Re: [Xen-devel] [PATCH] KVM, XEN: Fix potential race in pvclock code

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Jan 27, 2014 at 01:33:01PM +0100, Julian Stecklina wrote:
> On 01/24/2014 07:08 PM, Konrad Rzeszutek Wilk wrote:
> > On Thu, Jan 16, 2014 at 03:13:44PM +0100, Julian Stecklina wrote:
> >> The paravirtualized clock used in KVM and Xen uses a version field to
> >> allow the guest to see when the shared data structure is inconsistent.
> >> The code reads the version field twice (before and after the data
> >> structure is copied) and checks whether they haven't
> >> changed and that the lowest bit is not set. As the second access is not
> >> synchronized, the compiler could generate code that accesses version
> >> more than two times and you end up with inconsistent data.
> > 
> > Could you paste in the code that the 'bad' compiler generates
> > vs the compiler that generate 'good' code please?
> 
> At least 4.8 and probably older compilers compile this as intended. The
> point is that the standard does not guarantee the indented behavior,
> i.e. the code is wrong.

Perhaps I misunderstood Jan's response but it sounded to me like
that the compiler was not adhering to the standard?

> 
> I can refer to this lwn article:
> https://lwn.net/Articles/508991/
> 
> The whole point of ACCESS_ONCE is to avoid time bombs like that. There
> are lots of place where ACCESS_ONCE is used in the kernel:
> 
> http://lxr.free-electrons.com/ident?i=ACCESS_ONCE
> 
> See for example the check_zero function here:
> http://lxr.free-electrons.com/source/arch/x86/kernel/kvm.c#L559
> 

In other words, you don't have a sample of 'bad' compiler code.


> Julian
> 
> > 
> >>
> >> An example using pvclock_get_time_values:
> >>
> >> host starts updating data, sets src->version to 1
> >> guest reads src->version (1) and stores it into dst->version.
> >> guest copies inconsistent data
> >> guest reads src->version (1) and computes xor with dst->version.
> >> host finishes updating data and sets src->version to 2
> >> guest reads src->version (2) and checks whether lower bit is not set.
> >> while loop exits with inconsistent data!
> >>
> >> AFAICS the compiler is allowed to optimize the given code this way.
> >>
> >> Signed-off-by: Julian Stecklina <jsteckli@xxxxxxxxxxxxxxxxxxxx>
> >> ---
> >>  arch/x86/kernel/pvclock.c | 10 +++++++---
> >>  1 file changed, 7 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
> >> index 42eb330..f62b41c 100644
> >> --- a/arch/x86/kernel/pvclock.c
> >> +++ b/arch/x86/kernel/pvclock.c
> >> @@ -55,6 +55,8 @@ static u64 pvclock_get_nsec_offset(struct 
> >> pvclock_shadow_time *shadow)
> >>  static unsigned pvclock_get_time_values(struct pvclock_shadow_time *dst,
> >>                                    struct pvclock_vcpu_time_info *src)
> >>  {
> >> +  u32 nversion;
> >> +
> >>    do {
> >>            dst->version = src->version;
> >>            rmb();          /* fetch version before data */
> >> @@ -64,7 +66,8 @@ static unsigned pvclock_get_time_values(struct 
> >> pvclock_shadow_time *dst,
> >>            dst->tsc_shift         = src->tsc_shift;
> >>            dst->flags             = src->flags;
> >>            rmb();          /* test version after fetching data */
> >> -  } while ((src->version & 1) || (dst->version != src->version));
> >> +          nversion = ACCESS_ONCE(src->version);
> >> +  } while ((nversion & 1) || (dst->version != nversion));
> >>  
> >>    return dst->version;
> >>  }
> >> @@ -135,7 +138,7 @@ void pvclock_read_wallclock(struct pvclock_wall_clock 
> >> *wall_clock,
> >>                        struct pvclock_vcpu_time_info *vcpu_time,
> >>                        struct timespec *ts)
> >>  {
> >> -  u32 version;
> >> +  u32 version, nversion;
> >>    u64 delta;
> >>    struct timespec now;
> >>  
> >> @@ -146,7 +149,8 @@ void pvclock_read_wallclock(struct pvclock_wall_clock 
> >> *wall_clock,
> >>            now.tv_sec  = wall_clock->sec;
> >>            now.tv_nsec = wall_clock->nsec;
> >>            rmb();          /* fetch time before checking version */
> >> -  } while ((wall_clock->version & 1) || (version != wall_clock->version));
> >> +          nversion = ACCESS_ONCE(wall_clock->version);
> >> +  } while ((nversion & 1) || (version != nversion));
> >>  
> >>    delta = pvclock_clocksource_read(vcpu_time);    /* time since system 
> >> boot */
> >>    delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
> >> -- 
> >> 1.8.4.2
> >>
> >>
> >> _______________________________________________
> >> Xen-devel mailing list
> >> Xen-devel@xxxxxxxxxxxxx
> >> http://lists.xen.org/xen-devel
> 
> 



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