linux-kernel - Re: [RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if the refcount isn't 0

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Date:   Wed, 10 Mar 2021 11:09:58 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Filippo Sironi <sironi@...zon.de>
Cc:     <linux-kernel@...r.kernel.org>, <dwmw@...zon.co.uk>,
        <christian.brauner@...ntu.com>, <akpm@...ux-foundation.org>,
        <peterz@...radead.org>, <keescook@...omium.org>,
        <krisman@...labora.com>, <peterx@...hat.com>, <axboe@...nel.dk>,
        <surenb@...gle.com>, <shakeelb@...gle.com>, <guro@...com>,
        <elver@...gle.com>
Subject: Re: [RFC PATCH] mm: fork: Prevent a NULL deref by getting mm only if the refcount isn't 0

Filippo Sironi <sironi@...zon.de> writes:

> We've seen a number of crashes with the following signature:
>
>     BUG: kernel NULL pointer dereference, address: 0000000000000000
>     #PF: supervisor read access in kernel mode
>     #PF: error_code(0x0000) - not-present page
>     ...
>     Oops: 0000 [#1] SMP PTI
>     ...
>     RIP: 0010:__rb_erase_color+0xc2/0x260
>     ...
>     Call Trace:
>      unlink_file_vma+0x36/0x50
>      free_pgtables+0x62/0x110
>      exit_mmap+0xd5/0x160
>      ? put_dec+0x3a/0x90
>      ? num_to_str+0xa8/0xc0
>      mmput+0x11/0xb0
>      do_task_stat+0x940/0xc80
>      proc_single_show+0x49/0x80
>      ? __check_object_size+0xcc/0x1a0
>      seq_read+0xd3/0x400
>      vfs_read+0x72/0xb0
>      ksys_read+0x9c/0xd0
>      do_syscall_64+0x69/0x400
>      ? schedule+0x2a/0x90
>      entry_SYSCALL_64_after_hwframe+0x44/0xa9
>     ...
>
> This happens when a process goes through the tasks stats in procfs while
> another is exiting.  This looks like a race where the process that's
> exiting drops the last reference on the mm (with mmput) while the other
> increases it (with mmget).  By only increasing when the reference isn't
> 0 to begin with, we prevent this from happening.

For this to be a race with exit this would require racing with exit_mm
where current->mm is cleared.

Looking at exit_mm() the code does:

	struct mm_struct *mm = current->mm;

	mmap_read_lock(mm);
	mmgrab(mm);
        task_lock(current);
	local_irq_disable();
        current->mm = NULL;
        local_irq_enable();
        task_unlock(current);
        mmap_read_unlock(mm);

	mmput(mm);

Which seems to guarantee "mm_users > 0" if "task->mm != NULL" under
tasklist_lock.

So I suggest you instrument your failing kernels and find what is
improperly decrementing mm_users.

Eric