Re: [Xen-devel] [Patch 1/2] x86/vtd: Fix suspected data race condition in iommu_set_root_entry()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.10.13 at 11:48, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> Coverity ID: 1054967
> 
> Coverity spotted that iommu->root_maddr was optionally allocated within the
> protection of the iommu->lock, but was referenced with the protection of the
> iommu->register_lock, and freed without any lock.
> 
> Luckily, the code as-is is not vulnerable to the potential risks identified.
> 
> However, the alloc_pgtable_maddr() is far more appropriately done in
> iommu_alloc(), removing a set of spinlock calls, and a possibility for the
> iommu setup to fail later than iommu_alloc() with an -ENOMEM.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Keir Fraser <keir@xxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> CC: Xiantao Zhang <xiantao.zhang@xxxxxxxxx>
> ---
>  xen/drivers/passthrough/vtd/iommu.c |   19 +++----------------
>  1 file changed, 3 insertions(+), 16 deletions(-)
> 
> diff --git a/xen/drivers/passthrough/vtd/iommu.c 
> b/xen/drivers/passthrough/vtd/iommu.c
> index 12e0165..2dbe97a 100644
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -696,25 +696,9 @@ static void iommu_free_pagetable(u64 pt_maddr, int 
> level)
>  
>  static int iommu_set_root_entry(struct iommu *iommu)
>  {
> -    struct acpi_drhd_unit *drhd;
>      u32 sts;
>      unsigned long flags;
>  
> -    spin_lock(&iommu->lock);
> -
> -    if ( iommu->root_maddr == 0 )
> -    {
> -        drhd = iommu_to_drhd(iommu);
> -        iommu->root_maddr = alloc_pgtable_maddr(drhd, 1);
> -    }
> -
> -    if ( iommu->root_maddr == 0 )
> -    {
> -        spin_unlock(&iommu->lock);
> -        return -ENOMEM;
> -    }
> -
> -    spin_unlock(&iommu->lock);
>      spin_lock_irqsave(&iommu->register_lock, flags);
>      dmar_writeq(iommu->reg, DMAR_RTADDR_REG, iommu->root_maddr);
>  
> @@ -1144,6 +1128,9 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
>      iommu->intel->drhd = drhd;
>      drhd->iommu = iommu;
>  
> +    if ( !(iommu->root_maddr = alloc_pgtable_maddr(drhd, 1)) )
> +        return -ENOMEM;
> +
>      iommu->reg = ioremap(drhd->address, PAGE_SIZE);
>      if ( !iommu->reg )
>          return -ENOMEM;
> -- 
> 1.7.10.4




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