Re: [PATCH] tee: handle lookup of shm with reference count 0

From: Greg KH
Date: Tue Dec 14 2021 - 11:52:06 EST

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On Tue, Dec 14, 2021 at 05:31:44PM +0100, Jens Wiklander wrote:
> On Tue, Dec 14, 2021 at 04:25:11PM +0100, Greg KH wrote:
> > On Tue, Dec 14, 2021 at 03:59:57PM +0100, Jens Wiklander wrote:
> > > On Tue, Dec 14, 2021 at 02:44:30PM +0100, Greg KH wrote:
> > > > On Tue, Dec 14, 2021 at 01:35:40PM +0100, Jens Wiklander wrote:
> > > > > Since the tee subsystem does not keep a strong reference to its idle
> > > > > shared memory buffers, it races with other threads that try to destroy a
> > > > > shared memory through a close of its dma-buf fd or by unmapping the
> > > > > memory.
> > > > >
> > > > > In tee_shm_get_from_id() when a lookup in teedev->idr has been
> > > > > successful, it is possible that the tee_shm is in the dma-buf teardown
> > > > > path, but that path is blocked by the teedev mutex. Since we don't have
> > > > > an API to tell if the tee_shm is in the dma-buf teardown path or not we
> > > > > must find another way of detecting this condition.
> > > > >
> > > > > Fix this by doing the reference counting directly on the tee_shm using a
> > > > > new refcount_t refcount field. dma-buf is replaced by using
> > > > > anon_inode_getfd() instead, this separates the life-cycle of the
> > > > > underlying file from the tee_shm. tee_shm_put() is updated to hold the
> > > > > mutex when decreasing the refcount to 0 and then remove the tee_shm from
> > > > > teedev->idr before releasing the mutex. This means that the tee_shm can
> > > > > never be found unless it has a refcount larger than 0.
> > > >
> > > > So you are dropping dma-buf support entirely? And anon_inode_getfd()
> > > > works instead? Why do more people not do this as well?
> > >
> > > I don't know, but it should be noted that we're not doing very much with
> > > this file descriptor. We're only using it with mmap() and close().
> > >
> > > >
> > > > >
> > > > > Fixes: 967c9cca2cc5 ("tee: generic TEE subsystem")
> > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > Reviewed-by: Lars Persson <larper@xxxxxxxx>
> > > > > Reviewed-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > > > > Reported-by: Patrik Lantz <patrik.lantz@xxxxxxxx>
> > > > > Signed-off-by: Jens Wiklander <jens.wiklander@xxxxxxxxxx>
> > > > > ---
> > > > > drivers/tee/tee_shm.c | 174 +++++++++++++++-------------------------
> > > > > include/linux/tee_drv.h | 2 +-
> > > > > 2 files changed, 67 insertions(+), 109 deletions(-)
> > > > >
> > > > > diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
> > > > > index 8a8deb95e918..0c82cf981c46 100644
> > > > > --- a/drivers/tee/tee_shm.c
> > > > > +++ b/drivers/tee/tee_shm.c
> > > > > @@ -1,20 +1,17 @@
> > > > > // SPDX-License-Identifier: GPL-2.0-only
> > > > > /*
> > > > > - * Copyright (c) 2015-2016, Linaro Limited
> > > > > + * Copyright (c) 2015-2021, Linaro Limited
> > > >
> > > > Nit, did Linaro really make a copyrightable change in 2017, 2018, 2019
> > > > and 2020 as well? If not, please do not claim it.
> > >
> > > Fair enough, I was a bit lazy 2018 shouldn't be there now that I've
> > > checked the log. I'll fix.
> > >
> > > >
> > > > > */
> > > > > +#include <linux/anon_inodes.h>
> > > > > #include <linux/device.h>
> > > > > -#include <linux/dma-buf.h>
> > > > > -#include <linux/fdtable.h>
> > > > > #include <linux/idr.h>
> > > > > +#include <linux/mm.h>
> > > > > #include <linux/sched.h>
> > > > > #include <linux/slab.h>
> > > > > #include <linux/tee_drv.h>
> > > > > #include <linux/uio.h>
> > > > > -#include <linux/module.h>
> > > > > #include "tee_private.h"
> > > > >
> > > > > -MODULE_IMPORT_NS(DMA_BUF);
> > > > > -
> > > > > static void release_registered_pages(struct tee_shm *shm)
> > > > > {
> > > > > if (shm->pages) {
> > > > > @@ -31,16 +28,8 @@ static void release_registered_pages(struct tee_shm *shm)
> > > > > }
> > > > > }
> > > > >
> > > > > -static void tee_shm_release(struct tee_shm *shm)
> > > > > +static void tee_shm_release(struct tee_device *teedev, struct tee_shm *shm)
> > > > > {
> > > > > - struct tee_device *teedev = shm->ctx->teedev;
> > > > > -
> > > > > - if (shm->flags & TEE_SHM_DMA_BUF) {
> > > > > - mutex_lock(&teedev->mutex);
> > > > > - idr_remove(&teedev->idr, shm->id);
> > > > > - mutex_unlock(&teedev->mutex);
> > > > > - }
> > > > > -
> > > > > if (shm->flags & TEE_SHM_POOL) {
> > > > > struct tee_shm_pool_mgr *poolm;
> > > > >
> > > > > @@ -67,45 +56,6 @@ static void tee_shm_release(struct tee_shm *shm)
> > > > > tee_device_put(teedev);
> > > > > }
> > > > >
> > > > > -static struct sg_table *tee_shm_op_map_dma_buf(struct dma_buf_attachment
> > > > > - *attach, enum dma_data_direction dir)
> > > > > -{
> > > > > - return NULL;
> > > > > -}
> > > > > -
> > > > > -static void tee_shm_op_unmap_dma_buf(struct dma_buf_attachment *attach,
> > > > > - struct sg_table *table,
> > > > > - enum dma_data_direction dir)
> > > > > -{
> > > > > -}
> > > > > -
> > > > > -static void tee_shm_op_release(struct dma_buf *dmabuf)
> > > > > -{
> > > > > - struct tee_shm *shm = dmabuf->priv;
> > > > > -
> > > > > - tee_shm_release(shm);
> > > > > -}
> > > > > -
> > > > > -static int tee_shm_op_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> > > > > -{
> > > > > - struct tee_shm *shm = dmabuf->priv;
> > > > > - size_t size = vma->vm_end - vma->vm_start;
> > > > > -
> > > > > - /* Refuse sharing shared memory provided by application */
> > > > > - if (shm->flags & TEE_SHM_USER_MAPPED)
> > > > > - return -EINVAL;
> > > > > -
> > > > > - return remap_pfn_range(vma, vma->vm_start, shm->paddr >> PAGE_SHIFT,
> > > > > - size, vma->vm_page_prot);
> > > > > -}
> > > > > -
> > > > > -static const struct dma_buf_ops tee_shm_dma_buf_ops = {
> > > > > - .map_dma_buf = tee_shm_op_map_dma_buf,
> > > > > - .unmap_dma_buf = tee_shm_op_unmap_dma_buf,
> > > > > - .release = tee_shm_op_release,
> > > > > - .mmap = tee_shm_op_mmap,
> > > > > -};
> > > > > -
> > > > > struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
> > > > > {
> > > > > struct tee_device *teedev = ctx->teedev;
> > > > > @@ -140,6 +90,7 @@ struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
> > > > > goto err_dev_put;
> > > > > }
> > > > >
> > > > > + refcount_set(&shm->refcount, 1);
> > > > > shm->flags = flags | TEE_SHM_POOL;
> > > > > shm->ctx = ctx;
> > > > > if (flags & TEE_SHM_DMA_BUF)
> > > > > @@ -153,10 +104,7 @@ struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
> > > > > goto err_kfree;
> > > > > }
> > > > >
> > > > > -
> > > > > if (flags & TEE_SHM_DMA_BUF) {
> > > > > - DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> > > > > -
> > > > > mutex_lock(&teedev->mutex);
> > > > > shm->id = idr_alloc(&teedev->idr, shm, 1, 0, GFP_KERNEL);
> > > > > mutex_unlock(&teedev->mutex);
> > > > > @@ -164,28 +112,11 @@ struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
> > > > > ret = ERR_PTR(shm->id);
> > > > > goto err_pool_free;
> > > > > }
> > > > > -
> > > > > - exp_info.ops = &tee_shm_dma_buf_ops;
> > > > > - exp_info.size = shm->size;
> > > > > - exp_info.flags = O_RDWR;
> > > > > - exp_info.priv = shm;
> > > > > -
> > > > > - shm->dmabuf = dma_buf_export(&exp_info);
> > > > > - if (IS_ERR(shm->dmabuf)) {
> > > > > - ret = ERR_CAST(shm->dmabuf);
> > > > > - goto err_rem;
> > > > > - }
> > > > > }
> > > > >
> > > > > teedev_ctx_get(ctx);
> > > > >
> > > > > return shm;
> > > > > -err_rem:
> > > > > - if (flags & TEE_SHM_DMA_BUF) {
> > > > > - mutex_lock(&teedev->mutex);
> > > > > - idr_remove(&teedev->idr, shm->id);
> > > > > - mutex_unlock(&teedev->mutex);
> > > > > - }
> > > > > err_pool_free:
> > > > > poolm->ops->free(poolm, shm);
> > > > > err_kfree:
> > > > > @@ -246,6 +177,7 @@ struct tee_shm *tee_shm_register(struct tee_context *ctx, unsigned long addr,
> > > > > goto err;
> > > > > }
> > > > >
> > > > > + refcount_set(&shm->refcount, 1);
