Re: [PATCH v5 1/3] net: introduce helper sendpage_ok() in include/linux/net.h

[Coly Li]

On 2020/8/18 16:08, Eric Dumazet wrote:
> 
> 
> On 8/16/20 12:08 AM, Coly Li wrote:
>> The original problem was from nvme-over-tcp code, who mistakenly uses
>> kernel_sendpage() to send pages allocated by __get_free_pages() without
>> __GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
>> tail pages, sending them by kernel_sendpage() may trigger a kernel panic
>> from a corrupted kernel heap, because these pages are incorrectly freed
>> in network stack as page_count 0 pages.
>>
>> This patch introduces a helper sendpage_ok(), it returns true if the
>> checking page,
>> - is not slab page: PageSlab(page) is false.
>> - has page refcount: page_count(page) is not zero
>>
>> All drivers who want to send page to remote end by kernel_sendpage()
>> may use this helper to check whether the page is OK. If the helper does
>> not return true, the driver should try other non sendpage method (e.g.
>> sock_no_sendpage()) to handle the page.
>>
>>
>>
>> diff --git a/include/linux/net.h b/include/linux/net.h
>> index d48ff1180879..a807fad31958 100644
>> --- a/include/linux/net.h
>> +++ b/include/linux/net.h
>> @@ -21,6 +21,7 @@
>>  #include <linux/rcupdate.h>
>>  #include <linux/once.h>
>>  #include <linux/fs.h>
>> +#include <linux/mm.h>
>>  #include <linux/sockptr.h>
>>  
>>  #include <uapi/linux/net.h>
>> @@ -286,6 +287,21 @@ do {									\
>>  #define net_get_random_once_wait(buf, nbytes)			\
>>  	get_random_once_wait((buf), (nbytes))
>>  
>> +/*
>> + * E.g. XFS meta- & log-data is in slab pages, or bcache meta
>> + * data pages, or other high order pages allocated by
>> + * __get_free_pages() without __GFP_COMP, which have a page_count
>> + * of 0 and/or have PageSlab() set. We cannot use send_page for
>> + * those, as that does get_page(); put_page(); and would cause
>> + * either a VM_BUG directly, or __page_cache_release a page that
>> + * would actually still be referenced by someone, leading to some
>> + * obscure delayed Oops somewhere else.
>> + */
>> +static inline bool sendpage_ok(struct page *page)
>> +{
>> +	return  (!PageSlab(page) && page_count(page) >= 1);
>> +}
>>

Hi Eric,

> 
> return (A);
> 
> Can simply be written :
> 
> return A;
> 
> In this case :
> 
> return !PageSlab(page) && page_count(page) >= 1;

Sure, I update it in v6 series.


> 
> BTW, do you have plans to refine code added with commit a10674bf2406afc2554f9c7d31b2dc65d6a27fd9
> ("tcp: detecting the misuse of .sendpage for Slab objects")
> 

Thanks for the hint, I will remove this piece in v6 series.

Coly Li