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Re: [Qemu-devel] [PATCH 07/29] 9pfs: local: introduce symlink-attack saf

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 07/29] 9pfs: local: introduce symlink-attack safe xattr helpers
Date: Thu, 23 Feb 2017 09:02:39 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 02/20/2017 08:40 AM, Greg Kurz wrote:
> All operations dealing with extended attributes are vulnerable to symlink
> attacks because they use path-based syscalls which can traverse symbolic
> links while walking through the dirname part of the path.
> The solution is to introduce helpers based on opendir_nofollow(). This
> calls for "at" versions of the extended attribute syscalls, which don't
> exist unfortunately. This patch implement them by simulating the "at"
> behavior with fchdir(). Since the current working directory is process
> wide, and we don't want to confuse another thread in QEMU, all the work
> is done in a separate process.

Can you emulate *at using /proc/fd/nnn/xyz?  Coreutils was one of the
early adopters of the power of *at functions, and found that emulation
of *at via procfs was a LOT more efficient than emulation via fchdir
(although both emulations still exist in gnulib, since procfs is not

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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