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.)
Fix it by rejecting any firmware names containing ".." path components.
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>
---
Changes in v2:
- describe fix in commit message (dakr)
- write check more clearly and with comment in separate helper (dakr)
- document new restriction in comment above request_firmware() (dakr)
- warn when new restriction is triggered
- Link to v1: https://lore.kernel.org/r/20240820-firmware-traversal-v1-1-8699ffaa9276@xxxxxxxxxx
---
drivers/base/firmware_loader/main.c | 41 +++++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)
diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
index a03ee4b11134..dd47ce9a761f 100644
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -849,6 +849,37 @@ static void fw_log_firmware_info(const struct firmware *fw, const char *name,
{}
#endif
+/*
+ * Reject firmware file names with ".." path components.
+ * 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".
+ *
+ * Search for ".." surrounded by either '/' or start/end of string.
+ *
+ * This intentionally only looks at the firmware name, not at the firmware base
+ * directory or at symlink contents.
+ */
+static bool name_contains_dotdot(const char *name)
+{
+ size_t name_len = strlen(name);
+ size_t i;
+
+ if (name_len < 2)
+ return false;
+ for (i = 0; i < name_len - 1; i++) {
+ /* do we see a ".." sequence? */
+ if (name[i] != '.' || name[i+1] != '.')
+ continue;
+
+ /* is it a path component? */
+ if ((i == 0 || name[i-1] == '/') &&
+ (i == name_len - 2 || name[i+2] == '/'))
+ return true;
+ }
+ return false;
+}
+
/* called from request_firmware() and request_firmware_work_func() */
static int
_request_firmware(const struct firmware **firmware_p, const char *name,
@@ -869,6 +900,14 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
goto out;
}
+ if (name_contains_dotdot(name)) {
+ dev_warn(device,
+ "Firmware load for '%s' refused, path contains '..' component",
+ name);
+ ret = -EINVAL;
+ goto out;
+ }
+
ret = _request_firmware_prepare(&fw, name, device, buf, size,
offset, opt_flags);
if (ret <= 0) /* error or already assigned */
@@ -946,6 +985,8 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
* @name will be used as $FIRMWARE in the uevent environment and
* should be distinctive enough not to be confused with any other
* firmware image for this or any other device.
+ * It must not contain any ".." path components - "foo/bar..bin" is
+ * allowed, but "foo/../bar.bin" is not.
*
* Caller must hold the reference count of @device.
*
---
base-commit: b0da640826ba3b6506b4996a6b23a429235e6923
change-id: 20240820-firmware-traversal-6df8501b0fe4
--
Jann Horn <jannh@xxxxxxxxxx>