> > > > > shm->flags = flags | TEE_SHM_REGISTER;
> > > > > shm->ctx = ctx;
> > > > > shm->id = -1;
> > > > > @@ -306,22 +238,6 @@ struct tee_shm *tee_shm_register(struct tee_context *ctx, unsigned long addr,
> > > > > goto err;
> > > > > }
> > > > >
> > > > > - if (flags & TEE_SHM_DMA_BUF) {
> > > > > - DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> > > > > -
> > > > > - exp_info.ops = &tee_shm_dma_buf_ops;
> > > > > - exp_info.size = shm->size;
> > > > > - exp_info.flags = O_RDWR;
> > > > > - exp_info.priv = shm;
> > > > > -
> > > > > - shm->dmabuf = dma_buf_export(&exp_info);
> > > > > - if (IS_ERR(shm->dmabuf)) {
> > > > > - ret = ERR_CAST(shm->dmabuf);
> > > > > - teedev->desc->ops->shm_unregister(ctx, shm);
> > > > > - goto err;
> > > > > - }
> > > > > - }
> > > > > -
> > > > > return shm;
> > > > > err:
> > > > > if (shm) {
> > > > > @@ -339,6 +255,35 @@ struct tee_shm *tee_shm_register(struct tee_context *ctx, unsigned long addr,
> > > > > }
> > > > > EXPORT_SYMBOL_GPL(tee_shm_register);
> > > > >
> > > > > +static int tee_shm_fop_release(struct inode *inode, struct file *filp)
> > > > > +{
> > > > > + tee_shm_put(filp->private_data);
> > > > > + return 0;
> > > > > +}
> > > > > +
> > > > > +static int tee_shm_fop_mmap(struct file *filp, struct vm_area_struct *vma)
> > > > > +{
> > > > > + struct tee_shm *shm = filp->private_data;
> > > > > + size_t size = vma->vm_end - vma->vm_start;
> > > > > +
> > > > > + /* Refuse sharing shared memory provided by application */
> > > > > + if (shm->flags & TEE_SHM_USER_MAPPED)
> > > > > + return -EINVAL;
> > > > > +
> > > > > + /* check for overflowing the buffer's size */
> > > > > + if (vma->vm_pgoff + vma_pages(vma) > shm->size >> PAGE_SHIFT)
> > > > > + return -EINVAL;
> > > > > +
> > > > > + return remap_pfn_range(vma, vma->vm_start, shm->paddr >> PAGE_SHIFT,
> > > > > + size, vma->vm_page_prot);
> > > > > +}
> > > > > +
> > > > > +static const struct file_operations tee_shm_fops = {
> > > > > + .owner = THIS_MODULE,
> > > > > + .release = tee_shm_fop_release,
> > > > > + .mmap = tee_shm_fop_mmap,
> > > > > +};
> > > > > +
> > > > > /**
> > > > > * tee_shm_get_fd() - Increase reference count and return file descriptor
> > > > > * @shm: Shared memory handle
> > > > > @@ -351,10 +296,11 @@ int tee_shm_get_fd(struct tee_shm *shm)
> > > > > if (!(shm->flags & TEE_SHM_DMA_BUF))
> > > > > return -EINVAL;
> > > > >
> > > > > - get_dma_buf(shm->dmabuf);
> > > > > - fd = dma_buf_fd(shm->dmabuf, O_CLOEXEC);
> > > > > + /* matched by tee_shm_put() in tee_shm_op_release() */
> > > > > + refcount_inc(&shm->refcount);
> > > > > + fd = anon_inode_getfd("tee_shm", &tee_shm_fops, shm, O_RDWR);
> > > > > if (fd < 0)
> > > > > - dma_buf_put(shm->dmabuf);
> > > > > + tee_shm_put(shm);
> > > > > return fd;
> > > > > }
> > > > >
> > > > > @@ -364,17 +310,7 @@ int tee_shm_get_fd(struct tee_shm *shm)
> > > > > */
> > > > > void tee_shm_free(struct tee_shm *shm)
> > > > > {
> > > > > - /*
> > > > > - * dma_buf_put() decreases the dmabuf reference counter and will
> > > > > - * call tee_shm_release() when the last reference is gone.
> > > > > - *
> > > > > - * In the case of driver private memory we call tee_shm_release
> > > > > - * directly instead as it doesn't have a reference counter.
> > > > > - */
> > > > > - if (shm->flags & TEE_SHM_DMA_BUF)
> > > > > - dma_buf_put(shm->dmabuf);
> > > > > - else
> > > > > - tee_shm_release(shm);
> > > > > + tee_shm_put(shm);
> > > > > }
> > > > > EXPORT_SYMBOL_GPL(tee_shm_free);
> > > > >
> > > > > @@ -481,10 +417,15 @@ struct tee_shm *tee_shm_get_from_id(struct tee_context *ctx, int id)
> > > > > teedev = ctx->teedev;
> > > > > mutex_lock(&teedev->mutex);
> > > > > shm = idr_find(&teedev->idr, id);
> > > > > + /*
> > > > > + * If the tee_shm was found in the IDR it must have a refcount
> > > > > + * larger than 0 due to the guarantee in tee_shm_put() below. So
> > > > > + * it's safe to use refcount_inc().
