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    Subject[PATCH] drm/panfrost: Prevent race when handling page fault
    Date
    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
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