Date:   Tue, 16 Aug 2022 15:04:01 +0530
From:   Charan Teja Kalla <quic_charante@...cinc.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     <akpm@...ux-foundation.org>, <david@...hat.com>,
        <pasha.tatashin@...een.com>, <sieberf@...zon.com>,
        <shakeelb@...gle.com>, <sjpark@...zon.de>, <dhowells@...hat.com>,
        <willy@...radead.org>, <quic_pkondeti@...cinc.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH V3] mm: fix use-after free of page_ext after race with
 memory-offline

Thanks Michal!!

On 8/15/2022 8:36 PM, Michal Hocko wrote:
> On Tue 09-08-22 20:16:43, Charan Teja Kalla wrote:
> [...]
>> diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
>> index fabb2e1..0e259da 100644
>> --- a/include/linux/page_ext.h
>> +++ b/include/linux/page_ext.h
> [...]
>> @@ -87,5 +83,14 @@ static inline void page_ext_init_flatmem_late(void)
>>  static inline void page_ext_init_flatmem(void)
>>  {
>>  }
>> +
>> +static inline struct page *page_ext_get(struct page *page)
> struct page_ext *
> 
oops!! It didn't get caught as this is in !CONFIG_PAGE_EXTENSION.
>> +{
>> +	return NULL;
>> +}
>> +
>> +static inline void page_ext_put(void)
>> +{
>> +}
>>  #endif /* CONFIG_PAGE_EXTENSION */
>>  #endif /* __LINUX_PAGE_EXT_H */
> [...]
>> diff --git a/mm/page_ext.c b/mm/page_ext.c
>> index 3dc715d..91d7bd2 100644
>> --- a/mm/page_ext.c
>> +++ b/mm/page_ext.c
>> @@ -9,6 +9,7 @@
>>  #include <linux/page_owner.h>
>>  #include <linux/page_idle.h>
>>  #include <linux/page_table_check.h>
>> +#include <linux/rcupdate.h>
>>  
>>  /*
>>   * struct page extension
>> @@ -59,6 +60,10 @@
>>   * can utilize this callback to initialize the state of it correctly.
>>   */
>>  
>> +#ifdef CONFIG_SPARSEMEM
>> +#define PAGE_EXT_INVALID       (0x1)
>> +#endif
>> +
>>  #if defined(CONFIG_PAGE_IDLE_FLAG) && !defined(CONFIG_64BIT)
>>  static bool need_page_idle(void)
>>  {
>> @@ -84,6 +89,7 @@ static struct page_ext_operations *page_ext_ops[] __initdata = {
>>  unsigned long page_ext_size = sizeof(struct page_ext);
>>  
>>  static unsigned long total_usage;
>> +static struct page_ext *lookup_page_ext(const struct page *page);
>>  
>>  static bool __init invoke_need_callbacks(void)
>>  {
>> @@ -125,6 +131,37 @@ static inline struct page_ext *get_entry(void *base, unsigned long index)
>>  	return base + page_ext_size * index;
>>  }
>>  
>> +/*
>> + * This function gives proper page_ext of a memory section
>> + * during race with the offline operation on a memory block
>> + * this section falls into. Not using this function to get
>> + * page_ext of a page, in code paths where extra refcount
>> + * is not taken on that page eg: pfn walking, can lead to
>> + * use-after-free access of page_ext.
> I do not think this is really useful comment, it goes into way too much
> detail about memory hotplug yet not enough to actually understand the
> interaction because there are no references to the actual
> synchronization scheme. I would go with something like:
> 
> /*
>  * Get a page_ext associated with the given page. Returns NULL if
>  * no such page_ext exists otherwise ensures that the page_ext will
>  * stay alive until page_ext_put is called.
>  * This implies a non-sleeping context.
>  */

