* Re: tools/testing/selftests/ptrace/vmaccess.c hangs
2021-05-28 17:20 tools/testing/selftests/ptrace/vmaccess.c hangs Vitaly Chikunov
@ 2021-05-30 19:21 ` Bernd Edlinger
0 siblings, 0 replies; 2+ messages in thread
From: Bernd Edlinger @ 2021-05-30 19:21 UTC (permalink / raw)
To: Vitaly Chikunov, Eric W. Biederman, linux-kselftest, Oleg Nesterov
Cc: Kees Cook, Shuah Khan, Dmitry V. Levin
[-- Attachment #1: Type: text/plain, Size: 1640 bytes --]
On 5/28/21 7:20 PM, Vitaly Chikunov wrote:
> Hi,
>
> Possible ptrace related bug in tools/testing/selftests/ptrace/vmaccess.c -
> it timeout every time I run it on any kernel I try, in vm or bare metal.
>
> Does it run correctly for anybody?
>
No, that is an open issue, I wrote the test case in anticipation that all of
my patch series got accepted, but the last patch was not picked up for inclusion
in the linux kernel.
Well, actually I still have the patch, see attached patch file. This makes
the test case pass.
The idea is that PTRACE_ATTACH returns EAGAIN in this case, and the tracer
calls waitpid if that return value is received, which makes the zombie process die,
and then PTRACE_ATTACH will succeed.
Eric wanted to propose a different solution though.
Eric, have you meanwhile been able to come up with a better solution?
Thanks,
Bernd.
> Example vmaccess from v5.12 source tree run on 5.13.0+rc2 on bare metal
> Supermicro server (AMD EPYC):
>
> 5.10.35-rt-alt1.rt39:~# /usr/lib/kselftests/ptrace/vmaccess
> TAP version 13
> 1..2
> # Starting 2 tests from 1 test cases.
> # RUN global.vmaccess ...
> # OK global.vmaccess
> ok 1 global.vmaccess
> # RUN global.attach ...
> # attach: Test terminated by timeout
> # FAIL global.attach
> not ok 2 global.attach
> # FAILED: 1 / 2 tests passed.
> # Totals: pass:1 fail:1 xfail:0 xpass:0 skip:0 error:0
>
> Just to confirm for the latest kernel, it behaves the same [for drm-tip
> based] on 5.13rc2. Also, just tested on 5.11.21 with the same failure.
>
> Other ptrace tests pass.
>
> Thanks,
>
[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: 0001-exec-Fix-dead-lock-in-de_thread-with-ptrace_attach.patch --]
[-- Type: text/x-patch; name="0001-exec-Fix-dead-lock-in-de_thread-with-ptrace_attach.patch", Size: 13974 bytes --]
From c5afacba607c38f4c9d9bf106a071f9737ba1479 Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
Date: Tue, 10 Mar 2020 13:20:57 +0100
Subject: [PATCH 1/2] exec: Fix dead-lock in de_thread with ptrace_attach
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 <bernd.edlinger@hotmail.de>
---
fs/exec.c | 42 ++++++++++++++++++++++++++++++++++--------
fs/proc/base.c | 20 ++++++++++++++++++--
include/linux/sched/signal.h | 13 ++++++++++---
init/init_task.c | 2 +-
kernel/cred.c | 2 +-
kernel/fork.c | 2 +-
kernel/ptrace.c | 44 +++++++++++++++++++++++++++++++++++++++++---
kernel/seccomp.c | 25 +++++++++++++++++++------
8 files changed, 125 insertions(+), 25 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index 18594f1..6ab93c0 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1040,14 +1040,26 @@ 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
@@ -1438,7 +1450,10 @@ void setup_new_exec(struct linux_binprm * bprm)
*/
me->mm->task_size = TASK_SIZE;
up_write(&me->signal->exec_update_lock);
- mutex_unlock(&me->signal->cred_guard_mutex);
+ if (unlikely(me->signal->unsafe_execve_in_progress))
+ mutex_lock(&me->signal->exec_guard_mutex);
+ current->signal->unsafe_execve_in_progress = false;
+ mutex_unlock(&me->signal->exec_guard_mutex);
}
EXPORT_SYMBOL(setup_new_exec);
@@ -1453,22 +1468,30 @@ 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.
* setup_new_exec() commits the new creds and drops the lock.
* Or, if exec fails before, free_bprm() should release ->cred
* 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 = -EBUSY;
+ 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)
@@ -1479,7 +1502,10 @@ static void free_bprm(struct linux_binprm *bprm)
}
free_arg_pages(bprm);
if (bprm->cred) {
- 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) {
@@ -1542,7 +1568,7 @@ int bprm_change_interp(const char *interp, struct linux_binprm *bprm)
/*
* 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 3851bfc..fe2833c 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2735,14 +2735,30 @@ 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 -EBUSY in that case and
+ * unlock the mutex immediately.
+ */
+ rv = -EBUSY;
+ 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 3f6a0fc..407545a 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -214,6 +214,13 @@ 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
*/
@@ -224,11 +231,11 @@ 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
+ struct mutex exec_guard_mutex; /* guard against foreign influences on
* credential calculations
* (notably. ptrace)
- * Deprecated do not use in new code.
- * Use exec_update_lock instead.
+ * Held while execve runs, except when
+ * a sibling thread is being traced.
*/
struct rw_semaphore exec_update_lock; /* Held while task_struct is
* being updated during exec,
diff --git a/init/init_task.c b/init/init_task.c
index 3711cda..b917349 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_lock = __RWSEM_INITIALIZER(init_signals.exec_update_lock),
#ifdef CONFIG_POSIX_TIMERS
.posix_timers = LIST_HEAD_INIT(init_signals.posix_timers),
diff --git a/kernel/cred.c b/kernel/cred.c
index 421b114..f408024 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 426cd0c..5602226 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1601,7 +1601,7 @@ 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);
init_rwsem(&sig->exec_update_lock);
return 0;
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 61db50f..e052586 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -386,9 +386,26 @@ 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.
+ * The tracer is expected to call wait(2) and handle
+ * possible events before calling this API again.
+ */
+ 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);
@@ -441,7 +458,7 @@ 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) {
/*
@@ -466,10 +483,29 @@ 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);
/*
@@ -484,6 +520,8 @@ 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 1d60fc2..9292b25 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -466,7 +466,7 @@ 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
@@ -476,9 +476,22 @@ 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) {
@@ -564,7 +577,7 @@ void seccomp_filter_release(struct task_struct *tsk)
/**
* 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.
*
@@ -573,7 +586,7 @@ 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. */
@@ -1825,7 +1838,7 @@ 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);
@@ -1848,7 +1861,7 @@ 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) {
--
1.9.1
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