Messages in this thread | | | Date | Tue, 18 Dec 2012 14:10:56 -0800 | Subject | Re: [PATCH v2] firmware: make sure paths remain relative | From | Kees Cook <> |
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On Tue, Dec 18, 2012 at 2:02 PM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote: > On Tue, 18 Dec 2012 13:04:49 -0800 > Kees Cook <keescook@chromium.org> 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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