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Date: Tue, 12 Aug 2014 16:54:14 -0700
From: Andy Lutomirski <luto@...capital.net>
To: oss-security@...ts.openwall.com
Subject: Re: CVE Request: ro bind mount bypass using user namespaces

On 08/12/2014 02:48 PM, Kenton Varda wrote:
> Due to a bug in the Linux kernel's implementation of remount, on systems
> with unprivileged user namespaces enabled, it is possible for an
> unprivileged user to gain write access to any visible read-only bind mount.
> It is also possible to bypass flags like nodev, nosuid, and noexec.
> 
> This problem affects sandboxing / containerization systems that do not
> expose the regular filesystem to the sandboxed process, but do expose a
> bind-mounted view of that filesystem using these flags to enforce security.
> This bug may enable a sandbox break-out. Sandboxes which have used
> seccomp-bpf to disable the "mount" system call or to disable user
> namespaces are likely safe.

nosuid/nodev failures are probably exploitable for full root in many
common configurations.

--Andy

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