Re: [PATCH] futex: Fix futex lock the wrong page

Re: [PATCH] futex: Fix futex lock the wrong page

[zhangxiaoxu (A)]

This patch is for stable branch linux-4.4-y.

On 2019/6/12 9:54, ZhangXiaoxu wrote:
> The upstram commit 65d8fc777f6d ("futex: Remove requirement
> for lock_page() in get_futex_key()") use variable 'page' as
> the page head, when merge it to stable branch, the variable
> `page_head` is page head.
> 
> In the stable branch, the variable `page` not means the page
> head, when lock the page head, we should lock 'page_head',
> rather than 'page'.
> 
> It maybe lead a hung task problem.
> 
> Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
> Cc: stable@vger.kernel.org
> ---
>   kernel/futex.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index ec9df5b..15d850f 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -593,8 +593,8 @@ again:
>   		 * applies. If this is really a shmem page then the page lock
>   		 * will prevent unexpected transitions.
>   		 */
> -		lock_page(page);
> -		shmem_swizzled = PageSwapCache(page) || page->mapping;
> +		lock_page(page_head);
> +		shmem_swizzled = PageSwapCache(page_head) || page_head->mapping;
>   		unlock_page(page_head);
>   		put_page(page_head);
>   
> 

[PATCH] futex: Fix futex lock the wrong page

[ZhangXiaoxu]

The upstram commit 65d8fc777f6d ("futex: Remove requirement
for lock_page() in get_futex_key()") use variable 'page' as
the page head, when merge it to stable branch, the variable
`page_head` is page head.

In the stable branch, the variable `page` not means the page
head, when lock the page head, we should lock 'page_head',
rather than 'page'.

It maybe lead a hung task problem.

Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
Cc: stable@vger.kernel.org
---
 kernel/futex.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index ec9df5b..15d850f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -593,8 +593,8 @@ again:
 		 * applies. If this is really a shmem page then the page lock
 		 * will prevent unexpected transitions.
 		 */
-		lock_page(page);
-		shmem_swizzled = PageSwapCache(page) || page->mapping;
+		lock_page(page_head);
+		shmem_swizzled = PageSwapCache(page_head) || page_head->mapping;
 		unlock_page(page_head);
 		put_page(page_head);
 
-- 
2.7.4

Re: [PATCH] futex: Fix futex lock the wrong page

[Greg KH]

On Wed, Jun 12, 2019 at 09:50:25AM +0800, zhangxiaoxu (A) wrote:
> This patch is for stable branch linux-4.4-y.
> 
> On 2019/6/12 9:54, ZhangXiaoxu wrote:
> > The upstram commit 65d8fc777f6d ("futex: Remove requirement
> > for lock_page() in get_futex_key()") use variable 'page' as
> > the page head, when merge it to stable branch, the variable
> > `page_head` is page head.
> > 
> > In the stable branch, the variable `page` not means the page
> > head, when lock the page head, we should lock 'page_head',
> > rather than 'page'.
> > 
> > It maybe lead a hung task problem.
> > 
> > Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
> > Cc: stable@vger.kernel.org
> > ---
> >   kernel/futex.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)

I do not understand.

Please read
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to submit a patch to the stable trees properly.

If the commit is not in Linus's tree, then we can not take it, unless
something is _very_ broken and it is the only way it can be resolved.

thanks,

greg k-h

Re: [PATCH] futex: Fix futex lock the wrong page

[Thomas Gleixner]

On Wed, 12 Jun 2019, Greg KH wrote:
> On Wed, Jun 12, 2019 at 09:50:25AM +0800, zhangxiaoxu (A) wrote:
> > This patch is for stable branch linux-4.4-y.
> > 
> > On 2019/6/12 9:54, ZhangXiaoxu wrote:
> > > The upstram commit 65d8fc777f6d ("futex: Remove requirement
> > > for lock_page() in get_futex_key()") use variable 'page' as
> > > the page head, when merge it to stable branch, the variable
> > > `page_head` is page head.
> > > 
> > > In the stable branch, the variable `page` not means the page
> > > head, when lock the page head, we should lock 'page_head',
> > > rather than 'page'.
> > > 
> > > It maybe lead a hung task problem.
> > > 
> > > Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
> > > Cc: stable@vger.kernel.org
> > > ---
> > >   kernel/futex.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> I do not understand.
> 
> Please read
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to submit a patch to the stable trees properly.
> 
> If the commit is not in Linus's tree, then we can not take it, unless
> something is _very_ broken and it is the only way it can be resolved.

There is something _very_ broken. Upstream is correct but the 4.4. backport
of the above commit is broken (93dcb09e29bb24a86aa7b7eff65e424f7dc98af2) in
the way Zhang described. So it's a 4.4. only issue.

Thanks,

	tglx

Re: [PATCH] futex: Fix futex lock the wrong page

[Greg KH]

On Wed, Jun 12, 2019 at 09:29:48AM +0200, Thomas Gleixner wrote:
> On Wed, 12 Jun 2019, Greg KH wrote:
> > On Wed, Jun 12, 2019 at 09:50:25AM +0800, zhangxiaoxu (A) wrote:
> > > This patch is for stable branch linux-4.4-y.
> > > 
> > > On 2019/6/12 9:54, ZhangXiaoxu wrote:
> > > > The upstram commit 65d8fc777f6d ("futex: Remove requirement
> > > > for lock_page() in get_futex_key()") use variable 'page' as
> > > > the page head, when merge it to stable branch, the variable
> > > > `page_head` is page head.
> > > > 
> > > > In the stable branch, the variable `page` not means the page
> > > > head, when lock the page head, we should lock 'page_head',
> > > > rather than 'page'.
> > > > 
> > > > It maybe lead a hung task problem.
> > > > 
> > > > Signed-off-by: ZhangXiaoxu <zhangxiaoxu5@huawei.com>
> > > > Cc: stable@vger.kernel.org
> > > > ---
> > > >   kernel/futex.c | 4 ++--
> > > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > I do not understand.
> > 
> > Please read
> >     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> > for how to submit a patch to the stable trees properly.
> > 
> > If the commit is not in Linus's tree, then we can not take it, unless
> > something is _very_ broken and it is the only way it can be resolved.
> 
> There is something _very_ broken. Upstream is correct but the 4.4. backport
> of the above commit is broken (93dcb09e29bb24a86aa7b7eff65e424f7dc98af2) in
> the way Zhang described. So it's a 4.4. only issue.

That wasn't obvious at all, and wasn't cc:ed to stable to start with...

Anyway, I'll go queue this up to the 4.4.y tree if no one objects...

thanks,

greg k-h