On Sun, Mar 01, 2020 at 05:46:08PM +0000, Bernd Edlinger wrote: > 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/$pid/mem)", I don't know for sure how that can be done, but if > that is possible, it would probably work as a test case. > > What do you think? Yeah, anything that calls ptrace_may_access() is fine and open(/proc/$pid/mem) will work so long as $pid is not in the same thread-group as the caller. A polished version of the reproducer you linked in would probably be good.
