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Mon, 30 Mar 2020 18:26:09 +0000 Subject: [PATCH v7 15/16] exec: Fix dead-lock in de_thread with ptrace_attach From: Bernd Edlinger To: "Eric W. Biederman" Cc: "gregkh@linuxfoundation.org" , Kirill Tkhai , Christian Brauner , Kees Cook , "jannh@google.com" , Jonathan Corbet , Alexander Viro , Andrew Morton , "adobriyan@gmail.com" , Thomas Gleixner , Oleg Nesterov , Frederic Weisbecker , "avagin@gmail.com" , Ingo Molnar , "Peter Zijlstra (Intel)" , "duyuyang@gmail.com" , David Hildenbrand , Sebastian Andrzej Siewior , Anshuman Khandual , David Howells , James Morris , Shakeel Butt , Jason Gunthorpe , "christian@kellner.me" , Andrea Arcangeli , Aleksa Sarai , "Dmitry V. Levin" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "stable@vger.kernel.org" , "linux-api@vger.kernel.org" References: <077b63b7-6f5e-aa8e-bf96-a586b481cc46@hotmail.de> <87a7448q7t.fsf@x220.int.ebiederm.org> Message-ID: Date: Mon, 30 Mar 2020 20:26:04 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 In-Reply-To: Content-Language: en-US X-ClientProxiedBy: AM0PR10CA0045.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:20b:150::25) To AM6PR03MB5170.eurprd03.prod.outlook.com (2603:10a6:20b:ca::23) X-Microsoft-Original-Message-ID: MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.1.101] (92.77.140.102) by AM0PR10CA0045.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:20b:150::25) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2856.20 via Frontend Transport; Mon, 30 Mar 2020 18:26:06 +0000 X-Microsoft-Original-Message-ID: X-TMN: [y9ufXCBt4sr7XGb+WUkBtwVDEusu7VC3] X-MS-PublicTrafficType: Email X-IncomingHeaderCount: 50 X-EOPAttributedMessage: 0 X-MS-Office365-Filtering-Correlation-Id: 20199f2c-b72f-4876-2c8a-08d7d4d7d122 X-MS-TrafficTypeDiagnostic: DB8EUR05HT007: X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: uS7rbgjfuVIVdgnKcd2uan596kkpevqhgupHOUEPthl4XWcOvoWBIc6SdcmbpYdAJbQPCsoQ5+l3lIr7OB/7BfCWnKYhfOzw3imFzQb+dPie8sahw5jmloYRcbWreWW0L5FK9btRZPbfrQXJje9NO2asSFw1vjjziJAkqbqBXAAkinKRKPfYe5NRa7dALK1O X-MS-Exchange-AntiSpam-MessageData: 8GX4ZTJd49+7/svEgexSIVDmYXbhqnZQSGHJp6bHrA7oF+H9yJ0alQujonVxE77OutkC/Hd8PtV5fvObHNrewNy+KIsi/BazBn1kf6pyE0gpZ2H8M4XzyHIlj4Ykh4elrfX/rG4MjrZf3hiGAdq2Og== X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 20199f2c-b72f-4876-2c8a-08d7d4d7d122 X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 Mar 2020 18:26:09.3636 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-FromEntityHeader: Internet X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB8EUR05HT007 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: This removes the last users of cred_guard_mutex and replaces it with a new mutex exec_guard_mutex, and a boolean unsafe_execve_in_progress. This addresses the case when at least one of the sibling threads is traced, and therefore the trace process may dead-lock in ptrace_attach, but de_thread will need to wait for the tracer to continue execution. The solution is to detect this situation and make ptrace_attach and similar functions return -EAGAIN, but only in a situation where a dead-lock is imminent. This means this is an API change, but only when the process is traced while execve happens in a multi-threaded application. See tools/testing/selftests/ptrace/vmaccess.c for a test case that gets fixed by this change. Signed-off-by: Bernd Edlinger --- fs/exec.c | 44 +++++++++++++++++++++++++++++++++++--------- fs/proc/base.c | 20 ++++++++++++++++++-- include/linux/sched/signal.h | 14 +++++++++----- init/init_task.c | 2 +- kernel/cred.c | 2 +- kernel/fork.c | 2 +- kernel/ptrace.c | 42 +++++++++++++++++++++++++++++++++++++++--- kernel/seccomp.c | 25 +++++++++++++++++++------ 8 files changed, 123 insertions(+), 28 deletions(-) v7: Added "big fat" warning comments, made the change in proc_pid_attr_write a bit more readable. diff --git a/fs/exec.c b/fs/exec.c index 0e46ec5..2056562 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1078,14 +1078,26 @@ static int de_thread(struct task_struct *tsk) struct signal_struct *sig = tsk->signal; struct sighand_struct *oldsighand = tsk->sighand; spinlock_t *lock = &oldsighand->siglock; + struct task_struct *t = tsk; if (thread_group_empty(tsk)) goto no_thread_group; + spin_lock_irq(lock); + while_each_thread(tsk, t) { + if (unlikely(t->ptrace)) + sig->unsafe_execve_in_progress = true; + } + + if (unlikely(sig->unsafe_execve_in_progress)) { + spin_unlock_irq(lock); + mutex_unlock(&sig->exec_guard_mutex); + spin_lock_irq(lock); + } + /* * Kill all other threads in the thread group. */ - spin_lock_irq(lock); if (signal_group_exit(sig)) { /* * Another group action in progress, just @@ -1429,22 +1441,30 @@ void finalize_exec(struct linux_binprm *bprm) EXPORT_SYMBOL(finalize_exec); /* - * Prepare credentials and lock ->cred_guard_mutex. + * Prepare credentials and lock ->exec_guard_mutex. * install_exec_creds() commits the new creds and drops the lock. * Or, if exec fails before, free_bprm() should release ->cred and * and unlock. */ static int prepare_bprm_creds(struct linux_binprm *bprm) { - if (mutex_lock_interruptible(¤t->signal->cred_guard_mutex)) + int ret; + + if (mutex_lock_interruptible(¤t->signal->exec_guard_mutex)) return -ERESTARTNOINTR; + ret = -EAGAIN; + if (unlikely(current->signal->unsafe_execve_in_progress)) + goto out; + bprm->cred = prepare_exec_creds(); if (likely(bprm->cred)) return 0; - mutex_unlock(¤t->signal->cred_guard_mutex); - return -ENOMEM; + ret = -ENOMEM; +out: + mutex_unlock(¤t->signal->exec_guard_mutex); + return ret; } static void free_bprm(struct linux_binprm *bprm) @@ -1453,7 +1473,10 @@ static void free_bprm(struct linux_binprm *bprm) if (bprm->cred) { if (bprm->called_exec_mmap) mutex_unlock(¤t->signal->exec_update_mutex); - mutex_unlock(¤t->signal->cred_guard_mutex); + if (unlikely(current->signal->unsafe_execve_in_progress)) + mutex_lock(¤t->signal->exec_guard_mutex); + current->signal->unsafe_execve_in_progress = false; + mutex_unlock(¤t->signal->exec_guard_mutex); abort_creds(bprm->cred); } if (bprm->file) { @@ -1497,19 +1520,22 @@ void install_exec_creds(struct linux_binprm *bprm) if (get_dumpable(current->mm) != SUID_DUMP_USER) perf_event_exit_task(current); /* - * cred_guard_mutex must be held at least to this point to prevent + * exec_guard_mutex must be held at least to this point to prevent * ptrace_attach() from altering our determination of the task's * 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); + if (unlikely(current->signal->unsafe_execve_in_progress)) + mutex_lock(¤t->signal->exec_guard_mutex); + current->signal->unsafe_execve_in_progress = false; + mutex_unlock(¤t->signal->exec_guard_mutex); } EXPORT_SYMBOL(install_exec_creds); /* * determine how safe it is to execute the proposed program - * - the caller must hold ->cred_guard_mutex to protect against + * - the caller must hold ->exec_guard_mutex to protect against * PTRACE_ATTACH or seccomp thread-sync */ static void check_unsafe_exec(struct linux_binprm *bprm) diff --git a/fs/proc/base.c b/fs/proc/base.c index 6b13fc4..eaca36e 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -2680,14 +2680,30 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf, } /* Guard against adverse ptrace interaction */ - rv = mutex_lock_interruptible(¤t->signal->cred_guard_mutex); + rv = mutex_lock_interruptible(¤t->signal->exec_guard_mutex); if (rv < 0) goto out_free; + /* + * BIG FAT WARNING - Fragile code ahead. + * Please do not insert any code between these two + * if statements. It may happen that execve has to + * release the exec_guard_mutex in order to prevent + * deadlocks. In that case unsafe_execve_in_progress + * will be set. If that happens you cannot assume that + * the usual guarantees implied by exec_guard_mutex + * are valid. Just return -EAGAIN in that case and + * unlock the mutex immediately. + */ + rv = -EAGAIN; + if (unlikely(current->signal->unsafe_execve_in_progress)) + goto out_unlock; + rv = security_setprocattr(PROC_I(inode)->op.lsm, file->f_path.dentry->d_name.name, page, count); - mutex_unlock(¤t->signal->cred_guard_mutex); +out_unlock: + mutex_unlock(¤t->signal->exec_guard_mutex); out_free: kfree(page); out: diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index a29df79..e83cef2 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -212,6 +212,13 @@ struct signal_struct { #endif /* + * Set while execve is executing but is *not* holding + * exec_guard_mutex to avoid possible dead-locks. + * Only valid when exec_guard_mutex is held. + */ + bool unsafe_execve_in_progress; + + /* * Thread is the potential origin of an oom condition; kill first on * oom */ @@ -222,11 +229,8 @@ struct signal_struct { struct mm_struct *oom_mm; /* recorded mm when the thread group got * killed by the oom killer */ - struct mutex cred_guard_mutex; /* guard against foreign influences on - * credential calculations - * (notably. ptrace) - * Deprecated do not use in new code. - * Use exec_update_mutex instead. + struct mutex exec_guard_mutex; /* Held while execve runs, except when + * a sibling thread is being traced. */ struct mutex exec_update_mutex; /* Held while task_struct is being * updated during exec, and may have diff --git a/init/init_task.c b/init/init_task.c index bd403ed..6f96327 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -25,7 +25,7 @@ }, .multiprocess = HLIST_HEAD_INIT, .rlim = INIT_RLIMITS, - .cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex), + .exec_guard_mutex = __MUTEX_INITIALIZER(init_signals.exec_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), diff --git a/kernel/cred.c b/kernel/cred.c index 71a7926..341ca59 100644 --- a/kernel/cred.c +++ b/kernel/cred.c @@ -295,7 +295,7 @@ struct cred *prepare_creds(void) /* * Prepare credentials for current to perform an execve() - * - The caller must hold ->cred_guard_mutex + * - The caller must hold ->exec_guard_mutex */ struct cred *prepare_exec_creds(void) { diff --git a/kernel/fork.c b/kernel/fork.c index e23ccac..98012f7 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1593,7 +1593,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk) sig->oom_score_adj = current->signal->oom_score_adj; sig->oom_score_adj_min = current->signal->oom_score_adj_min; - mutex_init(&sig->cred_guard_mutex); + mutex_init(&sig->exec_guard_mutex); mutex_init(&sig->exec_update_mutex); return 0; diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 43d6179..19bf69f 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -392,9 +392,24 @@ static int ptrace_attach(struct task_struct *task, long request, * under ptrace. */ retval = -ERESTARTNOINTR; - if (mutex_lock_interruptible(&task->signal->cred_guard_mutex)) + if (mutex_lock_interruptible(&task->signal->exec_guard_mutex)) goto out; + /* + * BIG FAT WARNING - Fragile code ahead. + * Please do not insert any code between these two + * if statements. It may happen that execve has to + * release the exec_guard_mutex in order to prevent + * deadlocks. In that case unsafe_execve_in_progress + * will be set. If that happens you cannot assume that + * the usual guarantees implied by exec_guard_mutex + * are valid. Just return -EAGAIN in that case and + * unlock the mutex immediately. + */ + retval = -EAGAIN; + if (unlikely(task->signal->unsafe_execve_in_progress)) + goto unlock_creds; + task_lock(task); retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS); task_unlock(task); @@ -447,7 +462,7 @@ static int ptrace_attach(struct task_struct *task, long request, unlock_tasklist: write_unlock_irq(&tasklist_lock); unlock_creds: - mutex_unlock(&task->signal->cred_guard_mutex); + mutex_unlock(&task->signal->exec_guard_mutex); out: if (!retval) { /* @@ -472,10 +487,29 @@ static int ptrace_attach(struct task_struct *task, long request, */ static int ptrace_traceme(void) { - int ret = -EPERM; + int ret; + + if (mutex_lock_interruptible(¤t->signal->exec_guard_mutex)) + return -ERESTARTNOINTR; + + /* + * BIG FAT WARNING - Fragile code ahead. + * Please do not insert any code between these two + * if statements. It may happen that execve has to + * release the exec_guard_mutex in order to prevent + * deadlocks. In that case unsafe_execve_in_progress + * will be set. If that happens you cannot assume that + * the usual guarantees implied by exec_guard_mutex + * are valid. Just return -EAGAIN in that case and + * unlock the mutex immediately. + */ + ret = -EAGAIN; + if (unlikely(current->signal->unsafe_execve_in_progress)) + goto unlock_creds; write_lock_irq(&tasklist_lock); /* Are we already being traced? */ + ret = -EPERM; if (!current->ptrace) { ret = security_ptrace_traceme(current->parent); /* @@ -490,6 +524,8 @@ static int ptrace_traceme(void) } write_unlock_irq(&tasklist_lock); +unlock_creds: + mutex_unlock(¤t->signal->exec_guard_mutex); return ret; } diff --git a/kernel/seccomp.c b/kernel/seccomp.c index b6ea3dc..7ebb194 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -329,7 +329,7 @@ static int is_ancestor(struct seccomp_filter *parent, /** * seccomp_can_sync_threads: checks if all threads can be synchronized * - * Expects sighand and cred_guard_mutex locks to be held. + * Expects sighand and exec_guard_mutex locks to be held. * * Returns 0 on success, -ve on error, or the pid of a thread which was * either not in the correct seccomp mode or did not have an ancestral @@ -339,9 +339,22 @@ static inline pid_t seccomp_can_sync_threads(void) { struct task_struct *thread, *caller; - BUG_ON(!mutex_is_locked(¤t->signal->cred_guard_mutex)); + BUG_ON(!mutex_is_locked(¤t->signal->exec_guard_mutex)); assert_spin_locked(¤t->sighand->siglock); + /* + * BIG FAT WARNING - Fragile code ahead. + * It may happen that execve has to release the + * exec_guard_mutex in order to prevent deadlocks. + * In that case unsafe_execve_in_progress will be set. + * If that happens you cannot assume that the usual + * guarantees implied by exec_guard_mutex are valid. + * Just return -EAGAIN in that case and unlock the mutex + * immediately. + */ + if (unlikely(current->signal->unsafe_execve_in_progress)) + return -EAGAIN; + /* Validate all threads being eligible for synchronization. */ caller = current; for_each_thread(caller, thread) { @@ -371,7 +384,7 @@ static inline pid_t seccomp_can_sync_threads(void) /** * seccomp_sync_threads: sets all threads to use current's filter * - * Expects sighand and cred_guard_mutex locks to be held, and for + * Expects sighand and exec_guard_mutex locks to be held, and for * seccomp_can_sync_threads() to have returned success already * without dropping the locks. * @@ -380,7 +393,7 @@ static inline void seccomp_sync_threads(unsigned long flags) { struct task_struct *thread, *caller; - BUG_ON(!mutex_is_locked(¤t->signal->cred_guard_mutex)); + BUG_ON(!mutex_is_locked(¤t->signal->exec_guard_mutex)); assert_spin_locked(¤t->sighand->siglock); /* Synchronize all threads. */ @@ -1319,7 +1332,7 @@ static long seccomp_set_mode_filter(unsigned int flags, * while another thread is in the middle of calling exec. */ if (flags & SECCOMP_FILTER_FLAG_TSYNC && - mutex_lock_killable(¤t->signal->cred_guard_mutex)) + mutex_lock_killable(¤t->signal->exec_guard_mutex)) goto out_put_fd; spin_lock_irq(¤t->sighand->siglock); @@ -1337,7 +1350,7 @@ static long seccomp_set_mode_filter(unsigned int flags, out: spin_unlock_irq(¤t->sighand->siglock); if (flags & SECCOMP_FILTER_FLAG_TSYNC) - mutex_unlock(¤t->signal->cred_guard_mutex); + mutex_unlock(¤t->signal->exec_guard_mutex); out_put_fd: if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) { if (ret) {