Re: [RFC PATCH] mm/mempolicy: Fix memory leak in set_mempolicy_home_node system call

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2022-12-15 02:51, Michal Hocko wrote:
> On Wed 14-12-22 17:21:10, Mathieu Desnoyers wrote:
>> When encountering any vma in the range with policy other than MPOL_BIND
>> or MPOL_PREFERRED_MANY, an error is returned without issuing a mpol_put
>> on the policy just allocated with mpol_dup().
>>
>> This allows arbitrary users to leak kernel memory.
>>
>> Fixes: c6018b4b2549 ("mm/mempolicy: add set_mempolicy_home_node syscall")
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> Cc: Ben Widawsky <ben.widawsky@intel.com>
>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>> Cc: Feng Tang <feng.tang@intel.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>> Cc: Randy Dunlap <rdunlap@infradead.org>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Andi Kleen <ak@linux.intel.com>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Huang Ying <ying.huang@intel.com>
>> Cc: <linux-api@vger.kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: stable@vger.kernel.org # 5.17+
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> Thanks for catching this!
> 
> Btw. looking at the code again it seems rather pointless to duplicate
> the policy just to throw it away anyway. A slightly bigger diff but this
> looks more reasonable to me. What do you think? I can also send it as a
> clean up on top of your fix.

I think it would be best if this comes as a cleanup on top of my fix. 
The diff is larger than the minimal change needed to fix the leak in 
stable branches.

Your approach looks fine, except for the vma_policy(vma) -> old change 
already spotted by Aneesh.

Thanks,

Mathieu

> ---
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 61aa9aedb728..918cdc8a7f0c 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1489,7 +1489,7 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
>   {
>   	struct mm_struct *mm = current->mm;
>   	struct vm_area_struct *vma;
> -	struct mempolicy *new;
> +	struct mempolicy *new. *old;
>   	unsigned long vmstart;
>   	unsigned long vmend;
>   	unsigned long end;
> @@ -1521,30 +1521,28 @@ SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, le
>   		return 0;
>   	mmap_write_lock(mm);
>   	for_each_vma_range(vmi, vma, end) {
> -		vmstart = max(start, vma->vm_start);
> -		vmend   = min(end, vma->vm_end);
> -		new = mpol_dup(vma_policy(vma));
> -		if (IS_ERR(new)) {
> -			err = PTR_ERR(new);
> -			break;
> -		}
> -		/*
> -		 * Only update home node if there is an existing vma policy
> -		 */
> -		if (!new)
> -			continue;
> -
>   		/*
>   		 * If any vma in the range got policy other than MPOL_BIND
>   		 * or MPOL_PREFERRED_MANY we return error. We don't reset
>   		 * the home node for vmas we already updated before.
>   		 */
> -		if (new->mode != MPOL_BIND && new->mode != MPOL_PREFERRED_MANY) {
> +		old = vma_policy(vma);
> +		if (!old)
> +			continue;
> +		if (old->mode != MPOL_BIND && old->mode != MPOL_PREFERRED_MANY) {
>   			err = -EOPNOTSUPP;
>   			break;
>   		}
>   
> +		new = mpol_dup(vma_policy(vma));
> +		if (IS_ERR(new)) {
> +			err = PTR_ERR(new);
> +			break;
> +		}
> +
>   		new->home_node = home_node;
> +		vmstart = max(start, vma->vm_start);
> +		vmend   = min(end, vma->vm_end);
>   		err = mbind_range(mm, vmstart, vmend, new);
>   		mpol_put(new);
>   		if (err)

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com