Will update as per the Matthew input @
https://lore.kernel.org/all/YvplthTjM8Ez5DIq@casper.infradead.org/
>> + */
>> +struct page_ext *page_ext_get(struct page *page)
>> +{
>> +	struct page_ext *page_ext;
>> +
>> +	rcu_read_lock();
>> +	page_ext = lookup_page_ext(page);
>> +	if (!page_ext) {
>> +		rcu_read_unlock();
>> +		return NULL;
>> +	}
>> +
>> +	return page_ext;
>> +}
>> +
>> +/*
>> + * Must be called after work is done with the page_ext received
>> + * with page_ext_get().
>> + */
>> +
>> +void page_ext_put(void)
>> +{
>> +	rcu_read_unlock();
>> +}
> Thinking about this some more I am not sure this is a good interface. It
> doesn't have any reference to the actual object this is called for. This
> is nicely visible in __folio_copy_owner which just calles page_ext_put()
> twice because there are 2 page_exts and I can already see how somebody
> might get confused this is just an error and send a patch to drop one of
> them.
> 
> I do understand why you went this way because having a parameter which
> is not used will likely lead to the same situation. On the other hand it
> could be annotated to not raise warnings. One potential way to
> workaround that would be
> 
> void page_ext_put(struct page_ext *page_ext)
> {
> 	if (unlikely(!page_ext))
> 		return;
> 	
> 	rcu_read_unlock();
> }
> 
> which would help to make the api slightly more robust in case somebody
> does page_ext_put in a branch where page_ext_get returns NULL.
> 
Looks better. Will change this accordingly.

> No strong opinion on that though. WDYI?
> 
>>  #ifndef CONFIG_SPARSEMEM
>>  
>>  
> [...]
>> @@ -183,19 +184,26 @@ static inline void __set_page_owner_handle(struct page_ext *page_ext,
>>  noinline void __set_page_owner(struct page *page, unsigned short order,
>>  					gfp_t gfp_mask)
>>  {
>> -	struct page_ext *page_ext = lookup_page_ext(page);
>> +	struct page_ext *page_ext = page_ext_get(page);
>>  	depot_stack_handle_t handle;
>>  
>>  	if (unlikely(!page_ext))
>>  		return;
> Either add a comment like this
> 	/* save_stack can sleep in general so we have to page_ext_put */


Vlastimil suggested to go for save stack first since !page_ext is mostly
unlikely.  Snip from his comments:
Why not simply do the save_stack() first and then page_ext_get() just
once? It should be really rare that it's NULL, so I don't think we save
much by avoiding an unnecessary save_stack(), while the overhead of
doing two get/put instead of one will affect every call.

https://lore.kernel.org/all/f5fd4942-b03e-1d1c-213b-9cd5283ced91@suse.cz/
>> +	page_ext_put();
>>  
>>  	handle = save_stack(gfp_mask);
> or just drop the initial page_ext_get altogether. This function is
> called only when page_ext is supposed to be initialized and !page_ext
> case above should be very unlikely. Or is there any reason to keep this?
> 


>> +
>> +	/* Ensure page_ext is valid after page_ext_put() above */
>> +	page_ext = page_ext_get(page);
>> +	if (unlikely(!page_ext))
>> +		return;
>>  	__set_page_owner_handle(page_ext, handle, order, gfp_mask);
>> +	page_ext_put();
>>  }
>>  
> [...]
>> @@ -558,13 +590,17 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>>  		 */
>>  		handle = READ_ONCE(page_owner->handle);
>>  		if (!handle)
>> -			continue;
>> +			goto loop;
>>  
>>  		/* Record the next PFN to read in the file offset */
>>  		*ppos = (pfn - min_low_pfn) + 1;
>>  
>> +		memcpy(&page_owner_tmp, page_owner, sizeof(struct page_owner));
>> +		page_ext_put();
> why not
> 		page_owner_tmp = *page_owner;

Done!!
> 
>>  		return print_page_owner(buf, count, pfn, page,
>> -				page_owner, handle);
>> +				&page_owner_tmp, handle);
>> +loop:
>> +		page_ext_put();
>>  	}
>>  
>>  	return 0;

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