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Subject: Re: [PATCH v2] iommu/sva: Fix PASID use-after-free issue
To:     Fenghua Yu <fenghua.yu@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
        Zhangfei Gao <zhangfei.gao@linaro.org>,
        Will Deacon <will@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Tony Luck <tony.luck@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, x86 <x86@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        iommu@lists.linux-foundation.org
References: <20220428180041.806809-1-fenghua.yu@intel.com>
From:   Zhangfei Gao <zhangfei.gao@linaro.org>
Message-ID: <8f50c673-fe92-3c42-993d-43e65fc7235c@linaro.org>
Date:   Fri, 29 Apr 2022 09:39:09 +0800
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On 2022/4/29 上午2:00, Fenghua Yu wrote:
> The PASID is being freed too early.  It needs to stay around until after
> device drivers that might be using it have had a chance to clear it out
> of the hardware.
>
> As a reminder:
>
> mmget() /mmput()  refcount the mm's address space
> mmgrab()/mmdrop() refcount the mm itself
>
> The PASID is currently tied to the life of the mm's address space and
> freed in __mmput().  This makes logical sense because the PASID can't be
> used once the address space is gone.
>
> But, this misses an important point: even after the address space is
> gone, the PASID will still be programmed into a device.  Device drivers
> might, for instance, still need to flush operations that are outstanding
> and need to use that PASID.  They do this at file->release() time.
>
> Device drivers call the IOMMU driver to hold a reference on the mm itself
> and drop it at file->release() time.  But, the IOMMU driver holds a
> reference on the mm itself, not the address space.  The address space
> (and the PASID) is long gone by the time the driver tries to clean up.
> This is effectively a use-after-free bug on the PASID.
>
> To fix this, move the PASID free operation from __mmput() to __mmdrop().
> This ensures that the IOMMU driver's existing mmgrab() keeps the PASID
> allocated until it drops its mm reference.
>
> Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit")
>
> Reported-by: Zhangfei Gao <zhangfei.gao@foxmail.com>
> Tested-by: Zhangfei Gao <zhangfei.gao@foxmail.com>

Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>

Use the formal email, thanks

> Suggested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Suggested-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> ---
>
> v2:
> - Dave Hansen rewrites the change log.
> - Add Tested-by: Zhangfei Gao <zhangfei.gao@foxmail.com>
> - Add Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
>
> The original patch was posted and discussed in:
> https://lore.kernel.org/lkml/YmdzFFx7fN586jcf@fyu1.sc.intel.com/
>
>   kernel/fork.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 9796897560ab..35a3beff140b 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -792,6 +792,7 @@ void __mmdrop(struct mm_struct *mm)
>   	mmu_notifier_subscriptions_destroy(mm);
>   	check_mm(mm);
>   	put_user_ns(mm->user_ns);
> +	mm_pasid_drop(mm);
>   	free_mm(mm);
>   }
>   EXPORT_SYMBOL_GPL(__mmdrop);
> @@ -1190,7 +1191,6 @@ static inline void __mmput(struct mm_struct *mm)
>   	}
>   	if (mm->binfmt)
>   		module_put(mm->binfmt->module);
> -	mm_pasid_drop(mm);
>   	mmdrop(mm);
>   }
>