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Message-ID: <2f4afb28-887d-2b49-570c-af933314de34@virtuozzo.com>
Date: Mon, 23 Mar 2020 13:51:35 +0300
From: Kirill Tkhai <ktkhai@...tuozzo.com>
To: Bernd Edlinger <bernd.edlinger@...mail.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Kees Cook <keescook@...omium.org>,
Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
Alexander Viro <viro@...iv.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
Alexey Dobriyan <adobriyan@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Oleg Nesterov <oleg@...hat.com>,
Frederic Weisbecker <frederic@...nel.org>,
Andrei Vagin <avagin@...il.com>,
Ingo Molnar <mingo@...nel.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Yuyang Du <duyuyang@...il.com>,
David Hildenbrand <david@...hat.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Anshuman Khandual <anshuman.khandual@....com>,
David Howells <dhowells@...hat.com>,
James Morris <jamorris@...ux.microsoft.com>,
Shakeel Butt <shakeelb@...gle.com>,
Jason Gunthorpe <jgg@...pe.ca>,
Christian Kellner <christian@...lner.me>,
Andrea Arcangeli <aarcange@...hat.com>,
Aleksa Sarai <cyphar@...har.com>,
"Dmitry V. Levin" <ldv@...linux.org>,
"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"stable@...r.kernel.org" <stable@...r.kernel.org>,
"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v6 05/16] exec: Add exec_update_mutex to replace
cred_guard_mutex
On 20.03.2020 23:25, Bernd Edlinger wrote:
> The cred_guard_mutex is problematic as it is held over possibly
> indefinite waits for userspace. The possible indefinite waits for
> userspace that I have identified are: The cred_guard_mutex is held in
> PTRACE_EVENT_EXIT waiting for the tracer. The cred_guard_mutex is
> held over "put_user(0, tsk->clear_child_tid)" in exit_mm(). The
> cred_guard_mutex is held over "get_user(futex_offset, ...") in
> exit_robust_list. The cred_guard_mutex held over copy_strings.
>
> The functions get_user and put_user can trigger a page fault which can
> potentially wait indefinitely in the case of userfaultfd or if
> userspace implements part of the page fault path.
>
> In any of those cases the userspace process that the kernel is waiting
> for might make a different system call that winds up taking the
> cred_guard_mutex and result in deadlock.
>
> Holding a mutex over any of those possibly indefinite waits for
> userspace does not appear necessary. Add exec_update_mutex that will
> just cover updating the process during exec where the permissions and
> the objects pointed to by the task struct may be out of sync.
>
> The plan is to switch the users of cred_guard_mutex to
> exec_update_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@...ssion.com>
> Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
Reviewed-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> ---
> fs/exec.c | 22 +++++++++++++++++++---
> include/linux/binfmts.h | 8 +++++++-
> include/linux/sched/signal.h | 9 ++++++++-
> init/init_task.c | 1 +
> kernel/fork.c | 1 +
> 5 files changed, 36 insertions(+), 5 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index d820a72..0e46ec5 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1010,16 +1010,26 @@ ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
> }
> EXPORT_SYMBOL(read_code);
>
> +/*
> + * Maps the mm_struct mm into the current task struct.
> + * On success, this function returns with the mutex
> + * exec_update_mutex locked.
> + */
> 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 +1041,11 @@ 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 +1300,12 @@ 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;
> bprm->mm = NULL;
>
> #ifdef CONFIG_POSIX_TIMERS
> @@ -1438,6 +1451,8 @@ static void free_bprm(struct linux_binprm *bprm)
> {
> free_arg_pages(bprm);
> if (bprm->cred) {
> + if (bprm->called_exec_mmap)
> + mutex_unlock(¤t->signal->exec_update_mutex);
> mutex_unlock(¤t->signal->cred_guard_mutex);
> abort_creds(bprm->cred);
> }
> @@ -1487,6 +1502,7 @@ void install_exec_creds(struct linux_binprm *bprm)
> * credentials; any time after this it may be unlocked.
> */
> security_bprm_committed_creds(bprm);
> + mutex_unlock(¤t->signal->exec_update_mutex);
> mutex_unlock(¤t->signal->cred_guard_mutex);
> }
> EXPORT_SYMBOL(install_exec_creds);
> @@ -1678,7 +1694,7 @@ 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;
> }
>
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