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Date:   Fri, 21 Feb 2020 15:10:48 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Jann Horn <jannh@google.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH] mm/tlb: Fix use_mm() vs TLB invalidate
Message-ID: <202002211506.2151CA26@keescook>
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On Fri, Feb 21, 2020 at 11:22:16AM -0800, Andy Lutomirski wrote:
> 
> 
> > On Feb 21, 2020, at 11:19 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > 
> > ﻿On Fri, Feb 21, 2020 at 3:11 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >> 
> >> +       BUG_ON(!(tsk->flags & PF_KTHREAD));
> >> +       BUG_ON(tsk->mm != NULL);
> > 
> > Stop this craziness.
> > 
> > There is absolutely ZERO excuse for this kind of garbage.
> > 
> > Making this a BUG_ON() will just cause all the possible debugging info
> > to be thrown away and lost, and you often have a dead machine.
> > 
> > For absolutely no good reason.
> > 
> > Make it a WARN_ON_ONCE(). If it triggers, everything works the way it
> > always did, but we get notified.
> > 
> > Stop with the stupid crazy BUG_ON() crap already. It is actively _bad_
> > for debugging.
> > 
> >  
> 
> In this particular case, if we actually flub this, we are very likely to cause data corruption — we’re about to do user access with the wrong mm.
> 
> So I suppose we could switch to init_mm and carry on. *Something* will crash, but it probably won’t corrupt data or take down the machine.

Why not just fail after the WARN -- I wrote the patch for the (very few)
callers to handle the errors, clean up, and carry on.

-- 
Kees Cook