Re: [PATCH] exec: Fix a deadlock in ptrace

[Christian Brauner]

On Mon, Mar 02, 2020 at 02:13:33AM +1100, Aleksa Sarai wrote:
> On 2020-03-01, Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> wrote:
> > 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@xxxxxxxxxx>
> 
> I can't comment on the validity of the patch, but I also found and
> reported this issue in 2016[1] and the discussion quickly veered into
> the problem being more complicated (and uglier) than it seems at first
> glance.
> 
> You should probably also Cc stable, given this has been a long-standing
> issue and your patch doesn't look (too) invasive.
> 
> [1]: https://lore.kernel.org/lkml/20160921152946.GA24210@xxxxxxxxxxxxxx/

Yeah, I remember you mentioning this a while back.

Bernd, we really want a reproducer for this sent alongside with this
patch added to:
tools/testing/selftests/ptrace/
Having a test for this bug irrespective of whether or not we go with
this as fix seems really worth it.

Oleg seems to have suggested that a potential alternative fix is to wait
in de_thread() until all other threads in the thread-group have passed
exit_notiy(). Right now we only kill them but don't wait. Currently
de_thread() only waits for the thread-group leader to pass exit_notify()
whenever a non-thread-group leader thread execs (because the exec'ing
thread becomes the new thread-group leader with the same pid as the
former thread-group leader).

Christian