linux-kernel - [RFC] vfio: remove useless judgement

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Message-Id: <20220623115603.22288-1-lizhe.67@bytedance.com>
Date:   Thu, 23 Jun 2022 19:56:03 +0800
From:   lizhe.67@...edance.com
To:     alex.williamson@...hat.com, cohuck@...hat.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        lizefan.x@...edance.com, lizhe.67@...edance.com
Subject: [RFC] vfio: remove useless judgement

From: Li Zhe <lizhe.67@...edance.com>

In function vfio_dma_do_unmap(), we currently prevent process to unmap
vfio dma region whose mm_struct is different from the vfio_dma->task.
In our virtual machine scenario which is using kvm and qemu, this
judgement stops us from liveupgrading our qemu, which uses fork() &&
exec() to load the new binary but the new process cannot do the
VFIO_IOMMU_UNMAP_DMA action during vm exit because of this judgement.

This judgement is added in commit 8f0d5bb95f76 ("vfio iommu type1: Add
task structure to vfio_dma") for the security reason. But it seems that
no other task who has no family relationship with old and new process
can get the same vfio_dma struct here for the reason of resource
isolation. So this patch delete it.

Signed-off-by: Li Zhe <lizhe.67@...edance.com>
---
 drivers/vfio/vfio_iommu_type1.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index c13b9290e357..a8ff00dad834 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -1377,12 +1377,6 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,

 		if (!iommu->v2 && iova > dma->iova)
 			break;
-		/*
-		 * Task with same address space who mapped this iova range is
-		 * allowed to unmap the iova range.
-		 */
-		if (dma->task->mm != current->mm)
-			break;

 		if (invalidate_vaddr) {
 			if (dma->vaddr_invalid) {
-- 
2.20.1