Re: [PATCH v2] firmware: make sure paths remain relative

[Kees Cook]

On Tue, Dec 18, 2012 at 2:02 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 18 Dec 2012 13:04:49 -0800
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> Some devices have configurable firmware locations. If these configuration
>> mechanisms are exposed to unprivileged userspace, it may be possible to
>> load firmware from an unexpected location. To minimize the risk of this,
>> make sure the string "../" does not appear in the firmware name. This
>> means that neither the users of request_firmware, nor the uevent handler
>> have to do this filtering themselves.
>
> Have you verified no existing distribution ever does this ?

As far as I've found, the only non-static firmware loader I've found
is on Chrome OS.

>> +     if (strstr(name, "../")) {
>
> and this is obviously buggy and hasn't been tested
>
> Hint foo/bar../baz
>
> You need to check for ^../ /../ and /..$

The distros that handle the firmware uevent all prepend the string
"/lib/firmware/".

/lib/firmware/ + "../anything" will leave the directory.
/lib/firmware/ + "anything/../anything" can potentially leave the directory.
/lib/firmware/ + "anything/.." can potentially leave the directory,
but does not refer to a file.

Only "../" needs to be filtered. Given what a tiny corner-case this is
(non-root-control over the firmware filename path), I feel it is
sufficient.

The alternative is for the uevent handler to do this filtering, but it
seemed more sensible to have the kernel not feed it a crazy string in
the first place.

-Kees

--
Kees Cook
Chrome OS Security
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