linux-kernel - Re: [PATCH] mm/mmap: Fix MAP_FIXED address return on VMA merge

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Date:   Wed, 19 Oct 2022 16:08:28 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Liu Zixian <liuzixian4@...wei.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] mm/mmap: Fix MAP_FIXED address return on VMA merge

>>>    mm/mmap.c | 15 +++++++--------
>>>    1 file changed, 7 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>> index 42cd2c260898..22010e13f1a1 100644
>>> --- a/mm/mmap.c
>>> +++ b/mm/mmap.c
>>> @@ -2625,14 +2625,14 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>>>    		if (error)
>>>    			goto unmap_and_free_vma;
>>> -		/* Can addr have changed??
>>> -		 *
>>> -		 * Answer: Yes, several device drivers can do it in their
>>> -		 *         f_op->mmap method. -DaveM
>>> +		/*
>>> +		 * Expansion is handled above, merging is handled below.
>>> +		 * Drivers should not alter the address of the VMA.
>>>    		 */
>>> -		WARN_ON_ONCE(addr != vma->vm_start);
>>> -
>>> -		addr = vma->vm_start;
>>> +		if (WARN_ON((addr != vma->vm_start))) {
>>> +			error = -EINVAL;
>>> +			goto close_and_free_vma;
>>> +		}
>>
>> If this is something that user space can trigger, WARN_* is the wrong
>> choice. But what I understand from the comment change is that this must not
>> happen at that point unless there is a real issue.
> 
> The VMA start address could be changed in call_mmap() which is a driver
> call.  I guess someone could write a driver to mmap by a users action?
> I don't think it can be reached other ways.  In any case, I'm changing a
> WARN_ON_ONCE() to a WARN_ON() and undoing the badness instead of
> marching forwards.

WARN_ON_ONCE() can also be used in conditionals if that's what you were 
concerned about, but ...

> 
>>
>> Why not "if (WARN_ON_ONCE)" ?
> 
> I was thinking it was harder to ignore if it happen more frequently?
> There isn't a driver that does this now, but I'm not picky over which
> variant to call.

.. I think the assumption really is that we won't see (m)any these 
calls. And if we do, it's a bad bad driver. So WARN_ON() might be just fine.

If this would be easy to trigger by any user space, WARN* would have 
been the wrong choice, that's why I was asking.

-- 
Thanks,

David / dhildenb