Thanks for your review.
On 6/14/21 6:42 PM, Eric W. Biederman wrote:
> Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> writes:
>
>> 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.
>
> Userspace processes hang waiting for each other. Not a proper kernel
> deadlock. Annoying but not horrible. Definitely worth fixing if we can.
>
I wonder if I am used a wrong term in the title.
Do you have a suggestion for better wording?
>> 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.
>
> Except you are not detecting this situation. Testing for t->ptrace
> finds tasks that have completed their ptrace attach and no longer need
> the cred_gaurd_mutex.
>
The first phase of de_thread needs co-operation from a user task,
if and only if any task t except the thread leader has t->ptrace.
Taking tasks from RUNNING->EXIT_ZOMBIE only needs co-operation from kernel code,
that is using mutex_wait_killable(&sig->cred_guard_mutex).
Tasks with !t->ptrace are childs of the thread leader, and are automatically
sent to EXIT_DEAD, see kernel/exit.c (exit_notify):
if (unlikely(tsk->ptrace)) {
[...]
} else if (thread_group_leader(tsk)) {
autoreap = thread_group_empty(tsk) &&
do_notify_parent(tsk, tsk->exit_signal);
} else {
autoreap = true;
}
if (autoreap) {
tsk->exit_state = EXIT_DEAD;
list_add(&tsk->ptrace_entry, &dead);
}
But tasks which are traced have a different parent, and will stay ZOMBIES
for as long as the tracer does not call waitpid or handle the SIGCHILD.
> You almost discover the related problem that involves PTRACE_EVENT_EXEC.
>
> It will probably help to have a full description of all of the
> processes and states involved in the hang in your description
> so you can show how your proposed change avoids the problem.
>
Ok, will try to do that.
>> 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.
>
> No. If you have killed other threads we are most definitely past the
> point where it is at all reasonable to return to userspace.
> Perfunctorily killing other threads may leave them with locks held and
> who knows what other problems. Certainly it leaves the application
> unable to process a failure from exec and continue on.
>
Yeah, I tend to agree. I had assumed that returning -ERESTARTSYS will always
bounce-back to the same execve syscall, and that the restarted execve call
must succeed. But especially the second assumption is not a given thing.
I wonder if that might work instead?
diff --git a/fs/exec.c b/fs/exec.c
index c7b1926..4490288 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1956,6 +1956,13 @@ static int do_execveat_common(int fd, struct filename *filename,
out_free:
free_bprm(bprm);
+ if (retval == -ERESTARTSYS && !fatal_signal_pending(current)) {
+ retval = do_execveat_common(fd, filename, argv, envp, flags);
+ if (retval < 0 && !fatal_signal_pending(current))
+ force_sigsegv(SIGSEGV);
+ return retval;
+ }
+
out_ret:
putname(filename);
return retval;
diff --git a/tools/testing/selftests/ptrace/vmaccess.c b/tools/testing/selftests/ptrace/vmaccess.c
index c7c2242..3b7d81fb 100644
--- a/tools/testing/selftests/ptrace/vmaccess.c
+++ b/tools/testing/selftests/ptrace/vmaccess.c
@@ -74,13 +74,13 @@ static void *thread(void *arg)
k = waitpid(-1, &s, 0);
ASSERT_EQ(k, pid);
ASSERT_EQ(WIFSTOPPED(s), 1);
- ASSERT_EQ(WSTOPSIG(s), SIGSTOP);
+ 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);
ASSERT_EQ(WIFSTOPPED(s), 1);
- ASSERT_EQ(WSTOPSIG(s), SIGTRAP);
+ ASSERT_EQ(WSTOPSIG(s), SIGSTOP);
k = ptrace(PTRACE_CONT, pid, 0L, 0L);
ASSERT_EQ(k, 0);
k = waitpid(-1, &s, 0);
Since the SIGSTOP from the PTRACE_ACCESS is not handled before the restart,
the SIGSTOP and SIGTRAP events in the test case are in reversed order, but
that is a possible outcome too.
>> From the user's point of view the whole execve was
>> simply delayed until after the ptrace_attach.
>
> Conceptually I like what you are trying to detect and do.
> However your description unfortunately does not match the code.
>
> If you can find a test for another process waiting to ptrace_attach
> one of our threads before we enter into de_thread that would be a
> reasonable time to do something, and would potentially make a nice
> fix.
>
No I don't see any way how to do that.
Unfortunately the tracer may or may not decide to do the ptrace_attach
at any time, and it is usually the same process that is unable to do
the waitpid because it is hanging in the ptrace_attach.
Bernd.
>
> Eric
>
>> 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@xxxxxxxxxx/
>>
>> 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 <bernd.edlinger@xxxxxxxxxx>
>> ---
>> fs/exec.c | 37 +++++++++++++++++++++++++++++--
>> 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, 89 insertions(+), 13 deletions(-)
>>
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 8344fba..c7b1926 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;
>> @@ -1062,6 +1064,17 @@ static int de_thread(struct task_struct *tsk)
>> if (!thread_group_leader(tsk))
>> sig->notify_count--;
>>
>> + while_each_thread(tsk, t) {
>> + if (unlikely(t->ptrace) && t != tsk->group_leader)
>> + sig->unsafe_execve_in_progress = true;
>> + }
>> +
>> + if (unlikely(sig->unsafe_execve_in_progress)) {
>> + spin_unlock_irq(lock);
>> + mutex_unlock(&sig->cred_guard_mutex);
>> + spin_lock_irq(lock);
>> + }
>> +
>> while (sig->notify_count) {
>> __set_current_state(TASK_KILLABLE);
>> spin_unlock_irq(lock);
>> @@ -1072,6 +1085,17 @@ static int de_thread(struct task_struct *tsk)
>> }
>> spin_unlock_irq(lock);
>>
>> + 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
>> * do is to wait for the thread group leader to become inactive,
>> @@ -1255,8 +1279,11 @@ 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 +1493,11 @@ 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 +1514,8 @@ 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);
