On Tue, Mar 10, 2020 at 02:43:41PM +0100, Bernd Edlinger 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
This changes mm_access to use the new exec_update_mutex instead of cred_guard_mutex.
This patch is based on the following patch by Eric W. Biederman: "[PATCH 0/5] Infrastructure to allow fixing exec deadlocks" Link: https://lore.kernel.org/lkml/87v9ne5y4y.fsf_-_@x220.int.ebiederm.org/
Signed-off-by: Bernd Edlinger bernd.edlinger@hotmail.de
Cool, yes, on top of the new infrastructure this looks correct to me -- the new mutex wraps mm changes and mm_access() is looking at *drum roll* the mm! :)
Reviewed-by: Kees Cook keescook@chromium.org
-Kees
kernel/fork.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c index c12595a..5720ff3 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1224,7 +1224,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) struct mm_struct *mm; int err;
- err = mutex_lock_killable(&task->signal->cred_guard_mutex);
- err = mutex_lock_killable(&task->signal->exec_update_mutex); if (err) return ERR_PTR(err);
@@ -1234,7 +1234,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) mmput(mm); mm = ERR_PTR(-EACCES); }
- mutex_unlock(&task->signal->cred_guard_mutex);
- mutex_unlock(&task->signal->exec_update_mutex);
return mm; } -- 1.9.1