linux-kernel - Re: x86/ldt: Prevent ldt inheritance on exec

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Date:   Fri, 8 Dec 2017 22:49:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: x86/ldt: Prevent ldt inheritance on exec

On Fri, 8 Dec 2017, Linus Torvalds wrote:

> On Fri, Dec 8, 2017 at 12:02 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > The reason why this happens is that init_new_context_ldt() is called from
> > init_new_context() which obviously needs to be called for both fork() and
> > exec().
> >
> > It would be surprising if anything relies on that behaviour, so it seems to
> > be safe to remove that misfeature.
> 
> Looks sane. That said, can't we separate this out into the copy_mm()
> phase only?
> 
> We have "arch_dup_mmap()" that is called on fork() only, so that could
> do the LDT copy from the old mm, and the actual init_new_context would
> just zero it out.
> 
> Then there wouldn't be any odd "check if this is an execve" because
> the copying would be done in the right place.

Yes, that should work. It just needs to change arch_dup_mmap() so it can
return an error code.

Thanks,

	tglx