The ima_measurements list is append-only and doesn't require
rcu_read_lock() protection. However, lockdep issues a warning when
traversing RCU lists without the read lock:

  security/integrity/ima/ima_kexec.c:40 RCU-list traversed in non-reader section!!

Fix this by using the variant of list_for_each_entry_rcu() with the last
argument set to true. This tells the RCU subsystem that traversing this
append-only list without the read lock is intentional and safe.

This change silences the lockdep warning while maintaining the correct
semantics for the append-only list traversal.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changes in v2:
- Do not hold the RCU read lock, but, annotate list_for_each_entry_rcu()
  that is OK to traverse the list without the RCU read lock.
- Link to v1: https://lore.kernel.org/r/20241104-ima_rcu-v1-1-5157460c5907@debian.org
---
 security/integrity/ima/ima_kexec.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_kexec.c b/security/integrity/ima/ima_kexec.c
index 52e00332defed39774c9e23e045f1377cfa30d0c..9d45f4d26f731658a79b94b9f95f4dcc4dcb6325 100644
--- a/security/integrity/ima/ima_kexec.c
+++ b/security/integrity/ima/ima_kexec.c
@@ -37,7 +37,8 @@ static int ima_dump_measurement_list(unsigned long *buffer_size, void **buffer,
 
 	memset(&khdr, 0, sizeof(khdr));
 	khdr.version = 1;
-	list_for_each_entry_rcu(qe, &ima_measurements, later) {
+	/* This is an append-only list, no need to hold the RCU read lock */
+	list_for_each_entry_rcu(qe, &ima_measurements, later, true) {
 		if (file.count < file.size) {
 			khdr.count++;
 			ima_measurements_show(&file, qe);

---
base-commit: ac24e26aa08fe026804f678599f805eb13374a5d
change-id: 20241104-ima_rcu-ee83da87d050

Best regards,
-- 
Breno Leitao <leitao@debian.org>

Hello Mimi,

On Thu, Nov 21, 2024 at 01:57:12AM -0800, Breno Leitao wrote:
> The ima_measurements list is append-only and doesn't require
> rcu_read_lock() protection. However, lockdep issues a warning when
> traversing RCU lists without the read lock:
> 
>   security/integrity/ima/ima_kexec.c:40 RCU-list traversed in non-reader section!!

What are the next steps in regarding this issue? I am still seeing this
problem on Linus' tree.

Thanks

On Thu, 2025-01-16 at 03:23 -0800, Breno Leitao wrote:
> Hello Mimi,
> 
> On Thu, Nov 21, 2024 at 01:57:12AM -0800, Breno Leitao wrote:
> > The ima_measurements list is append-only and doesn't require
> > rcu_read_lock() protection. However, lockdep issues a warning when
> > traversing RCU lists without the read lock:
> > 
> >   security/integrity/ima/ima_kexec.c:40 RCU-list traversed in non-reader
> > section!!
> 
> What are the next steps in regarding this issue? I am still seeing this
> problem on Linus' tree.

It's included in the v6.14 pull request, which I just sent.

thanks,

Mimi