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 have a second mutex that is used in mm_access, so it is allowed to continue while the dying threads are not yet terminated.
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 | 18 ++++++------ fs/exec.c | 9 ++++++ include/linux/binfmts.h | 6 +++- include/linux/sched/signal.h | 1 + init/init_task.c | 1 + kernel/cred.c | 2 +- kernel/fork.c | 5 ++-- mm/process_vm_access.c | 2 +- tools/testing/selftests/ptrace/Makefile | 4 +-- tools/testing/selftests/ptrace/vmaccess.c | 46 +++++++++++++++++++++++++++++++ 10 files changed, 79 insertions(+), 15 deletions(-) create mode 100644 tools/testing/selftests/ptrace/vmaccess.c
v2: adds a test case which passes when this patch is applied.
diff --git a/Documentation/security/credentials.rst b/Documentation/security/credentials.rst index 282e79f..c98e0a8 100644 --- a/Documentation/security/credentials.rst +++ b/Documentation/security/credentials.rst @@ -437,9 +437,13 @@ 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 the mutex +current->signal->cred_change_mutex is acquired later, while the credentials +and the process mmap are actually changed.
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 +470,8 @@ 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 +489,7 @@ 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..a6884e4 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(¤t->signal->cred_change_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 @@ -1420,6 +1426,8 @@ static void free_bprm(struct linux_binprm *bprm) { free_arg_pages(bprm); if (bprm->cred) { + if (bprm->called_flush_old_exec) + mutex_unlock(¤t->signal->cred_change_mutex); mutex_unlock(¤t->signal->cred_guard_mutex); abort_creds(bprm->cred); } @@ -1469,6 +1477,7 @@ 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->cred_change_mutex); mutex_unlock(¤t->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..37eeabe 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) */ + struct mutex cred_change_mutex; /* guard against credentials change */ } __randomize_layout;
/* diff --git a/init/init_task.c b/init/init_task.c index 9e5cbe5..6cd9a0f 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_change_mutex = __MUTEX_INITIALIZER(init_signals.cred_change_mutex), #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..0395154 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) struct mm_struct *mm; int err;
- err = mutex_lock_killable(&task->signal->cred_guard_mutex); + err = mutex_lock_killable(&task->signal->cred_change_mutex); if (err) return ERR_PTR(err);
@@ -1234,7 +1234,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) mmput(mm); mm = ERR_PTR(-EACCES); } - mutex_unlock(&task->signal->cred_guard_mutex); + mutex_unlock(&task->signal->cred_change_mutex);
return mm; } @@ -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->cred_change_mutex);
return 0; } 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..ef08c9f --- /dev/null +++ b/tools/testing/selftests/ptrace/vmaccess.c @@ -0,0 +1,46 @@ +// 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, 0, 0); + 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); + /* this is not fixed! ptrace(PTRACE_ATTACH, pid, 0,0); */ + f = kill(pid, SIGCONT); + ASSERT_EQ(0, f); +} + +TEST_HARNESS_MAIN