linux-stable.git/kernel/signal.c, branch v5.0.6

signal: Restore the stop PTRACE_EVENT_EXIT

2019-02-13T14:31:41+00:00

In the middle of do_exit() there is there is a call
"ptrace_event(PTRACE_EVENT_EXIT, code);" That call places the process
in TACKED_TRACED aka "(TASK_WAKEKILL | __TASK_TRACED)" and waits for
for the debugger to release the task or SIGKILL to be delivered.

Skipping past dequeue_signal when we know a fatal signal has already
been delivered resulted in SIGKILL remaining pending and
TIF_SIGPENDING remaining set.  This in turn caused the
scheduler to not sleep in PTACE_EVENT_EXIT as it figured
a fatal signal was pending.  This also caused ptrace_freeze_traced
in ptrace_check_attach to fail because it left a per thread
SIGKILL pending which is what fatal_signal_pending tests for.

This difference in signal state caused strace to report
strace: Exit of unknown pid NNNNN ignored

Therefore update the signal handling state like dequeue_signal
would when removing a per thread SIGKILL, by removing SIGKILL
from the per thread signal mask and clearing TIF_SIGPENDING.

Acked-by: Oleg Nesterov 
Reported-by: Oleg Nesterov 
Reported-by: Ivan Delalande 
Cc: stable@vger.kernel.org
Fixes: 35634ffa1751 ("signal: Always notice exiting tasks")
Signed-off-by: "Eric W. Biederman"

signal: Better detection of synchronous signals

2019-02-07T15:00:36+00:00

Recently syzkaller was able to create unkillablle processes by
creating a timer that is delivered as a thread local signal on SIGHUP,
and receiving SIGHUP SA_NODEFERER.  Ultimately causing a loop failing
to deliver SIGHUP but always trying.

When the stack overflows delivery of SIGHUP fails and force_sigsegv is
called.  Unfortunately because SIGSEGV is numerically higher than
SIGHUP next_signal tries again to deliver a SIGHUP.

From a quality of implementation standpoint attempting to deliver the
timer SIGHUP signal is wrong.  We should attempt to deliver the
synchronous SIGSEGV signal we just forced.

We can make that happening in a fairly straight forward manner by
instead of just looking at the signal number we also look at the
si_code.  In particular for exceptions (aka synchronous signals) the
si_code is always greater than 0.

That still has the potential to pick up a number of asynchronous
signals as in a few cases the same si_codes that are used
for synchronous signals are also used for asynchronous signals,
and SI_KERNEL is also included in the list of possible si_codes.

Still the heuristic is much better and timer signals are definitely
excluded.  Which is enough to prevent all known ways for someone
sending a process signals fast enough to cause unexpected and
arguably incorrect behavior.

Cc: stable@vger.kernel.org
Fixes: a27341cd5fcb ("Prioritize synchronous signals over 'normal' signals")
Tested-by: Dmitry Vyukov 
Reported-by: Dmitry Vyukov 
Signed-off-by: "Eric W. Biederman"

signal: Always notice exiting tasks

2019-02-07T14:59:50+00:00

Recently syzkaller was able to create unkillablle processes by
creating a timer that is delivered as a thread local signal on SIGHUP,
and receiving SIGHUP SA_NODEFERER.  Ultimately causing a loop
failing to deliver SIGHUP but always trying.

Upon examination it turns out part of the problem is actually most of
the solution.  Since 2.5 signal delivery has found all fatal signals,
marked the signal group for death, and queued SIGKILL in every threads
thread queue relying on signal->group_exit_code to preserve the
information of which was the actual fatal signal.

The conversion of all fatal signals to SIGKILL results in the
synchronous signal heuristic in next_signal kicking in and preferring
SIGHUP to SIGKILL.  Which is especially problematic as all
fatal signals have already been transformed into SIGKILL.

Instead of dequeueing signals and depending upon SIGKILL to
be the first signal dequeued, first test if the signal group
has already been marked for death.  This guarantees that
nothing in the signal queue can prevent a process that needs
to exit from exiting.

Cc: stable@vger.kernel.org
Tested-by: Dmitry Vyukov 
Reported-by: Dmitry Vyukov 
Ref: ebf5ebe31d2c ("[PATCH] signal-fixes-2.5.59-A4")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: "Eric W. Biederman"

signal: Always attempt to allocate siginfo for SIGSTOP

2019-02-05T14:18:17+00:00

Since 2.5.34 the code has had the potential to not allocate siginfo
for SIGSTOP signals.  Except for ptrace this is perfectly fine as only
ptrace can use PTRACE_PEEK_SIGINFO and see what the contents of
the delivered siginfo are.

Users of PTRACE_PEEK_SIGINFO that care about the contents siginfo
for SIGSTOP are rare, but they do exist.  A seccomp self test
has cared and lldb cares.

Jack Andersen  writes:

> The patch titled
> `signal: Never allocate siginfo for SIGKILL or SIGSTOP`
> created a regression for users of PTRACE_GETSIGINFO needing to
> discern signals that were raised via the tgkill syscall.
>
> A notable user of this tgkill+ptrace combination is lldb while
> debugging a multithreaded program. Without the ability to detect a
> SIGSTOP originating from tgkill, lldb does not have a way to
> synchronize on a per-thread basis and falls back to SIGSTOP-ing the
> entire process.

Everyone affected by this please note.  The kernel can still fail to
allocate a siginfo structure.  The allocation is with GFP_KERNEL and
is best effort only.  If memory is tight when the signal allocation
comes in this will fail to allocate a siginfo.

