This fixes a deadlock in the tracer when tracing a multi-threaded application that calls execve while more than one thread are running.

I observed that when running strace on the gcc test suite, it always blocks after a while, when expect calls execve, because other threads have to be terminated. They send ptrace events, but the strace is no longer able to respond, since it is blocked in vm_access.

The deadlock is always happening when strace needs to access the tracees process mmap, while another thread in the tracee starts to execve a child process, but that cannot continue until the PTRACE_EVENT_EXIT is handled and the WIFEXITED event is received:

strace D 0 30614 30584 0x00000000 Call Trace: __schedule+0x3ce/0x6e0 schedule+0x5c/0xd0 schedule_preempt_disabled+0x15/0x20 __mutex_lock.isra.13+0x1ec/0x520 __mutex_lock_killable_slowpath+0x13/0x20 mutex_lock_killable+0x28/0x30 mm_access+0x27/0xa0 process_vm_rw_core.isra.3+0xff/0x550 process_vm_rw+0xdd/0xf0 __x64_sys_process_vm_readv+0x31/0x40 do_syscall_64+0x64/0x220 entry_SYSCALL_64_after_hwframe+0x44/0xa9

expect D 0 31933 30876 0x80004003 Call Trace: __schedule+0x3ce/0x6e0 schedule+0x5c/0xd0 flush_old_exec+0xc4/0x770 load_elf_binary+0x35a/0x16c0 search_binary_handler+0x97/0x1d0 __do_execve_file.isra.40+0x5d4/0x8a0 __x64_sys_execve+0x49/0x60 do_syscall_64+0x64/0x220 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The proposed solution is to take the cred_guard_mutex only in a critical section at the beginning, and at the end of the execve function, and let PTRACE_ATTACH fail with EAGAIN while execve is not complete, but other functions like vm_access are allowed to complete normally.

I also took the opportunity to improve the documentation of prepare_creds, which is obviously out of sync.

Signed-off-by: Bernd Edlinger bernd.edlinger@hotmail.de --- Documentation/security/credentials.rst | 19 +++++---- fs/exec.c | 28 ++++++++++++-- include/linux/binfmts.h | 6 ++- include/linux/sched/signal.h | 1 + init/init_task.c | 1 + kernel/cred.c | 2 +- kernel/fork.c | 1 + kernel/ptrace.c | 4 ++ mm/process_vm_access.c | 2 +- tools/testing/selftests/ptrace/Makefile | 4 +- tools/testing/selftests/ptrace/vmaccess.c | 64 +++++++++++++++++++++++++++++++ 11 files changed, 115 insertions(+), 17 deletions(-) create mode 100644 tools/testing/selftests/ptrace/vmaccess.c

v2: adds a test case which passes when this patch is applied. v3: fixes the issue without introducing a new mutex.

diff --git a/Documentation/security/credentials.rst b/Documentation/security/credentials.rst index 282e79f..61d6704 100644 --- a/Documentation/security/credentials.rst +++ b/Documentation/security/credentials.rst @@ -437,9 +437,14 @@ new set of credentials by calling::

struct cred *prepare_creds(void);

-this locks current->cred_replace_mutex and then allocates and constructs a -duplicate of the current process's credentials, returning with the mutex still -held if successful. It returns NULL if not successful (out of memory). +this allocates and constructs a duplicate of the current process's credentials. +It returns NULL if not successful (out of memory). + +If called from __do_execve_file, the mutex current->signal->cred_guard_mutex +is acquired before this function gets called, and released after setting +current->signal->cred_locked_for_ptrace. The same mutex is acquired later, +while the credentials and the process mmap are actually changed, and +current->signal->cred_locked_for_ptrace is reset again.

