Re: [PATCH] Fix page corruption caused by racy check in __free_pages

[Vlastimil Babka]

On 2/9/23 18:48, David Chen wrote:
> When we upgraded our kernel, we started seeing some page corruption like
> the following consistently:
> 
>  BUG: Bad page state in process ganesha.nfsd  pfn:1304ca
>  page:0000000022261c55 refcount:0 mapcount:-128 mapping:0000000000000000 index:0x0 pfn:0x1304ca
>  flags: 0x17ffffc0000000()
>  raw: 0017ffffc0000000 ffff8a513ffd4c98 ffffeee24b35ec08 0000000000000000
>  raw: 0000000000000000 0000000000000001 00000000ffffff7f 0000000000000000
>  page dumped because: nonzero mapcount
>  CPU: 0 PID: 15567 Comm: ganesha.nfsd Kdump: loaded Tainted: P    B      O      5.10.158-1.nutanix.20221209.el7.x86_64 #1
>  Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/05/2016
>  Call Trace:
>   dump_stack+0x74/0x96
>   bad_page.cold+0x63/0x94
>   check_new_page_bad+0x6d/0x80
>   rmqueue+0x46e/0x970
>   get_page_from_freelist+0xcb/0x3f0
>   ? _cond_resched+0x19/0x40
>   __alloc_pages_nodemask+0x164/0x300
>   alloc_pages_current+0x87/0xf0
>   skb_page_frag_refill+0x84/0x110
>   ...
> 
> Sometimes, it would also show up as corruption in the free list pointer and
> cause crashes.
> 
> After bisecting the issue, we found the issue started from e320d3012d25:
> 
> 	if (put_page_testzero(page))
> 		free_the_page(page, order);
> 	else if (!PageHead(page))
> 		while (order-- > 0)
> 			free_the_page(page + (1 << order), order);
> 
> So the problem is the check PageHead is racy because at this point we
> already dropped our reference to the page. So even if we came in with
> compound page, the page can already be freed and PageHead can return
> false and we will end up freeing all the tail pages causing double free.
> 
> Fixes: e320d3012d25 ("mm/page_alloc.c: fix freeing non-compound pages")
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Chunwei Chen <david.chen@xxxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

That's nasty enough to go into 6.2, IMHO.

> ---
>  mm/page_alloc.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0745aedebb37..3bb3484563ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5631,9 +5631,12 @@ EXPORT_SYMBOL(get_zeroed_page);
>   */
>  void __free_pages(struct page *page, unsigned int order)
>  {
> +	/* get PageHead before we drop reference */
> +	int head = PageHead(page);
> +
>  	if (put_page_testzero(page))
>  		free_the_page(page, order);
> -	else if (!PageHead(page))
> +	else if (!head)
>  		while (order-- > 0)
>  			free_the_page(page + (1 << order), order);
>  }