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

On 13.03.2020 04:05, Bernd Edlinger wrote:

On 3/12/20 3:38 PM, Eric W. Biederman wrote:

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Kirill Tkhai ktkhai@virtuozzo.com writes:

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On 12.03.2020 15:24, Eric W. Biederman wrote:

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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@hotmail.de Date: Wed, 11 Mar 2020 15:31:07 +0100 Subject: [PATCH] Fix issues with exec_update_mutex

Signed-off-by: Bernd Edlinger bernd.edlinger@hotmail.de

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);
  

Despite this should fix the problem, this looks like a broken puzzle.

We can't use bprm->cred as an identifier whether the mutex was locked or not. We can check for bprm->cred in regard to cred_guard_mutex, because of there is strong rule: "cred_guard_mutex is becomes locked together with bprm->cred assignment (see prepare_bprm_creds()), and it becomes unlocked together with bprm->cred zeroing". Take attention on modularity of all this: there is no dependencies between anything else.

In regard to newly introduced exec_update_mutex, your fix and source patch way look like an obfuscation. The mutex becomes deadly glued to unrelated bprm->cred and bprm->mm, and this introduces the problems in the future modifications and support of all involved entities. If someone wants to move some functions in relation to each other, there will be a pain, and this person will have to go again the same dependencies and bug way, Eric stepped on in the original patch.