On 18.03.2020 23:06, Bernd Edlinger wrote: > On 3/18/20 1:22 PM, Kirill Tkhai wrote: >> On 18.03.2020 00:53, Bernd Edlinger wrote: >>> On 3/17/20 9:56 AM, Kirill Tkhai wrote: >>>> On 14.03.2020 12:11, Bernd Edlinger 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> >>>>> Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> >>>>> --- >>>>> fs/exec.c | 17 ++++++++++++++--- >>>>> include/linux/binfmts.h | 8 +++++++- >>>>> include/linux/sched/signal.h | 9 ++++++++- >>>>> init/init_task.c | 1 + >>>>> kernel/fork.c | 1 + >>>>> 5 files changed, 31 insertions(+), 5 deletions(-) >>>>> >>>>> v3: this update fixes lock-order and adds an explicit data member in linux_binprm >>>>> >>>>> diff --git a/fs/exec.c b/fs/exec.c >>>>> index d820a72..11974a1 100644 >>>>> --- a/fs/exec.c >>>>> +++ b/fs/exec.c >>>>> @@ -1014,12 +1014,17 @@ 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; >>>>> old_mm = current->mm; >>>>> exec_mm_release(tsk, old_mm); >>>>> >>>>> + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex); >>>>> + if (ret) >>>>> + return ret; >>>>> + >>>>> if (old_mm) { >>>>> sync_mm_rss(old_mm); >>>>> /* >>>>> @@ -1031,9 +1036,11 @@ static int exec_mmap(struct mm_struct *mm) >>>>> down_read(&old_mm->mmap_sem); >>>>> if (unlikely(old_mm->core_state)) { >>>>> up_read(&old_mm->mmap_sem); >>>>> + mutex_unlock(&tsk->signal->exec_update_mutex); >>>>> return -EINTR; >>>>> } >>>>> } >>>>> + >>>>> task_lock(tsk); >>>>> active_mm = tsk->active_mm; >>>>> membarrier_exec_mmap(mm); >>>>> @@ -1288,11 +1295,12 @@ int flush_old_exec(struct linux_binprm * bprm) >>>>> goto out; >>>>> >>>>> /* >>>>> - * After clearing bprm->mm (to mark that current is using the >>>>> - * prepared mm now), we have nothing left of the original >>>>> + * After setting bprm->called_exec_mmap (to mark that current is >>>>> + * using the prepared mm now), we have nothing left of the original >>>>> * process. If anything from here on returns an error, the check >>>>> * in search_binary_handler() will SEGV current. >>>>> */ >>>>> + bprm->called_exec_mmap = 1; >>>> >>>> The two below is non-breaking pair: >>>> >>>> exec_mmap(bprm->mm); >>>> bprm->called_exec_mmap = 1; >>>> >>>> Why not move this into exec_mmap(), so nobody definitely inserts something >>>> between them? >>>> >>> >>> Hmm, could be done, but then I would probably need a different name than >>> "called_exec_mmap". >>> >>> How about adding a nice function comment to exec_mmap that calls out the >>> changed behaviour that the exec_update_mutex is taken unless the function >>> fails? >> >> Not sure, I understand correct. >> >> Could you post this like a small patch hunk (on top of anything you want)? >> > > I was thinking of something like that: > > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1010,6 +1010,11 @@ ssize_t read_code(struct file *file, unsigned long addr, > } > EXPORT_SYMBOL(read_code); > > +/* > + * Maps the mm_struct mm into the current task struct. > + * On success, this function returns with the mutex > + * exec_update_mutex locked. > + */ Looks OK for me. > static int exec_mmap(struct mm_struct *mm) > { > struct task_struct *tsk; > > >>> Bernd. >>> >>> >>>>> bprm->mm = NULL; >>>>> >>>>> #ifdef CONFIG_POSIX_TIMERS >>>>> @@ -1438,6 +1446,8 @@ static