On 2020-03-01, Bernd Edlinger [off-list ref] 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 <redacted>

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@dhcp22.suse.cz/ (local)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>