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

From: Rob Herring
Date: Mon Sep 16 2019 - 18:53:26 EST

On Fri, Sep 13, 2019 at 12:25 PM Alyssa Rosenzweig
<alyssa.rosenzweig@xxxxxxxxxxxxx> wrote:
>
> I'm conflicted on this series.
>
> On the one hand, userspace should obviously not be able to crash the
> kernel. So the crash should be fixed in one way or another.
>
> On the other hand, userspace really has to supply all the BOs it uses
> for correctness. I realize the DDK doesn't do this but... it probably
> should, umm...
>
> Would it be possible to check for the NULL pointer in the kernel and
> skip printing information that would require a dereference? (Without
> having to play games with reference counts). Presumably that might fix
> crashes in other corner cases.

I don't think that prevents the use after free. I think the NULL is a
pointer within the BO, not the BO. So we'd still be retrieving the
NULL ptr from a freed object.

Rob

>
> On Thu, Sep 05, 2019 at 01:11:41PM +0100, Steven Price wrote:
> > 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@xxxxxxx>
> > ---
> >
> > 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
> >