Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex

[Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>]

On 3/12/20 3:38 PM, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
> 
>> On 12.03.2020 15:24, Eric W. Biederman wrote:
>>>
>>> I actually need to switch the lock ordering here, and I haven't yet
>>> because my son was sick yesterday.

All the best wishes to you and your son.  I hope he will get well soon.

And sorry for not missing the issue in the review.  The reason turns
out that bprm_mm_init is called after prepare_bprm_creds, but there
are error pathes between those where free_bprm is called up with
cred != NULL and mm == NULL, but the mutex not locked.

I figured out a possible fix for the problem that was pointed out:


>From ceb6f65b52b3a7f0280f4f20509a1564a439edf6 Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
Date: Wed, 11 Mar 2020 15:31:07 +0100
Subject: [PATCH] Fix issues with exec_update_mutex

Signed-off-by: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
---
 fs/exec.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index ffeebb1..cde4937 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1021,8 +1021,14 @@ static int exec_mmap(struct mm_struct *mm)
 	old_mm = current->mm;
 	exec_mm_release(tsk, old_mm);
 
-	if (old_mm) {
+	if (old_mm)
 		sync_mm_rss(old_mm);
+
+	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
+	if (ret)
+		return ret;
+
+	if (old_mm) {
 		/*
 		 * Make sure that if there is a core dump in progress
 		 * for the old mm, we get out and die instead of going
@@ -1032,14 +1038,11 @@ 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;
 		}
 	}
 
-	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
-	if (ret)
-		return ret;
-
 	task_lock(tsk);
 	active_mm = tsk->active_mm;
 	membarrier_exec_mmap(mm);
@@ -1444,8 +1447,6 @@ static void free_bprm(struct linux_binprm *bprm)
 {
 	free_arg_pages(bprm);
 	if (bprm->cred) {
-		if (!bprm->mm)
-			mutex_unlock(&current->signal->exec_update_mutex);
 		mutex_unlock(&current->signal->cred_guard_mutex);
 		abort_creds(bprm->cred);
 	}
@@ -1846,6 +1847,8 @@ static int __do_execve_file(int fd, struct filename *filename,
 	would_dump(bprm, bprm->file);
 
 	retval = exec_binprm(bprm);
+	if (bprm->cred && !bprm->mm)
+		mutex_unlock(&current->signal->exec_update_mutex);
 	if (retval < 0)
 		goto out;
 
-- 
1.9.1