[PATCH v2] ima: allow users to specify the pcr index with IMA_MEASURE_PCR_IDX

[Julian Braha <julianbraha@gmail.com>]

The IMA_MEASURE_PCR_IDX option is currently not visible in the kconfig
frontend, so it always uses its default, 10. This means that the
'range 8 14' is dead code, and users are unable to specify the pcr index
value.

In a previous discussion, Mimi explained that users should be able to use
this config option to specify the pcr index. [1]

Let's add a prompt for users to specify the pcr index, when EXPERT is
enabled.

This dead range was found by kconfirm, a static analysis tool for Kconfig.

Link: https://lore.kernel.org/all/1feff118-4afa-4b9c-86f1-271a7a88208f@gmail.com/T/#mc4efa2491b4937eb7c9e532c29ffba516a70e662 [1]
Signed-off-by: Julian Braha <julianbraha@gmail.com>
---
Changes since v1:
- updated help text to recommend using the default of 10

v1:
https://lore.kernel.org/all/20260824162215.1572367-1-julianbraha@gmail.com/
---
 security/integrity/ima/Kconfig | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
index b3a9f86809b0..72654cf797cd 100644
--- a/security/integrity/ima/Kconfig
+++ b/security/integrity/ima/Kconfig
@@ -46,12 +46,16 @@ config IMA_KEXEC
 
 config IMA_MEASURE_PCR_IDX
 	int
+	prompt "PCR Index for Aggregate" if EXPERT
 	range 8 14
 	default 10
 	help
 	  IMA_MEASURE_PCR_IDX determines the TPM PCR register index
 	  that IMA uses to maintain the integrity aggregate of the
-	  measurement list.  If unsure, use the default 10.
+	  measurement list. Most attestation tooling expects PCR 10.
+
+	  The default is almost always what you want. Only change this
+	  if you know what you are doing.
 
 config IMA_LSM_RULES
 	bool
-- 
2.55.0