On 09.03.2020 00:38, Eric W. Biederman wrote: > > The cred_guard_mutex is problematic. The cred_guard_mutex is held > over the userspace accesses as the arguments from userspace are read. > The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other > threads are killed. The cred_guard_mutex is held over > "put_user(0, tsk->clear_child_tid)" in exit_mm(). > > Any of those can result in deadlock, as the cred_guard_mutex is held > over a possible indefinite userspace waits for userspace. > > Add exec_update_mutex that is only held over exec updating process > with the new contents of exec, so that code that needs not to be > confused by exec changing the mm and the cred in ways that can not > happen during ordinary execution of a process. > > The plan is to switch the users of cred_guard_mutex to > exec_udpate_mutex one by one. This lets us move forward while still > being careful and not introducing any regressions. > > Link: https://lore.kernel.org/lkml/20160921152946.GA24210@xxxxxxxxxxxxxx/ > Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@xxxxxxxxxx/ > Link: https://lore.kernel.org/lkml/20160923095031.GA14923@xxxxxxxxxx/ > Link: https://lore.kernel.org/lkml/20170213141452.GA30203@xxxxxxxxxx/ > Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.") > Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2") > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> > --- > fs/exec.c | 9 +++++++++ > include/linux/sched/signal.h | 9 ++++++++- > init/init_task.c | 1 + > kernel/fork.c | 1 + > 4 files changed, 19 insertions(+), 1 deletion(-) > > diff --git a/fs/exec.c b/fs/exec.c > index d820a7272a76..ffeebb1f167b 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1014,6 +1014,7 @@ static int exec_mmap(struct mm_struct *mm) > { > struct task_struct *tsk; > struct mm_struct *old_mm, *active_mm; > + int ret; > > /* Notify parent that we're no longer interested in the old VM */ > tsk = current; > @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm) > return -EINTR; > } > } > + > + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex); > + if (ret) > + return ret; You missed old_mm->mmap_sem unlock. See here: diff --git a/fs/exec.c b/fs/exec.c index 47582cd97f86..d557bac3e862 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1063,8 +1063,11 @@ static int exec_mmap(struct mm_struct *mm) } ret = mutex_lock_killable(&tsk->signal->exec_update_mutex); - if (ret) + if (ret) { + if (old_mm) + up_read(&old_mm->mmap_sem); return ret; + } task_lock(tsk); active_mm = tsk->active_mm;
