[PATCH] ima: remove template "ima" as the compiled default

[GUO Zihua]

Template "ima" is a legacy template which limits the hash algorithm to
either sha1 or md5. None of them should be considered "strong" these
days. Besides, allowing template "ima" as the compiled default would
also cause the following issue: the cmdline option "ima_hash=" must be
behind "ima_template=", otherwise "ima_hash=" might be rejected.

The root cause of this issue is that during the processing of ima_hash,
we would try to check whether the hash algorithm is compatible with the
template. If the template is not set at the moment we do the check, we
check the algorithm against the compiled default template. If the
complied default template is "ima", then we reject any hash algorithm
other than sha1 and md5.

For example, if the compiled default template is "ima", and the default
algorithm is sha1 (which is the current default). In the cmdline, we put
in "ima_hash=sha256 ima_template=ima-ng". The expected behavior would be
that ima starts with ima-ng as the template and sha256 as the hash
algorithm. However, during the processing of "ima_hash=",
"ima_template=" has not been processed yet, and hash_setup would check
the configured hash algorithm against the compiled default: ima, and
reject sha256. So at the end, the hash algorithm that is actually used
will be sha1.

With template "ima" removed from the compiled default, we ensure that the
default tempalte would at least be "ima-ng" which allows for basically
any hash algorithm.

This change would not break the algorithm compatibility checking for
IMA.

Fixes: 4286587dccd43 ("ima: add Kconfig default measurement list template")
Signed-off-by: GUO Zihua <guozihua at huawei.com>
---
 security/integrity/ima/Kconfig | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
index f3a9cc201c8c..9513df2ac19e 100644
--- a/security/integrity/ima/Kconfig
+++ b/security/integrity/ima/Kconfig
@@ -65,14 +65,11 @@ choice
 	help
 	  Select the default IMA measurement template.
 
-	  The original 'ima' measurement list template contains a
-	  hash, defined as 20 bytes, and a null terminated pathname,
-	  limited to 255 characters.  The 'ima-ng' measurement list
-	  template permits both larger hash digests and longer
-	  pathnames.
-
-	config IMA_TEMPLATE
-		bool "ima"
+	  The 'ima-ng' measurement list template permits various hash
+	  digests and long pathnames. The compiled default template
+	  can be overwritten using the kernel command line
+	  'ima_template=' option.
+
 	config IMA_NG_TEMPLATE
 		bool "ima-ng (default)"
 	config IMA_SIG_TEMPLATE
@@ -82,7 +79,6 @@ endchoice
 config IMA_DEFAULT_TEMPLATE
 	string
 	depends on IMA
-	default "ima" if IMA_TEMPLATE
 	default "ima-ng" if IMA_NG_TEMPLATE
 	default "ima-sig" if IMA_SIG_TEMPLATE
 
-- 
2.17.1