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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Eric Blake <eblake@redhat.com> writes:

> [ Unknown signature status ]
> 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 <peter.maydell@linaro.org>
>> Signed-off-by: Greg Kurz <groug@kaod.org>
>> ---
>> 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,

We resolve "/foo" on the client side. So this is ok.

-aneesh