The mutex prevents ``ptrace()`` from altering the ptrace state of a process while security checks on credentials construction and changing is taking place @@ -466,9 +471,8 @@ by calling::

This will alter various aspects of the credentials and the process, giving the LSM a chance to do likewise, then it will use ``rcu_assign_pointer()`` to -actually commit the new credentials to ``current->cred``, it will release -``current->cred_replace_mutex`` to allow ``ptrace()`` to take place, and it -will notify the scheduler and others of the changes. +actually commit the new credentials to ``current->cred``, and it will notify +the scheduler and others of the changes.

This function is guaranteed to return 0, so that it can be tail-called at the end of such functions as ``sys_setresuid()``. @@ -486,8 +490,7 @@ invoked::

void abort_creds(struct cred *new);

-This releases the lock on ``current->cred_replace_mutex`` that -``prepare_creds()`` got and then releases the new credentials. +This releases the new credentials.

A typical credentials alteration function would look something like this:: diff --git a/fs/exec.c b/fs/exec.c index 74d88da..e466301 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1266,6 +1266,12 @@ int flush_old_exec(struct linux_binprm * bprm) if (retval) goto out;

+ retval = mutex_lock_killable(&current->signal->cred_guard_mutex); + if (retval) + goto out; + + bprm->called_flush_old_exec = 1; + /* * Must be called _before_ exec_mmap() as bprm->mm is * not visibile until then. This also enables the update @@ -1398,28 +1404,41 @@ void finalize_exec(struct linux_binprm *bprm) EXPORT_SYMBOL(finalize_exec);

/* - * Prepare credentials and lock ->cred_guard_mutex. + * Prepare credentials and set ->cred_locked_for_ptrace. * 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) { + int ret; + if (mutex_lock_interruptible(&current->signal->cred_guard_mutex)) return -ERESTARTNOINTR;

+ ret = -EAGAIN; + if (unlikely(current->signal->cred_locked_for_ptrace)) + goto out; + + ret = -ENOMEM; bprm->cred = prepare_exec_creds(); - if (likely(bprm->cred)) - return 0; + if (likely(bprm->cred)) { + current->signal->cred_locked_for_ptrace = true; + ret = 0; + }

+out: mutex_unlock(&current->signal->cred_guard_mutex); - return -ENOMEM; + return ret; }

static void free_bprm(struct linux_binprm *bprm) { free_arg_pages(bprm); if (bprm->cred) { + if (!bprm->called_flush_old_exec) + mutex_lock(&current->signal->cred_guard_mutex); + current->signal->cred_locked_for_ptrace = false; mutex_unlock(&current->signal->cred_guard_mutex); abort_creds(bprm->cred); } @@ -1469,6 +1488,7 @@ void install_exec_creds(struct linux_binprm *bprm) * credentials; any time after this it may be unlocked. */ security_bprm_committed_creds(bprm); + current->signal->cred_locked_for_ptrace = false; mutex_unlock(&current->signal->cred_guard_mutex); } EXPORT_SYMBOL(install_exec_creds); diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h index b40fc63..2e1318b 100644 --- a/include/linux/binfmts.h +++ b/include/linux/binfmts.h @@ -44,7 +44,11 @@ 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 the cred_change_mutex is taken. + */ + called_flush_old_exec:1; #ifdef __alpha__ unsigned int taso:1; #endif diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 8805025..073a2b7 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -225,6 +225,7 @@ struct signal_struct { struct mutex cred_guard_mutex; /* guard against foreign influences on * credential calculations * (notably. ptrace) */ + bool cred_locked_for_ptrace; /* set while in execve */ } __randomize_layout;

/* diff --git a/init/init_task.c b/init/init_task.c index 9e5cbe5..ecefff28 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), + .cred_locked_for_ptrace = false, #ifdef CONFIG_POSIX_TIMERS .posix_timers = LIST_HEAD_INIT(init_signals.posix_timers), .cputimer = { diff --git a/kernel/cred.c b/kernel/cred.c index 809a985..e4c78de 100644 --- a/kernel/cred.c +++ b/kernel/cred.c @@ -676,7 +676,7 @@ void __init cred_init(void) * * Returns the new credentials or NULL if out of memory. * - * Does not take, and does not return holding current->cred_replace_mutex. + * Does not take, and does not return holding ->cred_guard_mutex. */ struct cred *prepare_kernel_cred(struct task_struct *daemon) { diff --git a/kernel/fork.c b/kernel/fork.c index 0808095..a2b2ec8 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); + sig->cred_locked_for_ptrace = false;

return 0; } diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 43d6179..abf09ba 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -395,6 +395,10 @@ static int ptrace_attach(struct task_struct *task, long request, if (mutex_lock_interruptible(&task->signal->cred_guard_mutex)) goto out;

