Subject: Re: [PATCH kernel 5/9] KVM: PPC: Account TCE-containing pages in locked_vm

From: David Gibson <[email-masked]>
Date: Tue, 8 Dec 2015 16:18:38 +1100

On Tue, Sep 15, 2015 at 08:49:35PM +1000, Alexey Kardashevskiy wrote:
> At the moment pages used for TCE tables (in addition to pages addressed
> by TCEs) are not counted in locked_vm counter so a malicious userspace
> tool can call ioctl(KVM_CREATE_SPAPR_TCE) as many times as RLIMIT_NOFILE and
> lock a lot of memory.
> 
> This adds counting for pages used for TCE tables.
> 
> This counts the number of pages required for a table plus pages for
> the kvmppc_spapr_tce_table struct (TCE table descriptor) itself.

Hmm.  Does it make sense to account for the descriptor struct itself?
I mean there are lots of little structures the kernel will allocate on
a process's behalf, and I don't think most of them get accounted
against locked vm.

> This does not change the amount of (de)allocated memory.
> 
> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_64_vio.c | 51 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index 9526c34..b70787d 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -45,13 +45,56 @@ static long kvmppc_stt_npages(unsigned long window_size)
>  		     * sizeof(u64), PAGE_SIZE) / PAGE_SIZE;
>  }
>  
> +static long kvmppc_account_memlimit(long npages, bool inc)
> +{
> +	long ret = 0;
> +	const long bytes = sizeof(struct kvmppc_spapr_tce_table) +
> +			(abs(npages) * sizeof(struct page *));
> +	const long stt_pages = ALIGN(bytes, PAGE_SIZE) / PAGE_SIZE;

Overflow checks might be useful here, I'm not sure.

> +
> +	if (!current || !current->mm)
> +		return ret; /* process exited */
> +
> +	npages += stt_pages;
> +
> +	down_write(&current->mm->mmap_sem);
> +
> +	if (inc) {
> +		long locked, lock_limit;
> +
> +		locked = current->mm->locked_vm + npages;
> +		lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
> +		if (locked > lock_limit && !capable(CAP_IPC_LOCK))
> +			ret = -ENOMEM;
> +		else
> +			current->mm->locked_vm += npages;
> +	} else {
> +		if (npages > current->mm->locked_vm)

Should this be a WARN_ON?  It means something has gone wrong
previously in the accounting, doesn't it?

> +			npages = current->mm->locked_vm;
> +
> +		current->mm->locked_vm -= npages;
> +	}
> +
> +	pr_debug("[%d] RLIMIT_MEMLOCK KVM %c%ld %ld/%ld%s\n", current->pid,
> +			inc ? '+' : '-',
> +			npages << PAGE_SHIFT,
> +			current->mm->locked_vm << PAGE_SHIFT,
> +			rlimit(RLIMIT_MEMLOCK),
> +			ret ? " - exceeded" : "");
> +
> +	up_write(&current->mm->mmap_sem);
> +
> +	return ret;
> +}
> +
>  static void release_spapr_tce_table(struct rcu_head *head)
>  {
>  	struct kvmppc_spapr_tce_table *stt = container_of(head,
>  			struct kvmppc_spapr_tce_table, rcu);
>  	int i;
> +	long npages = kvmppc_stt_npages(stt->window_size);
>  
> -	for (i = 0; i < kvmppc_stt_npages(stt->window_size); i++)
> +	for (i = 0; i < npages; i++)
>  		__free_page(stt->pages[i]);
>  
>  	kfree(stt);
> @@ -89,6 +132,7 @@ static int kvm_spapr_tce_release(struct inode *inode, struct file *filp)
>  
>  	kvm_put_kvm(stt->kvm);
>  
> +	kvmppc_account_memlimit(kvmppc_stt_npages(stt->window_size), false);
>  	call_rcu(&stt->rcu, release_spapr_tce_table);
>  
>  	return 0;
> @@ -114,6 +158,11 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  	}
>  
>  	npages = kvmppc_stt_npages(args->window_size);
> +	ret = kvmppc_account_memlimit(npages, true);
> +	if (ret) {
> +		stt = NULL;
> +		goto fail;
> +	}
>  
>  	stt = kzalloc(sizeof(*stt) + npages * sizeof(struct page *),
>  		      GFP_KERNEL);

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson