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

Kirill Tkhai
2020-03-17T08:57:49Z

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?

> 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;
> }
>