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

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Date:   Wed, 27 Jun 2018 14:29:33 +0200
From:   Jann Horn <jannh@...gle.com>
To:     ben@...adent.org.uk, Al Viro <viro@...iv.linux.org.uk>
Cc:     rth@...ddle.net, ink@...assic.park.msu.ru, mattst88@...il.com,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        ebiederm@...ssion.com, linux-alpha@...r.kernel.org,
        sparclinux@...r.kernel.org, security@...nel.org, hch@...radead.org,
        serge@...lyn.com
Subject: Re: [PATCH] sys: don't hold uts_sem while accessing userspace memory

On Tue, Jun 26, 2018 at 7:43 PM Ben Hutchings <ben@...adent.org.uk> wrote:
>
> On Mon, 2018-06-25 at 20:16 +0200, Jann Horn wrote:
> > On Mon, Jun 25, 2018 at 6:41 PM Al Viro <viro@...iv.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().

Ah, you're right. Looks like this was previously fixed in commit
21c5977a836e ("alpha: fix several security issues", first in v3.0),
and then un-fixed in commit 9ba3eb5103cf ("osf_getdomainname(): use
copy_to_user()", first in v4.13).