On Tue, Aug 20, 2024 at 01:18:54AM +0200, Jann Horn wrote:
> Most firmware names are hardcoded strings, or are constructed from fairly
> constrained format strings where the dynamic parts are just some hex
> numbers or such.
>
> However, there are a couple codepaths in the kernel where firmware file
> names contain string components that are passed through from a device or
> semi-privileged userspace; the ones I could find (not counting interfaces
> that require root privileges) are:
>
> - lpfc_sli4_request_firmware_update() seems to construct the firmware
> filename from "ModelName", a string that was previously parsed out of
> some descriptor ("Vital Product Data") in lpfc_fill_vpd()
> - nfp_net_fw_find() seems to construct a firmware filename from a model
> name coming from nfp_hwinfo_lookup(pf->hwinfo, "nffw.partno"), which I
> think parses some descriptor that was read from the device.
> (But this case likely isn't exploitable because the format string looks
> like "netronome/nic_%s", and there shouldn't be any *folders* starting
> with "netronome/nic_". The previous case was different because there,
> the "%s" is *at the start* of the format string.)
> - module_flash_fw_schedule() is reachable from the
> ETHTOOL_MSG_MODULE_FW_FLASH_ACT netlink command, which is marked as
> GENL_UNS_ADMIN_PERM (meaning CAP_NET_ADMIN inside a user namespace is
> enough to pass the privilege check), and takes a userspace-provided
> firmware name.
> (But I think to reach this case, you need to have CAP_NET_ADMIN over a
> network namespace that a special kind of ethernet device is mapped into,
> so I think this is not a viable attack path in practice.)
>
> For what it's worth, I went looking and haven't found any USB device
> drivers that use the firmware loader dangerously.
Your commit message very well describes the status quo, but only implies the
problem, and skips how you intend to solve it.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: abb139e75c2c ("firmware: teach the kernel to load firmware files directly from the filesystem")
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> I wasn't sure whether to mark this one for stable or not - but I think
> since there seems to be at least one PCI device model which could
> trigger firmware loading with directory traversal, we should probably
> backport the fix?
> ---
> drivers/base/firmware_loader/main.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
> index a03ee4b11134..a32be64f3bf5 100644
> --- a/drivers/base/firmware_loader/main.c
> +++ b/drivers/base/firmware_loader/main.c
> @@ -864,7 +864,15 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
> if (!firmware_p)
> return -EINVAL;
>
> - if (!name || name[0] == '\0') {
> + /*
> + * Reject firmware file names with "/../" sequences in them.
> + * There are drivers that construct firmware file names from
> + * device-supplied strings, and we don't want some device to be able
> + * to tell us "I would like to be sent my firmware from
> + * ../../../etc/shadow, please".
> + */
> + if (!name || name[0] == '\0' ||
> + strstr(name, "/../") != NULL || strncmp(name, "../", 3) == 0) {
Seems reasonable, but are there any API users that rely on that?
I guess we can't just check for strstr(name, "../"), because "foo.." is a valid
file name? Maybe it would be worth adding a comment and / or a small
helper function for that.
I also suggest to update the documentation of the firmware loader API to let
people know that going back the path isn't tolerated by this API.
> ret = -EINVAL;
> goto out;
> }
>
> ---
> base-commit: b0da640826ba3b6506b4996a6b23a429235e6923
> change-id: 20240820-firmware-traversal-6df8501b0fe4
> --
> Jann Horn <jannh@xxxxxxxxxx>
>
