linux-kernel - Re: [RFC PATCH] mm/mmap: Fix uprobe anon page be overwritten when expanding vma during mremap

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Message-ID: <20250525095926.GA5391@redhat.com>
Date: Sun, 25 May 2025 11:59:27 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Pu Lehui <pulehui@...weicloud.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, mhiramat@...nel.org,
	peterz@...radead.org, akpm@...ux-foundation.org,
	Liam.Howlett@...cle.com, vbabka@...e.cz, jannh@...gle.com,
	pfalcato@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	pulehui@...wei.com, Andrii Nakryiko <andrii@...nel.org>,
	Jiri Olsa <jolsa@...nel.org>
Subject: Re: [RFC PATCH] mm/mmap: Fix uprobe anon page be overwritten when
 expanding vma during mremap

On 05/24, David Hildenbrand wrote:
>
> On 24.05.25 18:45, Oleg Nesterov wrote:
> >
> >To be honest, I can't even understand this part due to my ignorance.
> >What does "the old uprobe anon page to be orphan" actually mean?
> >How can the unnecessary uprobe_mmap() lead to an "unbalanced"
> >inc_mm_counter(MM_ANONPAGES) ? Or what else can explain the
> >"BUG: Bad rss-counter state" from check_mm() ? Or there are more problems?
>
> Essentially, we end up mapping an anonymous page (when install the uprobe)
> after preparing the new VMA, but before moving over the pages from the old
> VMA.
>
> So when we then move over the pages from the old VMA, we overwrite the PTE
> mapping an anonymous page (due to uprobe).
>
> As we simply overwrite the PTE that is mapping an anonymous page, we run
> into inconsistency later: RSS counter mismatch, memory leak, etc.

Ah, I seem to start understand... move_ptes() doesn't even check *new_pte,
I guess it assumes pte_none(ptep_get(new_pte), right? So the old anonymous
page is simply leaked after set_pte_at(mm, new_addr, new_pte, pte)...

Correct?

> We should never be installing an anonymous page (due to uprobe) into a VMA
> during mremap() before moving over the pages from the old VMA.

OK. But do you see any reason why uprobe_mmap() should be ever called during
mremap() ? not to mention munmap() ...

Thanks!

Oleg.