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From: Brian Gerst on 12 Aug 2010 07:40
On Thu, Aug 12, 2010 at 6:45 AM, Xiaotian Feng <dfeng(a)redhat.com> wrote:
> fpu.state is allocated from task_xstate_cachep, the size of
> task_xstate_cachep is xstate_size. But fpu.state is an union
> struct, which size is bigger than xstate_size if cpu_has_xsave,
> so if we want to visit fpu.state->xsave, the memory we allocated
> for fpu.state is not enough.
>
> This caused many poison/redzone overwritten alerts on task_xstate while using kvm.
>
> [ 1899.399373] =============================================================================
> [ 1899.399377] BUG task_xstate: Poison overwritten
> [ 1899.399378] -----------------------------------------------------------------------------
> [ 1899.399379]
> [ 1899.399382] INFO: 0xffff88020aca2100-0xffff88020aca217f. First byte 0x0 instead of 0x6b
> [ 1899.399385] INFO: Slab 0xffffea000725c300 objects=23 used=12 fp=0xffff88020aca2100 flags=0x200000000040c1
> [ 1899.399387] INFO: Object 0xffff88020aca2100 @offset=8448 fp=0xffff88020aca23c0
>
> With this patch applied, the poison overwritten alert disappeared.
>
> Signed-off-by: Xiaotian Feng <dfeng(a)redhat.com>
> Cc: Thomas Gleixner <tglx(a)linutronix.de>
> Cc: Ingo Molnar <mingo(a)redhat.com>
> Cc: "H. Peter Anvin" <hpa(a)zytor.com>
> Cc: Brian Gerst <brgerst(a)gmail.com>
> Cc: Avi Kivity <avi(a)redhat.com>
> Cc: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
> ---
>  arch/x86/kernel/process.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index d401f1d..609bee5 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -59,7 +59,7 @@ void free_thread_info(struct thread_info *ti)
>  void arch_task_cache_init(void)
>  {
>         task_xstate_cachep =
> -               kmem_cache_create("task_xstate", xstate_size,
> +               kmem_cache_create("task_xstate", sizeof(union thread_xstate),
>                                  __alignof__(union thread_xstate),
>                                  SLAB_PANIC | SLAB_NOTRACK, NULL);
>  }

This isn't the right solution. The point of having a variable
xstate_size is to only allocate the memory that is needed for the
capabilities of the cpu. The real bug is that some code is trying to
use xsave_struct when it either shouldn't be (because the cpu doesn't
support xsave), or because xstate_size < sizeof(xsave_struct).

Looking at xstate_enable_boot_cpu(), the xstate_size is set from
cpuid. It's possible that it is smaller than sizeof(xsave_strruct).
Maybe a sanity check should be added there?

--
Brian Gerst
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