On 12.03.2020 17:38, Eric W. Biederman wrote: > Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes: > >> On 12.03.2020 15:24, Eric W. Biederman wrote: >>> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes: >>> >>>> 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: >>> >>> Duh. Thank you. >>> >>> I actually need to switch the lock ordering here, and I haven't yet >>> because my son was sick yesterday. >> >> There is some fundamental problem with your patch, since the below fires in 100% cases >> on current linux-next: > > Thank you. > > I have just backed this out of linux-next for now because it is clearly > flawed. > > You make some good points about the recursion. I will go back to the > drawing board and see what I can work out. > > >> [ 22.838717] kernel BUG at fs/exec.c:1474! >> >> diff --git a/fs/exec.c b/fs/exec.c >> index 47582cd97f86..0f77f8c94905 100644 >> --- a/fs/exec.c >> +++ b/fs/exec.c >> @@ -1470,8 +1470,10 @@ static void free_bprm(struct linux_binprm *bprm) >> { >> free_arg_pages(bprm); >> if (bprm->cred) { >> - if (!bprm->mm) >> + if (!bprm->mm) { >> + BUG_ON(!mutex_is_locked(¤t->signal->exec_update_mutex)); >> mutex_unlock(¤t->signal->exec_update_mutex); >> + } >> mutex_unlock(¤t->signal->cred_guard_mutex); >> abort_creds(bprm->cred); >> } >> @@ -1521,6 +1523,7 @@ void install_exec_creds(struct linux_binprm *bprm) >> * credentials; any time after this it may be unlocked. >> */ >> security_bprm_committed_creds(bprm); >> + BUG_ON(!mutex_is_locked(¤t->signal->exec_update_mutex)); >> mutex_unlock(¤t->signal->exec_update_mutex); >> mutex_unlock(¤t->signal->cred_guard_mutex); >> } >> >> --------------------------------------------------------------------------------------------- >> >> First time the mutex is unlocked in: >> >> exec_binprm()->search_binary_handler()->.load_binary->install_exec_creds() >> >> Then exec_binprm()->search_binary_handler()->.load_binary->flush_old_exec() clears mm: >> >> bprm->mm = NULL; >> >> Second time the mutex is unlocked in free_bprm(): >> >> if (bprm->cred) { >> if (!bprm->mm) >> mutex_unlock(¤t->signal->exec_update_mutex); >> >> My opinion is we should not relay on side indicators like bprm->mm. Better you may >> introduce struct linux_binprm::exec_update_mutex_is_locked. So the next person dealing >> with this after you won't waste much time on diving into this. Also, if someone decides >> to change the place, where bprm->mm is set into NULL, this person will bump into hell >> of dependences between unrelated components like your newly introduced mutex. >> >> So, I'm strongly for *struct linux_binprm::exec_update_mutex_is_locked*, since this improves >> modularity. > > Am I wrong or is that also a problem with cred_guard_mutex? No, there is no a problem. cred_guard_mutex is locked in a pair with bprm->cred = prepare_exec_creds() assignment. cred_guard_mutex is unlocked in a pair with bprm->cred = NULL clearing (see install_exec_creds()). Further free_bprm() skip unlock in case of bprm->cred is NULL.
