Re: [PATCH v3 0/7] File Sealing & memfd_create()

[Florian Weimer]

On 06/13/2014 05:33 PM, David Herrmann wrote:
> On Fri, Jun 13, 2014 at 5:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > Isn't the point of SEAL_SHRINK to allow servers to mmap and read
> > safely without worrying about SIGBUS?
>
> No, I don't think so.
> The point of SEAL_SHRINK is to prevent a file from shrinking. SIGBUS
> is an effect, not a cause. It's only a coincidence that "OOM during
> reads" and "reading beyond file-boundaries" has the same effect:
> SIGBUS.
> We only protect against reading beyond file-boundaries due to
> shrinking. Therefore, OOM-SIGBUS is unrelated to SEAL_SHRINK.
>
> Anyone dealing with mmap() _has_ to use mlock() to protect against
> OOM-SIGBUS. Making SEAL_SHRINK protect against OOM-SIGBUS would be
> redundant, because you can achieve the same with SEAL_SHRINK+mlock().

I don't think this is what potential users expect because mlock requires 
capabilities which are not available to them.

A couple of weeks ago, sealing was to be applied to anonymous shared 
memory.  Has this changed?  Why should *reading* it trigger OOM?

--
Florian Weimer / Red Hat Product Security Team
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