On 2022/3/7 10:32, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>
>> rcu_read_lock is required by grabbing the task refcount but it's not
>> needed for ptrace_may_access. So we could release the rcu lock after
>> task refcount is successfully grabbed to reduce the rcu holding time.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>> mm/migrate.c | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index da5a81052468..26943bd819e8 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1907,17 +1907,16 @@ static struct mm_struct *find_mm_struct(pid_t pid, nodemask_t *mem_nodes)
>> return ERR_PTR(-ESRCH);
>> }
>> get_task_struct(task);
>> + rcu_read_unlock();
>>
>> /*
>> * Check if this process has the right to modify the specified
>> * process. Use the regular "ptrace_may_access()" checks.
>> */
>> if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
>> - rcu_read_unlock();
>> mm = ERR_PTR(-EPERM);
>> goto out;
>> }
>> - rcu_read_unlock();
>>
>> mm = ERR_PTR(security_task_movememory(task));
>> if (IS_ERR(mm))
>
> Digged some history via `git blame`, found that the RCU read lock is
> extended in the following commit,
>
> "
> 3268c63eded4612a3d07b56d1e02ce7731e6608e
> Author: Christoph Lameter <cl@xxxxxxxxx>
> AuthorDate: Wed Mar 21 16:34:06 2012 -0700
> Commit: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CommitDate: Wed Mar 21 17:54:58 2012 -0700
>
> mm: fix move/migrate_pages() race on task struct
>
> Migration functions perform the rcu_read_unlock too early. As a result
> the task pointed to may change from under us. This can result in an oops,
> as reported by Dave Hansen in https://lkml.org/lkml/2012/2/23/302.
>
> The following patch extend the period of the rcu_read_lock until after the
> permissions checks are done. We also take a refcount so that the task
> reference is stable when calling security check functions and performing
> cpuset node validation (which takes a mutex).
>
> The refcount is dropped before actual page migration occurs so there is no
> change to the refcounts held during page migration.
>
> Also move the determination of the mm of the task struct to immediately
> before the do_migrate*() calls so that it is clear that we switch from
> handling the task during permission checks to the mm for the actual
> migration. Since the determination is only done once and we then no
> longer use the task_struct we can be sure that we operate on a specific
> address space that will not change from under us.
> "
>
> After that, the permission checking has been changed from __task_cred()
> to ptrace_may_access(). So the situation may change somewhat. Cced
In ptrace_may_access, __task_cred is access while holding the rcu read lock.
It seems this is ensured by the ptrace_may_access itself.
> some names found in git history to verify.
Thanks for your carefulness.
>
> Best Regards,
> Huang, Ying
> .
>