> > > > > + */
> > > > > if (!shm || shm->ctx != ctx)
> > > > > shm = ERR_PTR(-EINVAL);
> > > > > - else if (shm->flags & TEE_SHM_DMA_BUF)
> > > > > - get_dma_buf(shm->dmabuf);
> > > > > + else
> > > > > + refcount_inc(&shm->refcount);
> > > > > mutex_unlock(&teedev->mutex);
> > > > > return shm;
> > > > > }
> > > > > @@ -496,7 +437,24 @@ EXPORT_SYMBOL_GPL(tee_shm_get_from_id);
> > > > > */
> > > > > void tee_shm_put(struct tee_shm *shm)
> > > > > {
> > > > > - if (shm->flags & TEE_SHM_DMA_BUF)
> > > > > - dma_buf_put(shm->dmabuf);
> > > > > + struct tee_device *teedev = shm->ctx->teedev;
> > > > > + bool do_release = false;
> > > > > +
> > > > > + mutex_lock(&teedev->mutex);
> > > > > + if (refcount_dec_and_test(&shm->refcount)) {
> > > > > + /*
> > > > > + * refcount has reached 0, we must now remove it from the
> > > > > + * IDR before releasing the mutex. This will guarantee that
> > > > > + * the refcount_inc() in tee_shm_get_from_id() never starts
> > > > > + * from 0.
> > > > > + */
> > > > > + if (shm->flags & TEE_SHM_DMA_BUF)
> > > > > + idr_remove(&teedev->idr, shm->id);
> > > > > + do_release = true;
> > > >
> > > > As you are using a refcount in the "traditional" way, why not just use a
> > > > kref instead? That solves your "do_release" mess here.
> > >
> > > Yes, but it adds another problem. I don't want to hold the mutex when
> > > calling tee_shm_release() so that would mean moving idr_remove() to
> > > tee_shm_release() again and then use kref_get_unless_zero() in
> > > tee_shm_get_from_id() instead.
> >
> > Why does the idr have anything to do with it here? Once you are "done"
> > with the object, remove the entry from the idr and that's it. You
> > should never have to mess with kref_get_unless_zero(), that's what locks
> > are for.
>
> I hope it becomes more clear below.
>
> > > With this approach the tee_shm is removed from the IDR so it cannot be
> > > seen any longer when the refcount is 0. I tried implementing it in both
> > > ways before and in my opinion this turned out better.
> >
> > Really? Something feels wrong here. tee_release_device() should drop
> > the reference from the idr structure as then you know that userspace can
> > not grab any new references to it and should not need to look it up
> > again from anything. Then in your final kref_put() call, in the release
> > callback for that, free the memory.
>
> I assume you mean tee_shm_release().
>
> As I understand it you're describing more or less how it worked before
> this patch, the only significant difference I see is that it was the
> release callback from the DMA-buf that called tee_shm_release() instead
> of kref_put() as you suggest. With that we have a window where the
> reference counter is 0, but the tee_shm is still in the IDR. If another
> thread, perhaps malicious, calls tee_shm_get_from_id() it may be able to
> grab the mutex before tee_shm_release(). If we just increase the
> reference counter without checking if it's 0 to start with or not then
> we're in trouble.

Your device has to have a lock outside of it in order for this to work
no matter what. Your patch doesn't change that, you can't grab a lock
for yourself to then check if that pointer was valid or not :)

Why not properly reference count your shm objects in the tee object?
That way when the last reference to a shm object is dropped, it cleans
itself up and gets rid of the idr entry (under the device mutex). Then
it drops the reference to the tee object, and if that was the last
reference on the tee object, it too will be freed.

That's what kobjects do, and while you probably don't want to use a full
kobject here, the same idea is relevant.

If people can randomly grab shm objects, then they need to be reference
counted properly.

thanks,

greg k-h

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In reply to: Jens Wiklander: "Re: [PATCH] tee: handle lookup of shm with reference count 0"
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