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Subject[PATCH] creds: kill __task_cred()->task_is_dead() validation
commit 8f92054e:

add the following validation condition:

task->exit_state >= 0

to permit the access if the target task is dead and therefore
unable to change its own credentials.

OK, but afaics currently this can only help wait_task_zombie() which
calls __task_cred() without rcu lock.

Remove this validation and change wait_task_zombie() to use task_uid()
instead. This means we do rcu_read_lock() only to shut up the lockdep,
but we already do the same in, say, wait_task_stopped().

task_is_dead() should die, task->exit_state != 0 means that this task
has passed exit_notify(), only do_wait-like code paths should use this.

Unfortunately, we can't kill task_is_dead() right now, it has already
found the users in drivers/staging/, and I bet the usage is wrong.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
include/linux/cred.h | 10 +++-------
kernel/exit.c | 2 +-
2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index adadf71..1b64c72 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -276,17 +276,13 @@ static inline void put_cred(const struct cred *_cred)
* @task: The task to query
*
* Access the objective credentials of a task. The caller must hold the RCU
- * readlock or the task must be dead and unable to change its own credentials.
+ * readlock.
*
* The result of this function should not be passed directly to get_cred();
* rather get_task_cred() should be used instead.
*/
-#define __task_cred(task) \
- ({ \
- const struct task_struct *__t = (task); \
- rcu_dereference_check(__t->real_cred, \
- task_is_dead(__t)); \
- })
+#define __task_cred(task) \
+ rcu_dereference((task)->real_cred)

/**
* get_current_cred - Get the current task's subjective credentials
diff --git a/kernel/exit.c b/kernel/exit.c
index 4b4042f..7b36288 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1206,7 +1206,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
unsigned long state;
int retval, status, traced;
pid_t pid = task_pid_vnr(p);
- uid_t uid = __task_cred(p)->uid;
+ uid_t uid = task_uid(p);
struct siginfo __user *infop;

if (!likely(wo->wo_flags & WEXITED))
--
1.5.5.1



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