Re: [Qemu-devel] [PATCH v4 1/3] 9pfs: forbid illegal path names

[Eric Blake]

On 08/30/2016 12:11 PM, Greg Kurz wrote:
> Empty path components don't make sense for most commands and may cause
> undefined behavior, depending on the backend.
> 
> Also, the walk request described in the 9P spec [1] clearly shows that
> the client is supposed to send individual path components: the official
> linux client never sends portions of path containing the / character for
> example.
> 
> Moreover, the 9P spec [2] also states that a system can decide to restrict
> the set of supported characters used in path components, with an explicit
> mention "to remove slashes from name components".
> 
> This patch introduces a new name_is_illegal() helper that checks the
> names sent by the client are not empty and don't contain unwanted chars.
> Since 9pfs is only supported on linux hosts, only the / character is
> checked at the moment. When support for other hosts (AKA. win32) is added,
> other chars may need to be blacklisted as well.
> 
> If a client sends an illegal path component, the request will fail and
> ENOENT is returned to the client.
> 
> [1] http://man.cat-v.org/plan_9/5/walk
> [2] http://man.cat-v.org/plan_9/5/intro
> 
> Suggested-by: Peter Maydell <address@hidden>
> Signed-off-by: Greg Kurz <address@hidden>
> ---
> v4: dropped the checking of the symbolic link target name: because a target
>     can be a full path and thus contain '/' and linux already complains if
>     it is an empty string. When the symlink gets dereferenced, slashes are
>     interpreted as the usual path component separator.

Can a symlink to "/foo" be used to escape the root (by being absolute
instead of relative)?  However, if the answer to that question requires
more code, I'm fine with it being a separate patch.  So for this email,

Reviewed-by: Eric Blake <address@hidden>

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

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