void free_bprm(struct linux_binprm *bprm) >>>>> { >>>>> free_arg_pages(bprm); >>>>> if (bprm->cred) { >>>>> + if (bprm->called_exec_mmap) >>>>> + mutex_unlock(¤t->signal->exec_update_mutex); >>>>> mutex_unlock(¤t->signal->cred_guard_mutex); >>>>> abort_creds(bprm->cred); >>>>> } >>>>> @@ -1487,6 +1497,7 @@ void install_exec_creds(struct linux_binprm *bprm) >>>>> * credentials; any time after this it may be unlocked. >>>>> */ >>>>> security_bprm_committed_creds(bprm); >>>>> + mutex_unlock(¤t->signal->exec_update_mutex); >>>>> mutex_unlock(¤t->signal->cred_guard_mutex); >>>>> } >>>>> EXPORT_SYMBOL(install_exec_creds); >>>>> @@ -1678,7 +1689,7 @@ int search_binary_handler(struct linux_binprm *bprm) >>>>> >>>>> read_lock(&binfmt_lock); >>>>> put_binfmt(fmt); >>>>> - if (retval < 0 && !bprm->mm) { >>>>> + if (retval < 0 && bprm->called_exec_mmap) { >>>>> /* we got to flush_old_exec() and failed after it */ >>>>> read_unlock(&binfmt_lock); >>>>> force_sigsegv(SIGSEGV); >>>>> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h >>>>> index b40fc63..a345d9f 100644 >>>>> --- a/include/linux/binfmts.h >>>>> +++ b/include/linux/binfmts.h >>>>> @@ -44,7 +44,13 @@ struct linux_binprm { >>>>> * exec has happened. Used to sanitize execution environment >>>>> * and to set AT_SECURE auxv for glibc. >>>>> */ >>>>> - secureexec:1; >>>>> + secureexec:1, >>>>> + /* >>>>> + * Set by flush_old_exec, when exec_mmap has been called. >>>>> + * This is past the point of no return, when the >>>>> + * exec_update_mutex has been taken. >>>>> + */ >>>>> + called_exec_mmap:1; >>>>> #ifdef __alpha__ >>>>> unsigned int taso:1; >>>>> #endif >>>>> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h >>>>> index 8805025..a29df79 100644 >>>>> --- a/include/linux/sched/signal.h >>>>> +++ b/include/linux/sched/signal.h >>>>> @@ -224,7 +224,14 @@ struct signal_struct { >>>>> >>>>> struct mutex cred_guard_mutex; /* guard against foreign influences on >>>>> * credential calculations >>>>> - * (notably. ptrace) */ >>>>> + * (notably. ptrace) >>>>> + * Deprecated do not use in new code. >>>>> + * Use exec_update_mutex instead. >>>>> + */ >>>>> + struct mutex exec_update_mutex; /* Held while task_struct is being >>>>> + * updated during exec, and may have >>>>> + * inconsistent permissions. >>>>> + */ >>>>> } __randomize_layout; >>>>> >>>>> /* >>>>> diff --git a/init/init_task.c b/init/init_task.c >>>>> index 9e5cbe5..bd403ed 100644 >>>>> --- a/init/init_task.c >>>>> +++ b/init/init_task.c >>>>> @@ -26,6 +26,7 @@ >>>>> .multiprocess = HLIST_HEAD_INIT, >>>>> .rlim = INIT_RLIMITS, >>>>> .cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex), >>>>> + .exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex), >>>>> #ifdef CONFIG_POSIX_TIMERS >>>>> .posix_timers = LIST_HEAD_INIT(init_signals.posix_timers), >>>>> .cputimer = { >>>>> diff --git a/kernel/fork.c b/kernel/fork.c >>>>> index 8642530..036b692 100644 >>>>> --- a/kernel/fork.c >>>>> +++ b/kernel/fork.c >>>>> @@ -1594,6 +1594,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk) >>>>> sig->oom_score_adj_min = current->signal->oom_score_adj_min; >>>>> >>>>> mutex_init(&sig->cred_guard_mutex); >>>>> + mutex_init(&sig->exec_update_mutex); >>>>> >>>>> return 0; >>>>> } >>>>> >>>> >>
