[PATCH] firmware: improve LSM/IMA security behaviour

[PATCH] firmware: improve LSM/IMA security behaviour

[Sven Van Asbroeck]

The firmware loader queries if LSM/IMA permits it to load firmware
via the sysfs fallback. Unfortunately, the code does the opposite:
it expressly permits sysfs fw loading if security_kernel_load_data(
LOADING_FIRMWARE) returns -EACCES. This happens because a
zero-on-success return value is cast to a bool that's true on success.

Fix the return value handling so we get the correct behaviour.

Fixes: 6e852651f28e ("firmware: add call to LSM hook before firmware sysfs fallback")
Cc: Stable <stable@vger.kernel.org>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Kees Cook <keescook@chromium.org>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
---
 drivers/base/firmware_loader/fallback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
index f962488546b6..103b5d37fa86 100644
--- a/drivers/base/firmware_loader/fallback.c
+++ b/drivers/base/firmware_loader/fallback.c
@@ -659,7 +659,7 @@ static bool fw_run_sysfs_fallback(enum fw_opt opt_flags)
 	/* Also permit LSMs and IMA to fail firmware sysfs fallback */
 	ret = security_kernel_load_data(LOADING_FIRMWARE);
 	if (ret < 0)
-		return ret;
+		return false;
 
 	return fw_force_sysfs_fallback(opt_flags);
 }
-- 
2.17.1

Re: [PATCH] firmware: improve LSM/IMA security behaviour

[Mimi Zohar]

On Mon, 2019-06-17 at 14:23 -0400, Sven Van Asbroeck wrote:
> The firmware loader queries if LSM/IMA permits it to load firmware
> via the sysfs fallback. Unfortunately, the code does the opposite:
> it expressly permits sysfs fw loading if security_kernel_load_data(
> LOADING_FIRMWARE) returns -EACCES. This happens because a
> zero-on-success return value is cast to a bool that's true on success.
> 
> Fix the return value handling so we get the correct behaviour.

Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>