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Subject[PATCH] umh: fix UAF when the process is being killed
Date
When the process is killed, wait_for_completion_state will return with
-ERESTARTSYS, and the completion variable in the stack will be freed.
If the user-mode thread is complete at the same time, there will be a UAF.

Please refer to the following scenarios.
T1 T2
------------------------------------------------------------------
call_usermodehelper_exec
call_usermodehelper_exec_async
<< do something >>
umh_complete(sub_info);
comp = xchg(&sub_info->complete, NULL);
/* we got the completion */
<< context switch >>

<< Being killed >>
retval = wait_for_completion_state(sub_info->complete, state);
if (!retval)
goto wait_done;

if (wait & UMH_KILLABLE) {
/* umh_complete() will see NULL and free sub_info */
if (xchg(&sub_info->complete, NULL))
goto unlock;
<< we can't got the completion >>
}
....
unlock:
helper_unlock();
return retval;
}

/**
* the completion variable in stack is end of life cycle.
* and maybe freed due to process is recycled.
*/
--------UAF here----------
if (comp)
complete(comp);

To fix it, we can put the completion variable in the subprocess_info
variable.

Reported-by: syzbot+10d19d528d9755d9af22@syzkaller.appspotmail.com
Reported-by: syzbot+70d5d5d83d03db2c813d@syzkaller.appspotmail.com
Reported-by: syzbot+83cb0411d0fcf0a30fc1@syzkaller.appspotmail.com

Signed-off-by: Schspa Shi <schspa@gmail.com>
---
include/linux/umh.h | 1 +
kernel/umh.c | 6 +++---
2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/umh.h b/include/linux/umh.h
index 5d1f6129b847..801f7efbc825 100644
--- a/include/linux/umh.h
+++ b/include/linux/umh.h
@@ -20,6 +20,7 @@ struct file;
struct subprocess_info {
struct work_struct work;
struct completion *complete;
+ struct completion done;
const char *path;
char **argv;
char **envp;
diff --git a/kernel/umh.c b/kernel/umh.c
index 850631518665..3ed39956c777 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -380,6 +380,7 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
sub_info->cleanup = cleanup;
sub_info->init = init;
sub_info->data = data;
+ init_completion(&sub_info->done);
out:
return sub_info;
}
@@ -405,7 +406,6 @@ EXPORT_SYMBOL(call_usermodehelper_setup);
int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
{
unsigned int state = TASK_UNINTERRUPTIBLE;
- DECLARE_COMPLETION_ONSTACK(done);
int retval = 0;

if (!sub_info->path) {
@@ -431,7 +431,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
* This makes it possible to use umh_complete to free
* the data structure in case of UMH_NO_WAIT.
*/
- sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
+ sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &sub_info->done;
sub_info->wait = wait;

queue_work(system_unbound_wq, &sub_info->work);
@@ -444,7 +444,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
if (wait & UMH_FREEZABLE)
state |= TASK_FREEZABLE;

- retval = wait_for_completion_state(&done, state);
+ retval = wait_for_completion_state(sub_info->complete, state);
if (!retval)
goto wait_done;

--
2.37.3
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