+ retval = -EAGAIN; + if (task->signal->cred_locked_for_ptrace) + goto unlock_creds; + task_lock(task); retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS); task_unlock(task); diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c index 357aa7b..b3e6eb5 100644 --- a/mm/process_vm_access.c +++ b/mm/process_vm_access.c @@ -204,7 +204,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, if (!mm || IS_ERR(mm)) { rc = IS_ERR(mm) ? PTR_ERR(mm) : -ESRCH; /* - * Explicitly map EACCES to EPERM as EPERM is a more a + * Explicitly map EACCES to EPERM as EPERM is a more * appropriate error code for process_vw_readv/writev */ if (rc == -EACCES) diff --git a/tools/testing/selftests/ptrace/Makefile b/tools/testing/selftests/ptrace/Makefile index c0b7f89..2f1f532 100644 --- a/tools/testing/selftests/ptrace/Makefile +++ b/tools/testing/selftests/ptrace/Makefile @@ -1,6 +1,6 @@ # SPDX-License-Identifier: GPL-2.0-only -CFLAGS += -iquote../../../../include/uapi -Wall +CFLAGS += -std=c99 -pthread -iquote../../../../include/uapi -Wall

-TEST_GEN_PROGS := get_syscall_info peeksiginfo +TEST_GEN_PROGS := get_syscall_info peeksiginfo vmaccess

include ../lib.mk diff --git a/tools/testing/selftests/ptrace/vmaccess.c b/tools/testing/selftests/ptrace/vmaccess.c new file mode 100644 index 0000000..63ff531 --- /dev/null +++ b/tools/testing/selftests/ptrace/vmaccess.c @@ -0,0 +1,64 @@ +// SPDX-License-Identifier: GPL-2.0+ +/* + * Copyright (c) 2020 Bernd Edlinger bernd.edlinger@hotmail.de + * All rights reserved. + * + * Check whether /proc/$pid/mem can be accessed without causing deadlocks + * when de_thread is blocked with ->cred_guard_mutex held. + */ + +#include "../kselftest_harness.h" +#include <stdio.h> +#include <fcntl.h> +#include <pthread.h> +#include <signal.h> +#include <unistd.h> +#include <sys/ptrace.h> + +static void *thread(void *arg) +{ + ptrace(PTRACE_TRACEME, 0, 0L, 0L); + return NULL; +} + +TEST(vmaccess) +{ + int f, pid = fork(); + char mm[64]; + + if (!pid) { + pthread_t pt; + pthread_create(&pt, NULL, thread, NULL); + pthread_join(pt, NULL); + execlp("true", "true", NULL); + } + + sleep(1); + sprintf(mm, "/proc/%d/mem", pid); + f = open(mm, O_RDONLY); + ASSERT_LE(0, f); + close(f); + f = kill(pid, SIGCONT); + ASSERT_EQ(0, f); +} + +TEST(attach) +{ + int f, pid = fork(); + + if (!pid) { + pthread_t pt; + pthread_create(&pt, NULL, thread, NULL); + pthread_join(pt, NULL); + execlp("true", "true", NULL); + } + + sleep(1); + f = ptrace(PTRACE_ATTACH, pid, 0L, 0L); + ASSERT_EQ(EAGAIN, errno); + ASSERT_EQ(f, -1); + f = kill(pid, SIGCONT); + ASSERT_EQ(0, f); +} + +TEST_HARNESS_MAIN