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Biederman" , Charles Haithcock , Suren Baghdasaryan , Yafang Shao , Helge Deller , YiFei Zhu , Adrian Reber , Thomas Gleixner , Jens Axboe , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , linux-kselftest@vger.kernel.org From: Bernd Edlinger Subject: [PATCHv8] exec: Fix dead-lock in de_thread with ptrace_attach Message-ID: Date: Thu, 10 Jun 2021 09:31:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 Content-Language: en-US X-TMN: [L+ONw775kzR9E9QlfNHbr9oNdi2aNLpr] X-ClientProxiedBy: PR3PR09CA0024.eurprd09.prod.outlook.com (2603:10a6:102:b7::29) To AM8PR10MB4708.EURPRD10.PROD.OUTLOOK.COM (2603:10a6:20b:364::23) X-Microsoft-Original-Message-ID: <54426f73-aa89-752f-aa5a-6bdf14ac288f@hotmail.de> MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.1.101] (84.57.61.94) by PR3PR09CA0024.eurprd09.prod.outlook.com (2603:10a6:102:b7::29) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4219.21 via Frontend Transport; Thu, 10 Jun 2021 07:31:43 +0000 X-MS-PublicTrafficType: Email X-IncomingHeaderCount: 45 X-EOPAttributedMessage: 0 X-MS-Office365-Filtering-Correlation-Id: 95165da8-abe6-4cbe-b138-08d92be1cc50 X-MS-TrafficTypeDiagnostic: AM5EUR02HT170: X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: RQ9eghDsUek4ekDbOn6HnFGN7EPHivTuWXgzVbqOzUYR+P5HAApj0stdulvngRdsTYRS3G8ffmHqTlT0GsRL3AB1Ke5YpFcpQvSKt0r/36zFl4YHap8fxHEfLdZwNBubM0hfa+ce1YFvjuYJuIx1fKgURjkvgb45/LtTzqVEU+byvj1Uy98m/0eEW1UeSg7nDgmBVqsYHC+TR8dW1No9pSu5kQRHgTcABA/6fTerYhqUNnNVHXZUzXzEmZ8zLwvKZtJXZswQSAOGJZ0GV1DlbRBY33xYzwefi/kgDDcoIVoUcrXeis7etCPiiLkCb15QNoBgDJLFZ581lHiTh8VTY6vAr2HrJS8vRunMkAQBOZbPiEoKeTTENeGkLppUSywE0Cbud0zjPxl3GCi1fWK2ER97z1Mq6/QLvEKRXNWbTrYyOFFVL20Qz2PQuA1REvO9SR4XumlSuJ/5CNfkNmkt1Iy3RGFdKGIeJ/uhOZzrHoU= X-MS-Exchange-AntiSpam-MessageData: BISle2dBfegWkjZEufPp+mE228pxQN+VTcb74rIVbtw8hc47/tIEoWWnH60RsafwvqoqeAegrn2IX1HrZ2EOMESUtMx8VVkyz18u3zxACuXc6FkFf4WtXx9zOfejCSNVwMeSk8MWp2Wz632oS37KTw== X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 95165da8-abe6-4cbe-b138-08d92be1cc50 X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Jun 2021 07:31:45.0986 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-AuthSource: AM5EUR02FT027.eop-EUR02.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: Internet X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM5EUR02HT170 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org This introduces signal->unsafe_execve_in_progress, which is used to fix 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 allow ptrace_attach to continue, while de_thread() is still waiting for traced zombies to be eventually released. When the current thread changed the ptrace status from non-traced to traced, we can simply abort the whole execve and restart it by returning -ERESTARTSYS. This needs to be done before changing the thread leader, because the PTRACE_EVENT_EXEC needs to know the old thread pid. Although it is technically after the point of no return, we just have to reset bprm->point_of_no_return here, since at this time only the other threads have received a fatal signal, not the current thread. From the user's point of view the whole execve was simply delayed until after the ptrace_attach. Other threads die quickly since the cred_guard_mutex is released, but a deadly signal is already pending. In case the mutex_lock_killable misses the signal, ->unsafe_execve_in_progress makes sure they release the mutex immediately and return with -ERESTARTNOINTR. This means there is no API change, unlike the previous version of this patch which was discussed here: https://lore.kernel.org/lkml/b6537ae6-31b1-5c50-f32b-8b8332ace882@hotmail.de/ See tools/testing/selftests/ptrace/vmaccess.c for a test case that gets fixed by this change. Note that since the test case was originally designed to test the ptrace_attach returning an error in this situation, the test expectation needed to be adjusted, to allow the API to succeed at the first attempt. Signed-off-by: Bernd Edlinger --- fs/exec.c | 45 ++++++++++++++++++++++++++----- fs/proc/base.c | 6 +++++ include/linux/sched/signal.h | 13 +++++++++ kernel/ptrace.c | 9 +++++++ kernel/seccomp.c | 12 ++++++--- tools/testing/selftests/ptrace/vmaccess.c | 25 +++++++++++------ 6 files changed, 92 insertions(+), 18 deletions(-) diff --git a/fs/exec.c b/fs/exec.c index 8344fba..ac3fec1 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1040,6 +1040,8 @@ 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; + unsigned int prev_ptrace = tsk->ptrace; + struct task_struct *t = tsk; if (thread_group_empty(tsk)) goto no_thread_group; @@ -1057,20 +1059,40 @@ static int de_thread(struct task_struct *tsk) return -EAGAIN; } + while_each_thread(tsk, t) { + if (unlikely(t->ptrace) && t != tsk->group_leader) + sig->unsafe_execve_in_progress = true; + } + sig->group_exit_task = tsk; sig->notify_count = zap_other_threads(tsk); if (!thread_group_leader(tsk)) sig->notify_count--; + spin_unlock_irq(lock); - while (sig->notify_count) { - __set_current_state(TASK_KILLABLE); - spin_unlock_irq(lock); + if (unlikely(sig->unsafe_execve_in_progress)) + mutex_unlock(&sig->cred_guard_mutex); + + for (;;) { + set_current_state(TASK_KILLABLE); + if (!sig->notify_count) + break; schedule(); if (__fatal_signal_pending(tsk)) goto killed; - spin_lock_irq(lock); } - spin_unlock_irq(lock); + __set_current_state(TASK_RUNNING); + + if (unlikely(sig->unsafe_execve_in_progress)) { + if (mutex_lock_killable(&sig->cred_guard_mutex)) + goto killed; + sig->unsafe_execve_in_progress = false; + if (!prev_ptrace && tsk->ptrace) { + sig->group_exit_task = NULL; + sig->notify_count = 0; + return -ERESTARTSYS; + } + } /* * At this point all other threads have exited, all we have to @@ -1255,8 +1277,11 @@ int begin_new_exec(struct linux_binprm * bprm) * Make this the only thread in the thread group. */ retval = de_thread(me); - if (retval) + if (retval) { + if (retval == -ERESTARTSYS) + bprm->point_of_no_return = false; goto out; + } /* * Cancel any io_uring activity across execve @@ -1466,6 +1491,11 @@ static int prepare_bprm_creds(struct linux_binprm *bprm) if (mutex_lock_interruptible(¤t->signal->cred_guard_mutex)) return -ERESTARTNOINTR; + if (unlikely(current->signal->unsafe_execve_in_progress)) { + mutex_unlock(¤t->signal->cred_guard_mutex); + return -ERESTARTNOINTR; + } + bprm->cred = prepare_exec_creds(); if (likely(bprm->cred)) return 0; @@ -1482,7 +1512,8 @@ static void free_bprm(struct linux_binprm *bprm) } free_arg_pages(bprm); if (bprm->cred) { - mutex_unlock(¤t->signal->cred_guard_mutex); + if (!current->signal->unsafe_execve_in_progress) + mutex_unlock(¤t->signal->cred_guard_mutex); abort_creds(bprm->cred); } if (bprm->file) { diff --git a/fs/proc/base.c b/fs/proc/base.c index 3851bfc..3b2a55c 100644 --- a/fs/proc/base.c +++ b/fs/proc/base.c @@ -2739,6 +2739,12 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf, if (rv < 0) goto out_free; + if (unlikely(current->signal->unsafe_execve_in_progress)) { + mutex_unlock(¤t->signal->cred_guard_mutex); + rv = -ERESTARTNOINTR; + goto out_free; + } + rv = security_setprocattr(PROC_I(inode)->op.lsm, file->f_path.dentry->d_name.name, page, count); diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 3f6a0fc..220a083 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -214,6 +214,17 @@ struct signal_struct { #endif /* + * Set while execve is executing but is *not* holding + * cred_guard_mutex to avoid possible dead-locks. + * The cred_guard_mutex is released *after* de_thread() has + * called zap_other_threads(), therefore a fatal signal is + * guaranteed to be already pending in the unlikely event, that + * current->signal->unsafe_execve_in_progress happens to be + * true after the cred_guard_mutex was acquired. + */ + bool unsafe_execve_in_progress; + + /* * Thread is the potential origin of an oom condition; kill first on * oom */ @@ -227,6 +238,8 @@ struct signal_struct { struct mutex cred_guard_mutex; /* guard against foreign influences on * credential calculations * (notably. ptrace) + * Held while execve runs, except when + * a sibling thread is being traced. * Deprecated do not use in new code. * Use exec_update_lock instead. */ diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 61db50f..0cbc1eb 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -468,6 +468,14 @@ static int ptrace_traceme(void) { int ret = -EPERM; + if (mutex_lock_interruptible(¤t->signal->cred_guard_mutex)) + return -ERESTARTNOINTR; + + if (unlikely(current->signal->unsafe_execve_in_progress)) { + mutex_unlock(¤t->signal->cred_guard_mutex); + return -ERESTARTNOINTR; + } + write_lock_irq(&tasklist_lock); /* Are we already being traced? */ if (!current->ptrace) { @@ -483,6 +491,7 @@ static int ptrace_traceme(void) } } write_unlock_irq(&tasklist_lock); + mutex_unlock(¤t->signal->cred_guard_mutex); return ret; } diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 1d60fc2..b1389ee 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -1824,9 +1824,15 @@ static long seccomp_set_mode_filter(unsigned int flags, * Make sure we cannot change seccomp or nnp state via TSYNC * while another thread is in the middle of calling exec. */ - if (flags & SECCOMP_FILTER_FLAG_TSYNC && - mutex_lock_killable(¤t->signal->cred_guard_mutex)) - goto out_put_fd; + if (flags & SECCOMP_FILTER_FLAG_TSYNC) { + if (mutex_lock_killable(¤t->signal->cred_guard_mutex)) + goto out_put_fd; + + if (unlikely(current->signal->unsafe_execve_in_progress)) { + mutex_unlock(¤t->signal->cred_guard_mutex); + goto out_put_fd; + } + } spin_lock_irq(¤t->sighand->siglock); diff --git a/tools/testing/selftests/ptrace/vmaccess.c b/tools/testing/selftests/ptrace/vmaccess.c index 4db327b..c7c2242 100644 --- a/tools/testing/selftests/ptrace/vmaccess.c +++ b/tools/testing/selftests/ptrace/vmaccess.c @@ -39,8 +39,15 @@ static void *thread(void *arg) f = open(mm, O_RDONLY); ASSERT_GE(f, 0); close(f); - f = kill(pid, SIGCONT); - ASSERT_EQ(f, 0); + f = waitpid(-1, NULL, 0); + ASSERT_NE(f, -1); + ASSERT_NE(f, 0); + ASSERT_NE(f, pid); + f = waitpid(-1, NULL, 0); + ASSERT_EQ(f, pid); + f = waitpid(-1, NULL, 0); + ASSERT_EQ(f, -1); + ASSERT_EQ(errno, ECHILD); } TEST(attach) @@ -57,22 +64,24 @@ static void *thread(void *arg) sleep(1); k = ptrace(PTRACE_ATTACH, pid, 0L, 0L); - ASSERT_EQ(errno, EAGAIN); - ASSERT_EQ(k, -1); + ASSERT_EQ(k, 0); k = waitpid(-1, &s, WNOHANG); ASSERT_NE(k, -1); ASSERT_NE(k, 0); ASSERT_NE(k, pid); ASSERT_EQ(WIFEXITED(s), 1); ASSERT_EQ(WEXITSTATUS(s), 0); - sleep(1); - k = ptrace(PTRACE_ATTACH, pid, 0L, 0L); - ASSERT_EQ(k, 0); k = waitpid(-1, &s, 0); ASSERT_EQ(k, pid); ASSERT_EQ(WIFSTOPPED(s), 1); ASSERT_EQ(WSTOPSIG(s), SIGSTOP); - k = ptrace(PTRACE_DETACH, pid, 0L, 0L); + k = ptrace(PTRACE_CONT, pid, 0L, 0L); + ASSERT_EQ(k, 0); + k = waitpid(-1, &s, 0); + ASSERT_EQ(k, pid); + ASSERT_EQ(WIFSTOPPED(s), 1); + ASSERT_EQ(WSTOPSIG(s), SIGTRAP); + k = ptrace(PTRACE_CONT, pid, 0L, 0L); ASSERT_EQ(k, 0); k = waitpid(-1, &s, 0); ASSERT_EQ(k, pid);