances that there are scripts which do this are pretty darned
> good - it's fairly easy to accidentally leave junk like this in strings
> when hacking stuff together in scripting languages.
>
>   

Yeah, maybe, but that is really tricky...
>   
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index 756ccaf..ff2ca5c 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -163,11 +163,14 @@ int strict_strtoul(const char *cp, unsigned int base, unsigned long *res)
> >  	char *tail;
> >  	unsigned long val;
> >  	size_t len;
> > +	char tmp[32];
> >  
> >  	*res = 0;
> >  	len = strlen(cp);
> >  	if (len == 0)
> >  		return -EINVAL;
> > +	if (len > snprintf(tmp, "%ld", ULONG_MAX))
> > +		return -EINVAL;
> >  
> >  	val = simple_strtoul(cp, &tail, base);
> >  	if (tail == cp)
> >     
>
> And here we're doing a check for that overflow, yes?
>
> - We don't need tmp[].  vsnprintf(NULL, ...) can be used to query the
>   length of an sprintf.  See how kvasprintf() does this.
>
> - The strict_strtoul() documentation should be updated?
>
> - The above change affects strict_strtol() too.
>
> - The same change should be made to strict_strtoull() and hence
>   strict_strtoll()?
>
>   
Good points!
I agree, so maybe we only need to change this part?
Hmm, I need to check the callers of strict_strtol()...

Thank you!


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/