linux.git/kernel/ptrace.c, branch v6.6.132

ptrace: Provide set/get interface for syscall user dispatch

2023-04-16T12:23:07+00:00

The syscall user dispatch configuration can only be set by the task itself,
but lacks a ptrace set/get interface which makes it impossible to implement
checkpoint/restore for it.

Add the required ptrace requests and the get/set functions in the syscall
user dispatch code to make that possible.

Signed-off-by: Gregory Price 
Signed-off-by: Thomas Gleixner 
Reviewed-by: Oleg Nesterov 
Link: https://lore.kernel.org/r/20230407171834.3558-4-gregory.price@memverge.com

rseq: Introduce extensible rseq ABI

2022-12-27T11:52:10+00:00

Introduce the extensible rseq ABI, where the feature size supported by
the kernel and the required alignment are communicated to user-space
through ELF auxiliary vectors.

This allows user-space to call rseq registration with a rseq_len of
either 32 bytes for the original struct rseq size (which includes
padding), or larger.

If rseq_len is larger than 32 bytes, then it must be large enough to
contain the feature size communicated to user-space through ELF
auxiliary vectors.

Signed-off-by: Mathieu Desnoyers 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lore.kernel.org/r/20221122203932.231377-4-mathieu.desnoyers@efficios.com

freezer,sched: Rewrite core freezer logic

2022-09-07T19:53:50+00:00

Rewrite the core freezer to behave better wrt thawing and be simpler
in general.

By replacing PF_FROZEN with TASK_FROZEN, a special block state, it is
ensured frozen tasks stay frozen until thawed and don't randomly wake
up early, as is currently possible.

As such, it does away with PF_FROZEN and PF_FREEZER_SKIP, freeing up
two PF_flags (yay!).

Specifically; the current scheme works a little like:

	freezer_do_not_count();
	schedule();
	freezer_count();

And either the task is blocked, or it lands in try_to_freezer()
through freezer_count(). Now, when it is blocked, the freezer
considers it frozen and continues.

However, on thawing, once pm_freezing is cleared, freezer_count()
stops working, and any random/spurious wakeup will let a task run
before its time.

That is, thawing tries to thaw things in explicit order; kernel
threads and workqueues before doing bringing SMP back before userspace
etc.. However due to the above mentioned races it is entirely possible
for userspace tasks to thaw (by accident) before SMP is back.

This can be a fatal problem in asymmetric ISA architectures (eg ARMv9)
where the userspace task requires a special CPU to run.

As said; replace this with a special task state TASK_FROZEN and add
the following state transitions:

	TASK_FREEZABLE	-> TASK_FROZEN
	__TASK_STOPPED	-> TASK_FROZEN
	__TASK_TRACED	-> TASK_FROZEN

The new TASK_FREEZABLE can be set on any state part of TASK_NORMAL
(IOW. TASK_INTERRUPTIBLE and TASK_UNINTERRUPTIBLE) -- any such state
is already required to deal with spurious wakeups and the freezer
causes one such when thawing the task (since the original state is
lost).

The special __TASK_{STOPPED,TRACED} states *can* be restored since
their canonical state is in ->jobctl.

With this, frozen tasks need an explicit TASK_FROZEN wakeup and are
free of undue (early / spurious) wakeups.

Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Ingo Molnar 
Acked-by: Rafael J. Wysocki 
Link: https://lore.kernel.org/r/20220822114649.055452969@infradead.org

ptrace: fix clearing of JOBCTL_TRACED in ptrace_unfreeze_traced()

2022-07-09T18:06:19+00:00

CI reported the following splat while running the strace testsuite:

  WARNING: CPU: 1 PID: 3570031 at kernel/ptrace.c:272 ptrace_check_attach+0x12e/0x178
  CPU: 1 PID: 3570031 Comm: strace Tainted: G           OE     5.19.0-20220624.rc3.git0.ee819a77d4e7.300.fc36.s390x #1
  Hardware name: IBM 3906 M04 704 (z/VM 7.1.0)
  Call Trace:
   [<00000000ab4b645a>] ptrace_check_attach+0x132/0x178
  ([<00000000ab4b6450>] ptrace_check_attach+0x128/0x178)
   [<00000000ab4b6cde>] __s390x_sys_ptrace+0x86/0x160
   [<00000000ac03fcec>] __do_syscall+0x1d4/0x200
   [<00000000ac04e312>] system_call+0x82/0xb0
  Last Breaking-Event-Address:
   [<00000000ab4ea3c8>] wait_task_inactive+0x98/0x190

This is because JOBCTL_TRACED is set, but the task is not in TASK_TRACED
state. Caused by ptrace_unfreeze_traced() which does:

	task->jobctl &= ~TASK_TRACED

but it should be:

	task->jobctl &= ~JOBCTL_TRACED

Fixes: 31cae1eaae4f ("sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state")
Signed-off-by: Sven Schnelle 
Tested-by: Alexander Gordeev 
Acked-by: Oleg Nesterov 
Acked-by: Peter Zijlstra 
Cc: Eric Biederman 
Cc: Steven Rostedt 
Cc: Kees Cook 
Signed-off-by: Linus Torvalds

Merge tag 'ptrace_stop-cleanup-for-v5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace

2022-06-03T23:13:25+00:00

Pull ptrace_stop cleanups from Eric Biederman:
 "While looking at the ptrace problems with PREEMPT_RT and the problems
  Peter Zijlstra was encountering with ptrace in his freezer rewrite I
  identified some cleanups to ptrace_stop that make sense on their own
  and move make resolving the other problems much simpler.

  The biggest issue is the habit of the ptrace code to change
  task->__state from the tracer to suppress TASK_WAKEKILL from waking up
  the tracee. No other code in the kernel does that and it is straight
  forward to update signal_wake_up and friends to make that unnecessary.

  Peter's task freezer sets frozen tasks to a new state TASK_FROZEN and
  then it stores them by calling "wake_up_state(t, TASK_FROZEN)" relying
  on the fact that all stopped states except the special stop states can
  tolerate spurious wake up and recover their state.

  The state of stopped and traced tasked is changed to be stored in
  task->jobctl as well as in task->__state. This makes it possible for
  the freezer to recover tasks in these special states, as well as
  serving as a general cleanup. With a little more work in that
  direction I believe TASK_STOPPED can learn to tolerate spurious wake
  ups and become an ordinary stop state.

  The TASK_TRACED state has to remain a special state as the registers
  for a process are only reliably available when the process is stopped
  in the scheduler. Fundamentally ptrace needs acess to the saved
  register values of a task.

  There are bunch of semi-random ptrace related cleanups that were found
  while looking at these issues.

  One cleanup that deserves to be called out is from commit 57b6de08b5f6
  ("ptrace: Admit ptrace_stop can generate spuriuos SIGTRAPs"). This
  makes a change that is technically user space visible, in the handling
  of what happens to a tracee when a tracer dies unexpectedly. According
  to our testing and our understanding of userspace nothing cares that
  spurious SIGTRAPs can be generated in that case"

* tag 'ptrace_stop-cleanup-for-v5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
  sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state
  ptrace: Always take siglock in ptrace_resume
  ptrace: Don't change __state
  ptrace: Admit ptrace_stop can generate spuriuos SIGTRAPs
  ptrace: Document that wait_task_inactive can't fail
  ptrace: Reimplement PTRACE_KILL by always sending SIGKILL
  signal: Use lockdep_assert_held instead of assert_spin_locked
  ptrace: Remove arch_ptrace_attach
  ptrace/xtensa: Replace PT_SINGLESTEP with TIF_SINGLESTEP
  ptrace/um: Replace PT_DTRACE with TIF_SINGLESTEP
  signal: Replace __group_send_sig_info with send_signal_locked
  signal: Rename send_signal send_signal_locked

sched,signal,ptrace: Rework TASK_TRACED, TASK_STOPPED state

2022-05-11T19:37:06+00:00

Currently ptrace_stop() / do_signal_stop() rely on the special states
TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
state exists only in task->__state and nowhere else.

There's two spots of bother with this:

 - PREEMPT_RT has task->saved_state which complicates matters,
   meaning task_is_{traced,stopped}() needs to check an additional
   variable.

 - An alternative freezer implementation that itself relies on a
   special TASK state would loose TASK_TRACED/TASK_STOPPED and will
   result in misbehaviour.

As such, add additional state to task->jobctl to track this state
outside of task->__state.

NOTE: this doesn't actually fix anything yet, just adds extra state.

--EWB
  * didn't add a unnecessary newline in signal.h
  * Update t->jobctl in signal_wake_up and ptrace_signal_wake_up
    instead of in signal_wake_up_state.  This prevents the clearing
    of TASK_STOPPED and TASK_TRACED from getting lost.
  * Added warnings if JOBCTL_STOPPED or JOBCTL_TRACED are not cleared

Signed-off-by: Peter Zijlstra (Intel) 
Link: https://lkml.kernel.org/r/20220421150654.757693825@infradead.org
Tested-by: Kees Cook 
Reviewed-by: Oleg Nesterov 
Link: https://lkml.kernel.org/r/20220505182645.497868-12-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman

ptrace: Always take siglock in ptrace_resume

2022-05-11T19:36:30+00:00

Make code analysis simpler and future changes easier by
always taking siglock in ptrace_resume.

Tested-by: Kees Cook 
Reviewed-by: Oleg Nesterov 
Link: https://lkml.kernel.org/r/20220505182645.497868-11-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman"

ptrace: Don't change __state

2022-05-11T19:35:32+00:00

Stop playing with tsk->__state to remove TASK_WAKEKILL while a ptrace
command is executing.

Instead remove TASK_WAKEKILL from the definition of TASK_TRACED, and
implement a new jobctl flag TASK_PTRACE_FROZEN.  This new flag is set
in jobctl_freeze_task and cleared when ptrace_stop is awoken or in
jobctl_unfreeze_task (when ptrace_stop remains asleep).

In signal_wake_up add __TASK_TRACED to state along with TASK_WAKEKILL
when the wake up is for a fatal signal.  Skip adding __TASK_TRACED
when TASK_PTRACE_FROZEN is not set.  This has the same effect as
changing TASK_TRACED to __TASK_TRACED as all of the wake_ups that use
TASK_KILLABLE go through signal_wake_up.

Handle a ptrace_stop being called with a pending fatal signal.
Previously it would have been handled by schedule simply failing to
sleep.  As TASK_WAKEKILL is no longer part of TASK_TRACED schedule
will sleep with a fatal_signal_pending.   The code in signal_wake_up
guarantees that the code will be awaked by any fatal signal that
codes after TASK_TRACED is set.

Previously the __state value of __TASK_TRACED was changed to
TASK_RUNNING when woken up or back to TASK_TRACED when the code was
left in ptrace_stop.  Now when woken up ptrace_stop now clears
JOBCTL_PTRACE_FROZEN and when left sleeping ptrace_unfreezed_traced
clears JOBCTL_PTRACE_FROZEN.

Tested-by: Kees Cook 
Reviewed-by: Oleg Nesterov 
Link: https://lkml.kernel.org/r/20220505182645.497868-10-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman"

ptrace: Document that wait_task_inactive can't fail

2022-05-11T19:34:47+00:00

After ptrace_freeze_traced succeeds it is known that the tracee
has a __state value of __TASK_TRACED and that no __ptrace_unlink will
happen because the tracer is waiting for the tracee, and the tracee is
in ptrace_stop.

The function ptrace_freeze_traced can succeed at any point after
ptrace_stop has set TASK_TRACED and dropped siglock.  The read_lock on
tasklist_lock only excludes ptrace_attach.

This means that the !current->ptrace which executes under a read_lock
of tasklist_lock will never see a ptrace_freeze_trace as the tracer
must have gone away before the tasklist_lock was taken and
ptrace_attach can not occur until the read_lock is dropped.  As
ptrace_freeze_traced depends upon ptrace_attach running before it can
run that excludes ptrace_freeze_traced until __state is set to
TASK_RUNNING.  This means that task_is_traced will fail in
ptrace_freeze_attach and ptrace_freeze_attached will fail.

On the current->ptrace branch of ptrace_stop which will be reached any
time after ptrace_freeze_traced has succeed it is known that __state
is __TASK_TRACED and schedule() will be called with that state.

Use a WARN_ON_ONCE to document that wait_task_inactive(TASK_TRACED)
should never fail.  Remove the stale comment about may_ptrace_stop.

Strictly speaking this is not true because if PREEMPT_RT is enabled
wait_task_inactive can fail because __state can be changed.  I don't
see this as a problem as the ptrace code is currently broken on
PREMPT_RT, and this is one of the issues.  Failing and warning when
the assumptions of the code are broken is good.

Tested-by: Kees Cook 
Reviewed-by: Oleg Nesterov 
Link: https://lkml.kernel.org/r/20220505182645.497868-8-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman"

ptrace: Reimplement PTRACE_KILL by always sending SIGKILL

2022-05-11T19:34:28+00:00

The current implementation of PTRACE_KILL is buggy and has been for
many years as it assumes it's target has stopped in ptrace_stop.  At a
quick skim it looks like this assumption has existed since ptrace
support was added in linux v1.0.

While PTRACE_KILL has been deprecated we can not remove it as
a quick search with google code search reveals many existing
programs calling it.

When the ptracee is not stopped at ptrace_stop some fields would be
set that are ignored except in ptrace_stop.  Making the userspace
visible behavior of PTRACE_KILL a noop in those case.

As the usual rules are not obeyed it is not clear what the
consequences are of calling PTRACE_KILL on a running process.
Presumably userspace does not do this as it achieves nothing.

Replace the implementation of PTRACE_KILL with a simple
send_sig_info(SIGKILL) followed by a return 0.  This changes the
observable user space behavior only in that PTRACE_KILL on a process
not stopped in ptrace_stop will also kill it.  As that has always
been the intent of the code this seems like a reasonable change.

Cc: stable@vger.kernel.org
Reported-by: Al Viro 
Suggested-by: Al Viro 
Tested-by: Kees Cook 
Reviewed-by: Oleg Nesterov 
Link: https://lkml.kernel.org/r/20220505182645.497868-7-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman"