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)? > 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; >>> } >>> >>
