Re: [PATCH v3 1/4] ima: fail file signature verification on non-init mounted filesystems

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Mimi Zohar (zohar@linux.vnet.ibm.com):
> FUSE can be mounted by unprivileged users either today with fusermount
> installed with setuid, or soon with the upcoming patches to allow FUSE
> mounts in a non-init user namespace.
> 
> This patch addresses the new unprivileged non-init mounted filesystems,
> which are untrusted, by failing the signature verification.
> 
> This patch defines two new flags SB_I_IMA_UNVERIFIABLE_SIGNATURE and
> SB_I_UNTRUSTED_MOUNTER.
> 
> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Seth Forshee <seth.forshee@canonical.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Dongsu Park <dongsu@kinvolk.io>
> Cc: Alban Crequy <alban@kinvolk.io>
> Cc: Serge E. Hallyn <serge@hallyn.com>

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

One comment below though,

> 
> ---
> Changelog v3:
> - Fix SB_IMA_UNVERIFIABLE_SIGNATURE & SB_I_UNTRUSTED_MOUNTER test.
> 
> Changelog v2:
> - Limit patch to non-init mounted filesystems.
> - Define 2 sb->s_iflags
> 
> Changelog v1:
> - Merged the unprivileged and privileged patches.
> - Dropped IMA fsname support.
> - Introduced a new IMA builtin policy named "untrusted_fs".
> - Replaced fs_type flag with sb->s_iflags flag.
> 
>  include/linux/fs.h                    |  2 ++
>  security/integrity/ima/ima_appraise.c | 15 ++++++++++++++-
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 2a815560fda0..4e1c76af7b68 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1320,6 +1320,8 @@ extern int send_sigurg(struct fown_struct *fown);
>  
>  /* sb->s_iflags to limit user namespace mounts */
>  #define SB_I_USERNS_VISIBLE		0x00000010 /* fstype already mounted */
> +#define SB_I_IMA_UNVERIFIABLE_SIGNATURE	0x00000020
> +#define SB_I_UNTRUSTED_MOUNTER		0x00000040
>  
>  /* Possible states of 'frozen' field */
>  enum {
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 1b177461f20e..4bafb397ee91 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -302,7 +302,19 @@ int ima_appraise_measurement(enum ima_hooks func,
>  	}
>  
>  out:
> -	if (status != INTEGRITY_PASS) {
> +	/*
> +	 * 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.
> +	 */
> +	if ((inode->i_sb->s_iflags &
> +	    (SB_I_IMA_UNVERIFIABLE_SIGNATURE | SB_I_UNTRUSTED_MOUNTER)) ==
> +	    (SB_I_IMA_UNVERIFIABLE_SIGNATURE | SB_I_UNTRUSTED_MOUNTER)) {

Heh, this is misleading combination of parentheses and indentation :)
I would recommend using a temporary variable like:

	cmpflags = SB_I_IMA_UNVERIFIABLE_SIGNATURE | SB_I_UNTRUSTED_MOUNTER;
	if ((inode->i_sb->s_iflags & cmpflags) == cmpflags) {

or maybe a helper function.

> +		status = INTEGRITY_FAIL;
> +		cause = "unverifiable-signature";
> +		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
> +				    op, cause, rc, 0);
> +	} else if (status != INTEGRITY_PASS) {
>  		if ((ima_appraise & IMA_APPRAISE_FIX) &&
>  		    (!xattr_value ||
>  		     xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
> @@ -319,6 +331,7 @@ int ima_appraise_measurement(enum ima_hooks func,
>  	} else {
>  		ima_cache_flags(iint, func);
>  	}
> +
>  	ima_set_cache_status(iint, func, status);
>  	return status;
>  }
> -- 
> 2.7.5