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Subject: Re: [PATCH v7 0/7] Add support for O_MAYEXEC
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From:   Deven Bowers <deven.desai@linux.microsoft.com>
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Date:   Tue, 11 Aug 2020 10:18:08 -0700
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On 8/11/2020 1:48 AM, Mickaël Salaün wrote:

[...snip]

>>> It is a
>>> good practice to check as soon as possible such properties, and it may
>>> enables to avoid (user space) time-of-check to time-of-use (TOCTOU)
>>> attacks (i.e. misuse of already open resources).
>>
>> The assumption that security checks should happen as early as possible
>> can actually cause security problems. For example, because seccomp was
>> designed to do its checks as early as possible, including before
>> ptrace, we had an issue for a long time where the ptrace API could be
>> abused to bypass seccomp filters.
>>
>> Please don't decide that a check must be ordered first _just_ because
>> it is a security check. While that can be good for limiting attack
>> surface, it can also create issues when the idea is applied too
>> broadly.
> 
> I'd be interested with such security issue examples.
> 
> I hope that delaying checks will not be an issue for mechanisms such as
> IMA or IPE:
> https://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.com/
> 
> Any though Mimi, Deven, Chrome OS folks?
> 

I don't see an issue with IPE. As long as the hypothetical new syscall
and associated security hook have the file struct available in the
hook, it should integrate fairly easily.

[...snip]