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From:   Steven Price <steven.price@arm.com>
To:     Rob Herring <robh@kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
        Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
        Steven Price <steven.price@arm.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: [PATCH] drm/panfrost: Prevent race when handling page fault
Date:   Thu,  5 Sep 2019 13:11:41 +0100
Message-Id: <20190905121141.42820-1-steven.price@arm.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When handling a GPU page fault addr_to_drm_mm_node() is used to
translate the GPU address to a buffer object. However it is possible for
the buffer object to be freed after the function has returned resulting
in a use-after-free of the BO.

Change addr_to_drm_mm_node to return the panfrost_gem_object with an
extra reference on it, preventing the BO from being freed until after
the page fault has been handled.

Signed-off-by: Steven Price <steven.price@arm.com>
---

I've managed to trigger this, generating the following stack trace.

Unable to handle kernel NULL pointer dereference at virtual address 00000090
pgd = 33a6a181
[00000090] *pgd=00000000
Internal error: Oops: 5 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 81 Comm: irq/60-mmu Not tainted 5.3.0-rc1+ #4
Hardware name: Rockchip (Device Tree)
PC is at panfrost_mmu_map_fault_addr+0x140/0x3a0
LR is at _raw_spin_unlock+0x20/0x3c
pc : [<c0508944>]    lr : [<c07ced10>]    psr: 20030013
sp : ec643e88  ip : ea70ce24  fp : ec5fe840
r10: 00000001  r9 : 00000000  r8 : 000178c9
r7 : 00000000  r6 : 178c9f00  r5 : ec5fe940  r4 : ea70ce08
r3 : 00000000  r2 : 00000000  r1 : ec640e00  r0 : ec5fe940
Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 10c5387d  Table: 29f8406a  DAC: 00000051
Process irq/60-mmu (pid: 81, stack limit = 0x4b907106)
Stack: (0xec643e88 to 0xec644000)
3e80:                   ec640e00 c07cede0 c0c0b200 ec640e00 00000000 00000000
3ea0: 000178c9 00000000 c0c0b200 c07cede0 eef91040 ec5fe840 00000001 00000000
3ec0: 00010001 010003c3 000000c3 00000001 178c9f00 c0508c98 eef91050 00000000
3ee0: ec643f34 c07c9188 00000001 c0167854 00000000 ec643ef8 c0c08448 c07c933c
3f00: 00000000 ec643f04 fffefffe 3d182b17 ee193468 ee193400 ec5db3c0 ec5db3c0
3f20: c0183840 ec5db3e4 c0cb2874 c0183b08 00000001 c018385c ffffe000 ee193400
3f40: ec5db3c0 c0183b8c c0c08448 00000000 60000013 00000000 c01839b8 3d182b17
3f60: ffffe000 ec5d0500 ec5db380 ec642000 ec5db3c0 c0183a64 00000000 ed025cd8
3f80: ec5d0538 c0146cfc ec642000 ec5db380 c0146bc0 00000000 00000000 00000000
3fa0: 00000000 00000000 00000000 c01010b4 00000000 00000000 00000000 00000000
3fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
3fe0: 00000000 00000000 00000000 00000000 00000013 00000000 00000000 00000000
[<c0508944>] (panfrost_mmu_map_fault_addr) from [<c0508c98>] (panfrost_mmu_irq_handler_thread+0xf4/0x248)
[<c0508c98>] (panfrost_mmu_irq_handler_thread) from [<c018385c>] (irq_thread_fn+0x1c/0x58)
[<c018385c>] (irq_thread_fn) from [<c0183b8c>] (irq_thread+0x128/0x240)
[<c0183b8c>] (irq_thread) from [<c0146cfc>] (kthread+0x13c/0x154)
[<c0146cfc>] (kthread) from [<c01010b4>] (ret_from_fork+0x14/0x20)
Exception stack(0xec643fb0 to 0xec643ff8)
3fa0:                                     00000000 00000000 00000000 00000000
3fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
3fe0: 00000000 00000000 00000000 00000000 00000013 00000000
Code: 1affffec eaffffe8 e5143004 e59d2014 (e5933090)
---[ end trace 04eadc3009b8f507 ]---
---
 drivers/gpu/drm/panfrost/panfrost_mmu.c | 38 ++++++++++++++++---------
 1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
index 842bdd7cf6be..115925e7460a 100644
--- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
+++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
@@ -392,9 +392,11 @@ void panfrost_mmu_pgtable_free(struct panfrost_file_priv *priv)
 	free_io_pgtable_ops(mmu->pgtbl_ops);
 }
 
