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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