On Wed, 2024-11-20 at 19:44 +0000, Breno Leitao wrote:
> Hello Mimi,
>
> On Tue, Nov 19, 2024 at 01:10:10PM -0500, Mimi Zohar wrote:
> > Hi Breno,
> >
> > On Mon, 2024-11-04 at 02:47 -0800, Breno Leitao wrote:
> > > Fix a potential RCU issue where ima_measurements list is traversed using
> > > list_for_each_entry_rcu() without proper RCU read lock protection. This
> > > caused warnings when CONFIG_PROVE_RCU was enabled:
> > >
> > > security/integrity/ima/ima_kexec.c:40 RCU-list traversed in non-reader section!!
> > >
> > > Add rcu_read_lock() before iterating over ima_measurements list to ensure
> > > proper RCU synchronization, consistent with other RCU list traversals in
> > > the codebase.
> >
> > The synchronization is to prevent freeing of data while walking the RCU list. In
> > this case, new measurements are only appended to the IMA measurement list. So
> > there shouldn't be an issue.
> >
> > The IMA measurement list is being copied during kexec "load", while other
> > processes are still running. Depending on the IMA policy, the kexec "load",
> > itself, and these other processes may result in additional measurements, which
> > should be copied across kexec. Adding the rcu_read_{lock, unlock} would
> > unnecessarily prevent them from being copied.
>
> Thank you for the detailed explanation. Since rcu_read_lock() operations are
> lightweight, I believe keeping them wouldn't impact performance significantly.
It's not a question of performance, but of missing measurements in the IMA
measurement list.
>
> However, if you prefer the lockless approach, I would suggest adding an
> argument to list_for_each_entry_rcu() to keep the warning out. What are
> your thoughts on this?
Yes, this is better.
thanks,
Mimi
>
> Author: Breno Leitao <leitao@xxxxxxxxxx>
> Date: Mon Nov 4 02:26:45 2024 -0800
>
> ima: kexec: silence RCU list traversal warning
>
> 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 lockless 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@xxxxxxxxxx>
>
> diff --git a/security/integrity/ima/ima_kexec.c b/security/integrity/ima/ima_kexec.c
> index 52e00332defed..9d45f4d26f731 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);
>
>
