Re: [PATCH v3 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

Kirill Tkhai
2020-03-18T12:23:23Z

On 18.03.2020 00:53, Bernd Edlinger wrote:
> On 3/17/20 9:56 AM, Kirill Tkhai wrote:
>> On 14.03.2020 12:11, Bernd Edlinger 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>
>>> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
>>> ---
>>> fs/exec.c | 17 ++++++++++++++---
>>> include/linux/binfmts.h | 8 +++++++-
>>> include/linux/sched/signal.h | 9 ++++++++-
>>> init/init_task.c | 1 +
>>> kernel/fork.c | 1 +
>>> 5 files changed, 31 insertions(+), 5 deletions(-)
>>>
>>> v3: this update fixes lock-order and adds an explicit data member in linux_binprm
>>>
>>> diff --git a/fs/exec.c b/fs/exec.c
>>> index d820a72..11974a1 100644
>>> --- a/fs/exec.c
>>> +++ b/fs/exec.c
>>> @@ -1014,12 +1014,17 @@ 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;
>>> old_mm = current->mm;
>>> exec_mm_release(tsk, old_mm);
>>>
>>> + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
>>> + if (ret)
>>> + return ret;
>>> +
>>> if (old_mm) {
>>> sync_mm_rss(old_mm);
>>> /*
>>> @@ -1031,9 +1036,11 @@ 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;
>>> }
>>> }
>>> +
>>> task_lock(tsk);
>>> active_mm = tsk->active_mm;
>>> membarrier_exec_mmap(mm);
>>> @@ -1288,11 +1295,12 @@ int flush_old_exec(struct linux_binprm * bprm)
>>> goto out;
>>>
>>> /*
>>> - * After clearing bprm->mm (to mark that current is using the
>>> - * prepared mm now), we have nothing left of the original
>>> + * After setting bprm->called_exec_mmap (to mark that current is
>>> + * using the prepared mm now), we have nothing left of the original
>>> * process. If anything from here on returns an error, the check
>>> * in search_binary_handler() will SEGV current.
>>> */
>>> + bprm->called_exec_mmap = 1;
>>
>> The two below is non-breaking pair:
>>
>> exec_mmap(bprm->mm);
>> bprm->called_exec_mmap = 1;
>>
>> Why not move this into exec_mmap(), so nobody definitely inserts something
>> between them?
>>
>
> Hmm, could be done, but then I would probably need a different name than
> "called_exec_mmap".
>
> How about adding a nice function comment to exec_mmap that calls out the
> changed behaviour that the exec_update_mutex is taken unless the function
> fails?

Not sure, I understand correct.

Could you post this like a small patch hunk (on top of anything you want)?

> Bernd.
>
>
>>> bprm->mm = NULL;
>>>
>>> #ifdef CONFIG_POSIX_TIMERS
>>> @@ -1438,6 +1446,8 @@ static void free_bprm(struct linux_binprm *bprm)
>>> {
>>> free_arg_pages(bprm);
>>> if (bprm->cred) {
>>> + if (bprm->called_exec_mmap)
>>> + mutex_unlock(&current->signal->exec_update_mutex);
>>> mutex_unlock(&current->signal->cred_guard_mutex);
>>> abort_creds(bprm->cred);
>>> }
>>> @@ -1487,6 +1497,7 @@ void install_exec_creds(struct linux_binprm *bprm)
>>> * credentials; any time after this it may be unlocked.
>>> */
>>> security_bprm_committed_creds(bprm);
>>> + mutex_unlock(&current->signal->exec_update_mutex);
>>> mutex_unlock(&current->signal->cred_guard_mutex);
>>> }
>>> EXPORT_SYMBOL(install_exec_creds);
>>> @@ -1678,7 +1689,7 @@ int search_binary_handler(struct linux_binprm *bprm)
>>>
>>> read_lock(&binfmt_lock);
>>> put_binfmt(fmt);
>>> - if (retval < 0 && !bprm->mm) {
>>> + if (retval < 0 && bprm->called_exec_mmap) {
>>> /* we got to flush_old_exec() and failed after it */
>>> read_unlock(&binfmt_lock);
>>> force_sigsegv(SIGSEGV);
>>> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
>>> index b40fc63..a345d9f 100644
>>> --- a/include/linux/binfmts.h
>>> +++ b/include/linux/binfmts.h
>>> @@ -44,7 +44,13 @@ struct linux_binprm {
>>> * exec has happened. Used to sanitize execution environment
>>> * and to set AT_SECURE auxv for glibc.
>>> */
>>> - secureexec:1;
>>> + secureexec:1,
>>> + /*
>>> + * Set by flush_old_exec, when exec_mmap has been called.
>>> + * This is past the point of no return, when the
>>> + * exec_update_mutex has been taken.
>>> + */
>>> + called_exec_mmap:1;
>>> #ifdef __alpha__
>>> unsigned int taso:1;
>>> #endif
>>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
>>> index 8805025..a29df79 100644
>>> --- a/include/linux/sched/signal.h
>>> +++ b/include/linux/sched/signal.h
>>> @@ -224,7 +224,14 @@ struct signal_struct {
>>>
>>> struct mutex cred_guard_mutex; /* guard against foreign influences on
>>> * credential calculations
>>> - * (notably. ptrace) */
>>> + * (notably. ptrace)
>>> + * Deprecated do not use in new code.
>>> + * Use exec_update_mutex instead.
>>> + */
>>> + struct mutex exec_update_mutex; /* Held while task_struct is being
>>> + * updated during exec, and may have
>>> + * inconsistent permissions.
>>> + */
>>> } __randomize_layout;
>>>
>>> /*
>>> diff --git a/init/init_task.c b/init/init_task.c
>>> index 9e5cbe5..bd403ed 100644
>>> --- a/init/init_task.c
>>> +++ b/init/init_task.c
>>> @@ -26,6 +26,7 @@
>>> .multiprocess = HLIST_HEAD_INIT,
>>> .rlim = INIT_RLIMITS,
>>> .cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex),
>>> + .exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex),
>>> #ifdef CONFIG_POSIX_TIMERS
>>> .posix_timers = LIST_HEAD_INIT(init_signals.posix_timers),
>>> .cputimer = {
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 8642530..036b692 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -1594,6 +1594,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
>>> sig->oom_score_adj_min = current->signal->oom_score_adj_min;
>>>
>>> mutex_init(&sig->cred_guard_mutex);
>>> + mutex_init(&sig->exec_update_mutex);
>>>
>>> return 0;
>>> }
>>>
>>