Re: [PATCH] sys: don't hold uts_sem while accessing userspace memory

[Ben Hutchings <ben@decadent.org.uk>]

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On Mon, 2018-06-25 at 20:16 +0200, Jann Horn wrote:
> On Mon, Jun 25, 2018 at 6:41 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > 
> > On Mon, Jun 25, 2018 at 06:34:10PM +0200, Jann Horn wrote:
> > 
> > > +     char tmp[32];
> > > 
> > > -     if (namelen > 32)
> > > +     if (namelen < 0 || namelen > 32)
> > >               namelen = 32;
> > > 
> > >       down_read(&uts_sem);
> > >       kname = utsname()->domainname;
> > >       len = strnlen(kname, namelen);
> > > -     if (copy_to_user(name, kname, min(len + 1, namelen)))
> > > -             err = -EFAULT;
> > > +     len = min(len + 1, namelen);
> > > +     memcpy(tmp, kname, len);
> > >       up_read(&uts_sem);
> > > 
> > > -     return err;
> > > +     if (copy_to_user(name, tmp, len))
> > > +             return -EFAULT;
> > 
> > Infoleak, and similar in a lot of other places.
> 
> I don't see a problem. copy_to_user() copies "len" bytes from "tmp".
[...]
> Can you please explain why there is an infoleak here?

I think you're *fixing* information leaks in the Alpha syscalls,
because a negative value of namelen used to result in a huge length
argument to copy_to_user().

Ben.

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.


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