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. + */ 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; >>>> } >>>> >>> >