-static struct drm_mm_node *addr_to_drm_mm_node(struct panfrost_device *pfdev, int as, u64 addr)
+static struct panfrost_gem_object *
+addr_to_drm_mm_node(struct panfrost_device *pfdev, int as, u64 addr)
 {
-	struct drm_mm_node *node = NULL;
+	struct panfrost_gem_object *bo = NULL;
+	struct drm_mm_node *node;
 	u64 offset = addr >> PAGE_SHIFT;
 	struct panfrost_mmu *mmu;
 
@@ -406,14 +408,17 @@ static struct drm_mm_node *addr_to_drm_mm_node(struct panfrost_device *pfdev, in
 
 		priv = container_of(mmu, struct panfrost_file_priv, mmu);
 		drm_mm_for_each_node(node, &priv->mm) {
-			if (offset >= node->start && offset < (node->start + node->size))
+			if (offset >= node->start &&
+			    offset < (node->start + node->size)) {
+				bo = drm_mm_node_to_panfrost_bo(node);
+				drm_gem_object_get(&bo->base.base);
 				goto out;
+			}
 		}
 	}
-
 out:
 	spin_unlock(&pfdev->as_lock);
-	return node;
+	return bo;
 }
 
 #define NUM_FAULT_PAGES (SZ_2M / PAGE_SIZE)
@@ -421,29 +426,28 @@ static struct drm_mm_node *addr_to_drm_mm_node(struct panfrost_device *pfdev, in
 int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as, u64 addr)
 {
 	int ret, i;
-	struct drm_mm_node *node;
 	struct panfrost_gem_object *bo;
 	struct address_space *mapping;
 	pgoff_t page_offset;
 	struct sg_table *sgt;
 	struct page **pages;
 
-	node = addr_to_drm_mm_node(pfdev, as, addr);
-	if (!node)
+	bo = addr_to_drm_mm_node(pfdev, as, addr);
+	if (!bo)
 		return -ENOENT;
 
-	bo = drm_mm_node_to_panfrost_bo(node);
 	if (!bo->is_heap) {
 		dev_WARN(pfdev->dev, "matching BO is not heap type (GPU VA = %llx)",
-			 node->start << PAGE_SHIFT);
-		return -EINVAL;
+			 bo->node.start << PAGE_SHIFT);
+		ret = -EINVAL;
+		goto err_bo;
 	}
 	WARN_ON(bo->mmu->as != as);
 
 	/* Assume 2MB alignment and size multiple */
 	addr &= ~((u64)SZ_2M - 1);
 	page_offset = addr >> PAGE_SHIFT;
-	page_offset -= node->start;
+	page_offset -= bo->node.start;
 
 	mutex_lock(&bo->base.pages_lock);
 
@@ -452,7 +456,8 @@ int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as, u64 addr)
 				     sizeof(struct sg_table), GFP_KERNEL | __GFP_ZERO);
 		if (!bo->sgts) {
 			mutex_unlock(&bo->base.pages_lock);
-			return -ENOMEM;
+			ret = -ENOMEM;
+			goto err_bo;
 		}
 
 		pages = kvmalloc_array(bo->base.base.size >> PAGE_SHIFT,
@@ -461,7 +466,8 @@ int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as, u64 addr)
 			kfree(bo->sgts);
 			bo->sgts = NULL;
 			mutex_unlock(&bo->base.pages_lock);
-			return -ENOMEM;
+			ret = -ENOMEM;
+			goto err_bo;
 		}
 		bo->base.pages = pages;
 		bo->base.pages_use_count = 1;
@@ -499,12 +505,16 @@ int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as, u64 addr)
 
 	dev_dbg(pfdev->dev, "mapped page fault @ AS%d %llx", as, addr);
 
+	drm_gem_object_put_unlocked(&bo->base.base);
+
 	return 0;
 
 err_map:
 	sg_free_table(sgt);
 err_pages:
 	drm_gem_shmem_put_pages(&bo->base);
+err_bo:
+	drm_gem_object_put_unlocked(&bo->base.base);
 	return ret;
 }
 
-- 
2.20.1