linux-kernel - Re: [PATCH v1 2/4] mm: Expose abnormal new_pte during move

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Message-ID: <ed72c0c8-a511-483a-be44-edf7432a4782@lucifer.local>
Date: Fri, 30 May 2025 11:21:33 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pu Lehui <pulehui@...weicloud.com>
Cc: mhiramat@...nel.org, oleg@...hat.com, 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, stable@...r.kernel.org,
        pulehui@...wei.com
Subject: Re: [PATCH v1 2/4] mm: Expose abnormal new_pte during move_ptes

On Thu, May 29, 2025 at 03:56:48PM +0000, Pu Lehui wrote:
> From: Pu Lehui <pulehui@...wei.com>
>
> When executing move_ptes, the new_pte must be NULL, otherwise it will be
> overwritten by the old_pte, and cause the abnormal new_pte to be leaked.
> In order to make this problem to be more explicit, let's add
> WARN_ON_ONCE when new_pte is not NULL.
>
> Suggested-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Pu Lehui <pulehui@...wei.com>

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

(both this and the amended version :)

> ---
>  mm/mremap.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 83e359754961..4e2491f8c2ce 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -237,6 +237,8 @@ static int move_ptes(struct pagetable_move_control *pmc,
>
>  	for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
>  				   new_pte++, new_addr += PAGE_SIZE) {
> +		WARN_ON_ONCE(!pte_none(*new_pte));
> +

I mean, we really really should not ever be seeing a mapped PTE here, so I think
a WARN_ON_ONCE() is fine.

We unmap anything ahead of time, and only I think this uprobe breakpoint
installation would ever cause this to be the case.

We can make this a VM_WARN_ON_ONCE() too I suppose, just in case there's
something we're not thinking of, but I'd say at some point we'd want to change
it to a WARN_ON_ONCE().


>  		if (pte_none(ptep_get(old_pte)))
>  			continue;
>
> --
> 2.34.1
>