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SubjectRe: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex
Kirill Tkhai <ktkhai@virtuozzo.com> writes:

> On 09.03.2020 00:38, Eric W. Biederman wrote:
>>
>> The cred_guard_mutex is problematic. The cred_guard_mutex is held
>> over the userspace accesses as the arguments from userspace are read.
>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
>> threads are killed. The cred_guard_mutex is held over
>> "put_user(0, tsk->clear_child_tid)" in exit_mm().
>>
>> Any of those can result in deadlock, as the cred_guard_mutex is held
>> over a possible indefinite userspace waits for userspace.
>>
>> Add exec_update_mutex that is only held over exec updating process
>> with the new contents of exec, so that code that needs not to be
>> confused by exec changing the mm and the cred in ways that can not
>> happen during ordinary execution of a process.
>>
>> The plan is to switch the users of cred_guard_mutex to
>> exec_udpate_mutex one by one. This lets us move forward while still
>> being careful and not introducing any regressions.
>>
>> Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/
>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/
>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/
>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/
>> Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
>> Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> ---
>> fs/exec.c | 9 +++++++++
>> include/linux/sched/signal.h | 9 ++++++++-
>> init/init_task.c | 1 +
>> kernel/fork.c | 1 +
>> 4 files changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index d820a7272a76..ffeebb1f167b 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1014,6 +1014,7 @@ static int exec_mmap(struct mm_struct *mm)
>> {
>> struct task_struct *tsk;
>> struct mm_struct *old_mm, *active_mm;
>> + int ret;
>>
>> /* Notify parent that we're no longer interested in the old VM */
>> tsk = current;
>> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm)
>> return -EINTR;
>> }
>> }
>> +
>> + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>> + if (ret)
>> + return ret;
>
> You missed old_mm->mmap_sem unlock. See here:

Duh. Thank you.

I actually need to switch the lock ordering here, and I haven't yet
because my son was sick yesterday.

Something like this.

diff --git a/fs/exec.c b/fs/exec.c
index 96f89401b4d1..03d50c27ec01 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1020,9 +1020,14 @@ static int exec_mmap(struct mm_struct *mm)
tsk = current;
old_mm = current->mm;
exec_mm_release(tsk, old_mm);
+ if (old_mm)
+ sync_mm_rss(old_mm);
+
+ ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
+ if (ret)
+ return ret;

if (old_mm) {
- sync_mm_rss(old_mm);
/*
* Make sure that if there is a core dump in progress
* for the old mm, we get out and die instead of going
@@ -1032,14 +1037,11 @@ static int exec_mmap(struct mm_struct *mm)
down_read(&old_mm->mmap_sem);
if (unlikely(old_mm->core_state)) {
up_read(&old_mm->mmap_sem);
+ mutex_unlock(&tsk->signal->exec_update_mutex);
return -EINTR;
}
}

- ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
- if (ret)
- return ret;
-
task_lock(tsk);
active_mm = tsk->active_mm;
membarrier_exec_mmap(mm);


> diff --git a/fs/exec.c b/fs/exec.c
> index 47582cd97f86..d557bac3e862 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1063,8 +1063,11 @@ static int exec_mmap(struct mm_struct *mm)
> }
>
> ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
> - if (ret)
> + if (ret) {
> + if (old_mm)
> + up_read(&old_mm->mmap_sem);
> return ret;
> + }
>
> task_lock(tsk);
> active_mm = tsk->active_mm;

Eric

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