[PATCH] timer: prevent overflow in apply_slack

[PATCH] timer: prevent overflow in apply_slack

[Jiri Bohac]

Prevent overflow in the computation of timer expiry time inside
apply_slack().

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Suggested-by: Deborah Townsend <dstownse@us.ibm.com>

diff --git a/kernel/timer.c b/kernel/timer.c
index 87bd529..4c36c91 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
 
 	bit = find_last_bit(&mask, BITS_PER_LONG);
 
-	mask = (1 << bit) - 1;
+	mask = (1LL << bit) - 1;
 
 	expires_limit = expires_limit & ~(mask);
 
-- 
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ

Re: [PATCH] timer: prevent overflow in apply_slack

[Thomas Gleixner]

On Thu, 17 Apr 2014, Jiri Bohac wrote:

> Prevent overflow in the computation of timer expiry time inside
> apply_slack().

What's the impact of this overflow?

We don't want changelogs which describe what they do, we want them to
describe WHY and WHAT kind of problem the patch fixes.
 
Thanks,

	tglx

[PATCH v2] timer: prevent overflow in apply_slack

[Jiri Bohac]

On architectures with sizeof(int) < sizeof (long), the
computation of mask inside apply_slack() can be undefined if the
computed bit is > 32.

E.g. with: expires = 0xffffe6f5 and slack = 25, we get:

expires_limit = 0x20000000e
bit = 33
mask = (1 << 33) - 1  /* undefined */

On x86, mask becomes 1 and and the slack is not applied properly.
On s390, mask is -1, expires is set to 0 and the timer fires immediately.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Suggested-by: Deborah Townsend <dstownse@us.ibm.com>

diff --git a/kernel/timer.c b/kernel/timer.c
index 87bd529..4c36c91 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
 
 	bit = find_last_bit(&mask, BITS_PER_LONG);
 
-	mask = (1 << bit) - 1;
+	mask = (1LL << bit) - 1;
 
 	expires_limit = expires_limit & ~(mask);
 
-- 

-- 
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ

Re: [PATCH v2] timer: prevent overflow in apply_slack

[Jiri Bohac]

Thomas, does this make sense now, with the new description?

On Fri, Apr 18, 2014 at 05:23:11PM +0200, Jiri Bohac wrote:
> On architectures with sizeof(int) < sizeof (long), the
> computation of mask inside apply_slack() can be undefined if the
> computed bit is > 32.
> 
> E.g. with: expires = 0xffffe6f5 and slack = 25, we get:
> 
> expires_limit = 0x20000000e
> bit = 33
> mask = (1 << 33) - 1  /* undefined */
> 
> On x86, mask becomes 1 and and the slack is not applied properly.
> On s390, mask is -1, expires is set to 0 and the timer fires immediately.
> 
> Signed-off-by: Jiri Bohac <jbohac@suse.cz>
> Suggested-by: Deborah Townsend <dstownse@us.ibm.com>
> 
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 87bd529..4c36c91 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
>  
>  	bit = find_last_bit(&mask, BITS_PER_LONG);
>  
> -	mask = (1 << bit) - 1;
> +	mask = (1LL << bit) - 1;
>  
>  	expires_limit = expires_limit & ~(mask);
>  

Thanks,

-- 
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ

Re: [PATCH v2] timer: prevent overflow in apply_slack

[Thomas Gleixner]

On Tue, 29 Apr 2014, Jiri Bohac wrote:

> Thomas, does this make sense now, with the new description?

Yep, except
 
> On Fri, Apr 18, 2014 at 05:23:11PM +0200, Jiri Bohac wrote:
> > On architectures with sizeof(int) < sizeof (long), the
> > computation of mask inside apply_slack() can be undefined if the
> > computed bit is > 32.
> > 
> > E.g. with: expires = 0xffffe6f5 and slack = 25, we get:
> > 
> > expires_limit = 0x20000000e
> > bit = 33
> > mask = (1 << 33) - 1  /* undefined */
> > 
> > On x86, mask becomes 1 and and the slack is not applied properly.
> > On s390, mask is -1, expires is set to 0 and the timer fires immediately.
> > 
> > Signed-off-by: Jiri Bohac <jbohac@suse.cz>
> > Suggested-by: Deborah Townsend <dstownse@us.ibm.com>
> > 
> > diff --git a/kernel/timer.c b/kernel/timer.c
> > index 87bd529..4c36c91 100644
> > --- a/kernel/timer.c
> > +++ b/kernel/timer.c
> > @@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
> >  
> >  	bit = find_last_bit(&mask, BITS_PER_LONG);
> >  
> > -	mask = (1 << bit) - 1;
> > +	mask = (1LL << bit) - 1;

This should be 1UL, shouldn't it?

> >  	expires_limit = expires_limit & ~(mask);
> >  
> 
> Thanks,
> 
> -- 
> Jiri Bohac <jbohac@suse.cz>
> SUSE Labs, SUSE CZ
> 
> 

Re: [PATCH v3] timer: prevent overflow in apply_slack

[Jiri Bohac]

On Tue, Apr 29, 2014 at 07:22:25PM +0200, Thomas Gleixner wrote:
> > > +	mask = (1LL << bit) - 1;
> 
> This should be 1UL, shouldn't it?

yes, good catch, thanks!




On architectures with sizeof(int) < sizeof (long), the
computation of mask inside apply_slack() can be undefined if the
computed bit is > 32.

E.g. with: expires = 0xffffe6f5 and slack = 25, we get:

expires_limit = 0x20000000e
bit = 33
mask = (1 << 33) - 1  /* undefined */

On x86, mask becomes 1 and and the slack is not applied properly.
On s390, mask is -1, expires is set to 0 and the timer fires immediately.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Suggested-by: Deborah Townsend <dstownse@us.ibm.com>

diff --git a/kernel/timer.c b/kernel/timer.c
index 87bd529..4c36c91 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
 
 	bit = find_last_bit(&mask, BITS_PER_LONG);
 
-	mask = (1 << bit) - 1;
+	mask = (1UL << bit) - 1;
 
 	expires_limit = expires_limit & ~(mask);
 
-- 
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ

[tip:timers/urgent] timer: Prevent overflow in apply_slack

[tip-bot for Jiri Bohac]

Commit-ID:  98a01e779f3c66b0b11cd7e64d531c0e41c95762
Gitweb:     http://git.kernel.org/tip/98a01e779f3c66b0b11cd7e64d531c0e41c95762
Author:     Jiri Bohac <jbohac@suse.cz>
AuthorDate: Fri, 18 Apr 2014 17:23:11 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 30 Apr 2014 13:46:17 +0200

timer: Prevent overflow in apply_slack

On architectures with sizeof(int) < sizeof (long), the
computation of mask inside apply_slack() can be undefined if the
computed bit is > 32.

E.g. with: expires = 0xffffe6f5 and slack = 25, we get:

expires_limit = 0x20000000e
bit = 33
mask = (1 << 33) - 1  /* undefined */

On x86, mask becomes 1 and and the slack is not applied properly.
On s390, mask is -1, expires is set to 0 and the timer fires immediately.

Use 1UL << bit to solve that issue.

Suggested-by: Deborah Townsend <dstownse@us.ibm.com>
Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20140418152310.GA13654@midget.suse.cz
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 kernel/timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index 87bd529..3bb01a3 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
 
 	bit = find_last_bit(&mask, BITS_PER_LONG);
 
-	mask = (1 << bit) - 1;
+	mask = (1UL << bit) - 1;
 
 	expires_limit = expires_limit & ~(mask);