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From: Kees Cook <>
To: Linus Torvalds <>
Cc: Rob Landley <>,
	Linux Kernel Mailing List <>,,
Subject: Re: Regression: commit da029c11e6b1 broke toybox xargs.
Date: Fri, 3 Nov 2017 18:37:27 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Nov 3, 2017 at 6:22 PM, Linus Torvalds
<> wrote:
> On Fri, Nov 3, 2017 at 5:42 PM, Kees Cook <> wrote:
>> If we didn't do the "but no more than 75% of _STK_LIM", and moved to
>> something like "check stack utilization after loading the binary", we
>> end up in the position where the kernel is past the point of no return
>> (so instead of E2BIG, the execve()ing process just SEGVs), which is
>> much harder to debug or recover from (i.e. there's no process left to
>> return from the execve() from).
> Yeah, we've had that problem in the past, and it's the worst of all worlds.
> You can still trigger it (set RLIMIT_DATA to something much too small,
> for example, and then generate more than that by just repeating the
> same argument multiple times so that the execve() user doesn't trigger
> the limit, but the newly executed process does).
> But it should really be something that you need to be truly insane to trigger.
> I think we still don't know whether we're going to be suid at the time
> we copy the arguments, do we?

We don't. (In fact, arg copying happens before we've even figured out
which binfmt is involved.) I lifted it to just before the point of no
return, but moving it before arg copying looks very hard (which
contributed to why we went with the implementation we did).

> So it's pretty painful to make the limits different for suid and
> non-suid binaries.

I would agree.


Kees Cook
Pixel Security

  reply	other threads:[~2017-11-04  1:37 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-01 23:34 Regression: commit da029c11e6b1 broke toybox xargs Rob Landley
2017-11-02  3:30 ` Kees Cook
     [not found] ` <>
2017-11-02 15:40   ` Linus Torvalds
2017-11-03 23:58     ` Rob Landley
2017-11-04  0:03       ` [Toybox] " enh
2017-11-04  0:42       ` Kees Cook
2017-11-04  1:22         ` Linus Torvalds
2017-11-04  1:37           ` Kees Cook [this message]
2017-11-05  1:10             ` Rob Landley
2017-11-04  1:07       ` Linus Torvalds
2017-11-05  0:39         ` Rob Landley
2017-11-05 20:46           ` Linus Torvalds
2017-11-15 22:10             ` enh
2017-11-15 22:45               ` Linus Torvalds
2017-11-15 21:12           ` Pavel Machek

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