[PATCH] firmware_loader: Block path traversal

[Jann Horn]

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.

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) {
 		ret = -EINVAL;
 		goto out;
 	}

---
base-commit: b0da640826ba3b6506b4996a6b23a429235e6923
change-id: 20240820-firmware-traversal-6df8501b0fe4
-- 
Jann Horn <jannh@xxxxxxxxxx>