On 3/1/20 4:58 PM, Christian Brauner wrote: > 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. > I ran into this issue, because I wanted to fix an issue in the gcc testsuite, namely why it forgets to remove some temp files, so I did the following: strace -ftt -o trace.txt make check-gcc-c -k -j4 I reproduced with v4.20 and v5.5 kernel, and I don't know why but it is not happening on all systems I tested, maybe it is something that the expect program does, because, always when I try to reproduce this, the deadlock was always in "expect". I use expect version 5.45 on the computer where the above test freezes after a couple of minutes. I think the issue with strace is that it is using vm_access to get the parameters of a syscall that is going on in one thread, and that races with another thread that calls execve, and blocks the cred_guard_mutex. While Olg's test case here, will certainly not be fixed: https://lore.kernel.org/lkml/20160923095031.GA14923@xxxxxxxxxx/ he mentions the access to "anything else which needs ->cred_guard_mutex, say open(/proc/$pid/mem)", I don't know for sure how that can be done, but if that is possible, it would probably work as a test case. What do you think? Bernd. > 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 >