So I strongly recommend looking at more robust solutions for
synchronizing with a single thread such as PTRACE_INTERRUPT.  Or if
that does not work persuading your friendly local kernel developer to
build the interface you need.

Reported-by: Tycho Andersen 
Reported-by: Kees Cook 
Reported-by: Jack Andersen 
Acked-by: Linus Torvalds 
Reviewed-by: Christian Brauner 
Cc: stable@vger.kernel.org
Fixes: f149b3155744 ("signal: Never allocate siginfo for SIGKILL or SIGSTOP")
Fixes: 6dfc88977e42 ("[PATCH] shared thread signals")
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: "Eric W. Biederman"

Remove 'type' argument from access_ok() function

2019-01-04T02:57:57+00:00

Nobody has actually used the type (VERIFY_READ vs VERIFY_WRITE) argument
of the user address range verification function since we got rid of the
old racy i386-only code to walk page tables by hand.

It existed because the original 80386 would not honor the write protect
bit when in kernel mode, so you had to do COW by hand before doing any
user access.  But we haven't supported that in a long time, and these
days the 'type' argument is a purely historical artifact.

A discussion about extending 'user_access_begin()' to do the range
checking resulted this patch, because there is no way we're going to
move the old VERIFY_xyz interface to that model.  And it's best done at
the end of the merge window when I've done most of my merges, so let's
just get this done once and for all.

This patch was mostly done with a sed-script, with manual fix-ups for
the cases that weren't of the trivial 'access_ok(VERIFY_xyz' form.

There were a couple of notable cases:

 - csky still had the old "verify_area()" name as an alias.

 - the iter_iov code had magical hardcoded knowledge of the actual
   values of VERIFY_{READ,WRITE} (not that they mattered, since nothing
   really used it)

 - microblaze used the type argument for a debug printout

but other than those oddities this should be a total no-op patch.

I tried to fix up all architectures, did fairly extensive grepping for
access_ok() uses, and the changes are trivial, but I may have missed
something.  Any missed conversion should be trivially fixable, though.

Signed-off-by: Linus Torvalds

y2038: signal: Add compat_sys_rt_sigtimedwait_time64

2018-12-18T15:13:04+00:00

Now that 32-bit architectures have two variants of sys_rt_sigtimedwaid()
for 32-bit and 64-bit time_t, we also need to have a second compat system
call entry point on the corresponding 64-bit architectures.

The traditional system call keeps getting handled
by compat_sys_rt_sigtimedwait(), and this adds a new
compat_sys_rt_sigtimedwait_time64() that differs only in the timeout
argument type.

The naming remains a bit asymmetric for the moment. Ideally we would
want to have compat_sys_rt_sigtimedwait_time32() for the old version
and compat_sys_rt_sigtimedwait() for the new one to mirror the names
of the native entry points, but renaming the existing system call
tables causes unnecessary churn. I would suggest renaming all such
system calls together at a later point.

Signed-off-by: Arnd Bergmann

y2038: signal: Add sys_rt_sigtimedwait_time32

2018-12-18T15:13:04+00:00

Once sys_rt_sigtimedwait() gets changed to a 64-bit time_t, we have
to provide compatibility support for existing binaries.

An earlier version of this patch reused the compat_sys_rt_sigtimedwait
entry point to avoid code duplication, but this newer approach
duplicates the existing native entry point instead, which seems
a bit cleaner.

Signed-off-by: Arnd Bergmann

signal: Add restore_user_sigmask()

2018-12-06T16:22:53+00:00

Refactor the logic to restore the sigmask before the syscall
returns into an api.
This is useful for versions of syscalls that pass in the
sigmask and expect the current->sigmask to be changed during
the execution and restored after the execution of the syscall.

With the advent of new y2038 syscalls in the subsequent patches,
we add two more new versions of the syscalls (for pselect, ppoll
and io_pgetevents) in addition to the existing native and compat
versions. Adding such an api reduces the logic that would need to
be replicated otherwise.

Signed-off-by: Deepa Dinamani 
Signed-off-by: Arnd Bergmann

signal: Add set_user_sigmask()

2018-12-06T16:22:38+00:00

Refactor reading sigset from userspace and updating sigmask
into an api.

This is useful for versions of syscalls that pass in the
sigmask and expect the current->sigmask to be changed during,
and restored after, the execution of the syscall.

With the advent of new y2038 syscalls in the subsequent patches,
we add two more new versions of the syscalls (for pselect, ppoll,
and io_pgetevents) in addition to the existing native and compat
versions. Adding such an api reduces the logic that would need to
be replicated otherwise.

Note that the calls to sigprocmask() ignored the return value
from the api as the function only returns an error on an invalid
first argument that is hardcoded at these call sites.
The updated logic uses set_current_blocked() instead.

Signed-off-by: Deepa Dinamani 
Signed-off-by: Arnd Bergmann

kernel/signal.c: fix a comment error

2018-10-31T15:54:14+00:00

Because get_signal_to_deliver() was renamed to get_signal() the
comment should be fixed.

Link: http://lkml.kernel.org/r/1539179128-45709-1-git-send-email-swkhack@gmail.com
Signed-off-by: Weikang Shi 
Reported-by: Christian Brauner 
Cc: Eric W. Biederman 
Cc: Oleg Nesterov 
Cc: Anna-Maria Gleixner 
Cc: "Luck, Tony" 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds