[PATCH v3 3/4] ima: fail signature verification based on policy

[Serge E. Hallyn]

Quoting Mimi Zohar (zohar at linux.vnet.ibm.com):
> This patch addresses the fuse privileged mounted filesystems in
> environments which are unwilling to accept the risk of trusting the
> signature verification and want to always fail safe, but are for example
> using a pre-built kernel.
> 
> This patch defines a new builtin policy named "fail_securely", which can
> be specified on the boot command line as an argument to "ima_policy=".
> 
> Signed-off-by: Mimi Zohar <zohar at linux.vnet.ibm.com>
> Cc: Miklos Szeredi <miklos at szeredi.hu>
> Cc: Seth Forshee <seth.forshee at canonical.com>
> Cc: Eric W. Biederman <ebiederm at xmission.com>
> Cc: Dongsu Park <dongsu at kinvolk.io>
> Cc: Alban Crequy <alban at kinvolk.io>
> Cc: Serge E. Hallyn <serge at hallyn.com>

Acked-by: Serge Hallyn <serge at hallyn.com>

but,

> 
> ---
> Changelog v3:
> - Rename the builtin policy name
> 
> Changelog v2:
> - address the fail safe environement
> 
>  Documentation/admin-guide/kernel-parameters.txt |  8 +++++++-
>  security/integrity/ima/ima_appraise.c           | 11 ++++++-----
>  security/integrity/ima/ima_main.c               |  3 ++-
>  security/integrity/ima/ima_policy.c             |  5 +++++
>  security/integrity/integrity.h                  |  1 +
>  5 files changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1d1d53f85ddd..2cc17dc7ab84 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1525,7 +1525,8 @@
>  
>  	ima_policy=	[IMA]
>  			The builtin policies to load during IMA setup.
> -			Format: "tcb | appraise_tcb | secure_boot"
> +			Format: "tcb | appraise_tcb | secure_boot |
> +				 fail_securely"
>  
>  			The "tcb" policy measures all programs exec'd, files
>  			mmap'd for exec, and all files opened with the read
> @@ -1540,6 +1541,11 @@
>  			of files (eg. kexec kernel image, kernel modules,
>  			firmware, policy, etc) based on file signatures.
>  
> +			The "fail_securely" policy forces file signature
> +			verification failure also on privileged mounted
> +			filesystems with the SB_I_UNVERIFIABLE_SIGNATURE
> +			flag.
> +
>  	ima_tcb		[IMA] Deprecated.  Use ima_policy= instead.
>  			Load a policy which meets the needs of the Trusted
>  			Computing Base.  This means IMA will measure all
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 4bafb397ee91..3034935e1eb3 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -304,12 +304,13 @@ int ima_appraise_measurement(enum ima_hooks func,
>  out:
>  	/*
>  	 * File signatures on some filesystems can not be properly verified.
> -	 * On these filesytems, that are mounted by an untrusted mounter,
> -	 * fail the file signature verification.
> +	 * On these filesytems, that are mounted by an untrusted mounter or

How about "When such filesystems are mounted by an untrusted mounter or
on a system not willing to accept such a risk, ..." ?

(also filesytems is misspelled :)

> +	 * for systems not willing to accept the risk, fail the file signature
> +	 * verification.
>  	 */
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