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_-_@xxxxxxxxxxxxxxxxxxxxx/ > > Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> 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@xxxxxxxxxxxx> -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 -- Kees Cook
