lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date:	Tue, 12 Aug 2014 15:24:25 +0800
From:	"Zhang, Yanmin" <>
To:	Peter Zijlstra <>,
	"Sha, Ruibin" <>
CC:	Chintan Pandya <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"Zhang, Yanmin" <>,
	"He, Bo" <>
Subject: Re: [PATCH]  export the function kmap_flush_unused.

On 2014/8/11 19:54, Peter Zijlstra wrote:
> On Mon, Aug 11, 2014 at 01:26:45AM +0000, Sha, Ruibin wrote:
>> Hi Chintan,
>> Thank you very much for your timely and kindly response and comments.
>> Here is more detail about our Scenario:
>>      We have a big driver on Android product. The driver allocates lots of
>>      DDR pages. When applications mmap a file exported from the driver,
>>      driver would mmap the pages to the application space, usually with
>>      uncachable prot.
>>      On ia32/x86_64 arch, we have to avoid page cache alias issue. When
>>      driver allocates the pages, it would change page original mapping in
>>      page table with uncachable prot. Sometimes, the allocated page was
>>      used by kmap/kunmap. After kunmap, the page is still mapped in KMAP
>>      space. The entries in KMAP page table are not cleaned up until a
>>      kernel thread flushes the freed KMAP pages(usually it is woken up by kunmap).
>>      It means the driver need  force to flush the KMAP page table entries before mapping pages to
>>      application space to be used. Otherwise, there is a race to create
>>      cache alias.
>>      To resolve this issue, we need export function kmap_flush_unused as
>>      the driver is compiled as module. Then, the driver calls
>>      kmap_flush_unused if the allocated pages are in HIGHMEM and being
>>      used by kmap.
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?

Sorry, Peter. Ruibin is a new guy in LKML community. He uses outlook
to send emails. He would improve that.

> That said, it sounds like you want set_memory_() to call
> kmap_flush_unused(). Because this race it not at all specific to your
> usage, it could happen to any set_memory_() site, right?
No. set_memory_() assumes the memory is not in HIGHMEM.
This scenario is driver allocates HIGHMEM pages, which are kmapped before.
Kernel uses a lazy method when kunmap a HIGHMEM page.
The pages are not unmapped from KMAP page table entries immediately.
When next kmap calling uses the same entry, kernel would change pte.
Or when change_page_attr_set_clr is called.

Our big driver doesn't call change_page_attr_set_clr when mmap the
pages with UNCACHABLE prot. It need call kmap_flush_unused directly after
allocating HIGHMEM pages.

Thanks for the kind comments.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

Powered by blists - more mailing lists