[RFC PATCH v2 06/11] kexec: replace call to copy_file_from_fd() with kernel version

[Mimi Zohar]

On Mon, 2016-01-25 at 14:37 +0800, Dave Young wrote:
> Hi, Mimi
> 
> Besides of code issues, I have several thing to be understand:
> 
> What is the effect to kexec behavior with this patchset?
>   - without IMA enabled (kconfig or kernel cmdline) it will be same as before?

Yes, without IMA configured or an IMA policy, it is the same as before.

>   - with IMA enabled for kernel bzImage, kexec_file_load will check both ima
>     signature and original pe file signature, those two mechanisms are
>     somehow duplicated. I'm not sure if we need both for bzImage.

IMA provides a uniform method of measuring and appraising all files on
the system, based on policy.  The IMA policy could prevent the original
kexec syscall.  On systems without MODULE_SIG_FORCE, the IMA policy
would require an IMA signature as well.  (The current patch would
require both, even when MODULE_SIG_FORCE is enabled.)

The pe format is supported on x86.  Why require the pe file signature
format on all platforms?

> Do you have a simple usage documentation about how to test it?

The wiki[1] and ima-evm-ctl package[2] have directions for enabling
IMA/IMA-appraisal.

To include just the kexec image and initramfs file hashes in the IMA
measurement list, create a file containing the following IMA policy
rules.  "cat" the policy and redirect the output
to /sys/kernel/security/ima/policy.   After loading the kexec image and
initramfs, the IMA measurements will be included in the measurement list
(/sys/kernel/security/ima/ascii_runtime_measurements)

IMA policy: 
measure func=KEXEC_CHECK
measure func=INITRAMFS_CHECK

Appraising the kexec image and initramfs is a bit more complicated as it
requires creating a key, which is signed by a key on the system keyring,
and loading the key onto the trusted IMA keyring.  To simplify testing,
without CONFIG_IMA_TRUSTED_KEYRING enabled, the key being loaded onto
the IMA keyring does not need to be signed.  The evmctl man page[2]
contains directions for creating and loading the key onto the IMA
keyring. 

To appraise just the kexec image and initramfs files, add the following
two rules to the IMA policy and load the policy as before.  (The policy
can only be loaded once per boot, unless IMA_WRITE_POLICY is configured.
With the default appraisal policy, the policy would need to signed.)
Sign the kexec image and initramfs with evmctl before loading them.

# evmctl ima_sign -k <private key> -a sha256 <VM image>
# evmctl ima_sign -k <private key> -a sha256 <initramfs>

IMA appraise policy:
appraise func=KEXEC_CHECK appraise_type=imasig
appraise func=INITRAMFS_CHECK appraise_type=imasig

[1] http://sourceforge.net/p/linux-ima/wiki/Home
[2] http://linux-ima.sourceforge.net/evmctl.1.html

> > +{
> > +	struct fd f = fdget(fd);
> > +	int ret = -ENOEXEC;
> 
> -EBADF looks better?

Sure.

Mimi