linux-kernel - Re: [PATCH v2 1/3] mm/swapfile: make security_vm_enough_memory

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Date:   Fri, 17 Jun 2022 09:33:51 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] mm/swapfile: make security_vm_enough_memory_mm()
 work as expected

On 08.06.22 16:40, Miaohe Lin wrote:
> security_vm_enough_memory_mm() checks whether a process has enough memory
> to allocate a new virtual mapping. And total_swap_pages is considered as
> available memory while swapoff tries to make sure there's enough memory
> that can hold the swapped out memory. But total_swap_pages contains the
> swap space that is being swapoff. So security_vm_enough_memory_mm() will
> success even if there's no memory to hold the swapped out memory because

s/success/succeed/

> total_swap_pages always greater than or equal to p->pages.
> 
> In order to fix it, p->pages should be retracted from total_swap_pages

s/retracted/subtracted/

> first and then check whether there's enough memory for inuse swap pages.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/swapfile.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index ec4c1b276691..d2bead7b8b70 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2398,6 +2398,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  	struct filename *pathname;
>  	int err, found = 0;
>  	unsigned int old_block_size;
> +	unsigned int inuse_pages;
>  
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
> @@ -2428,9 +2429,13 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		spin_unlock(&swap_lock);
>  		goto out_dput;
>  	}
> -	if (!security_vm_enough_memory_mm(current->mm, p->pages))
> -		vm_unacct_memory(p->pages);
> +
> +	total_swap_pages -= p->pages;
> +	inuse_pages = READ_ONCE(p->inuse_pages);
> +	if (!security_vm_enough_memory_mm(current->mm, inuse_pages))
> +		vm_unacct_memory(inuse_pages);
>  	else {
> +		total_swap_pages += p->pages;

That implies that whenever we fail in security_vm_enough_memory_mm(),
that other concurrent users might see a wrong total_swap_pages.

Assume 4 GiB memory and 8 GiB swap. Let's assume 10 GiB are in use.

Temporarily, we'd have

CommitLimit    4 GiB
Committed_AS  10 GiB

Not sure if relevant, but I wonder if it could be avoided somehow?


Apart from that, LGTM.

-- 
Thanks,

David / dhildenb