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From:   <guangming.cao@mediatek.com>
To:     <christian.koenig@amd.com>, <sumit.semwal@linaro.org>
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Subject: Re: [PATCH] dma-buf: add kernel count for dma_buf
Date:   Wed, 14 Jul 2021 20:03:05 +0800
Message-ID: <20210714120305.70720-1-guangming.cao@mediatek.com>
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References: <371c6f09-2bba-a9d6-18e8-114bea97a18d@amd.com>
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From: Guangming.Cao <guangming.cao@mediatek.com>

On Wed, 2021-07-14 at 12:43 +0200, Christian K鰊ig wrote:
> Am 14.07.21 um 11:44 schrieb guangming.cao@mediatek.com:
> > From: Guangming Cao <Guangming.Cao@mediatek.com>
> > 
> > On Wed, 2021-07-14 at 10:46 +0200, Christian K鰊ig wrote:
> > > Am 14.07.21 um 09:11 schrieb guangming.cao@mediatek.com:
> > > > From: Guangming Cao <Guangming.Cao@mediatek.com>
> > > > 
> > > > Add a refcount for kernel to prevent UAF(Use After Free) issue.
> > > 
> > > Well NAK on so many levels.
> > > 
> > > > We can assume a case like below:
> > > >       1. kernel space alloc dma_buf(file count = 1)
> > > >       2. kernel use dma_buf to get fd(file count = 1)
> > > >       3. userspace use fd to do mapping (file count = 2)
> > > 
> > > Creating an userspace mapping increases the reference count for
> > > the
> > > underlying file object.
> > > 
> > > See the implementation of mmap_region():
> > > ...
> > >                   vma->vm_file = get_file(file);
> > >                   error = call_mmap(file, vma);
> > > ...
> > > 
> > > What can happen is the the underlying exporter redirects the mmap
> > > to
> > > a
> > > different file, e.g. TTM or GEM drivers do that all the time.
> > > 
> > > But this is fine since then the VA mapping is independent of the
> > > DMA-
> > > buf.
> > > 
> > > >       4. kernel call dma_buf_put (file count = 1)
> > > >       5. userpsace close buffer fd(file count = 0)
> > > >       6. at this time, buffer is released, but va is valid!!
> > > >          So we still can read/write buffer via mmap va,
> > > >          it maybe cause memory leak, or kernel exception.
> > > >          And also, if we use "ls -ll" to watch corresponding
> > > > process
> > > >              fd link info, it also will cause kernel exception.
> > > > 
> > > > Another case:
> > > >        Using dma_buf_fd to generate more than 1 fd, because
> > > >        dma_buf_fd will not increase file count, thus, when
> > > > close
> > > >        the second fd, it maybe occurs error.
> > > 
> > > Each opened fd will increase the reference count so this is
> > > certainly
> > > not correct what you describe here.
> > > 
> > > Regards,
> > > Christian.
> > > 
> > 
> > Yes, mmap will increase file count by calling get_file, so step[2]
> > ->
> > step[3], file count increase 1.
> > 
> > But, dma_buf_fd() will not increase file count.
> > function "dma_buf_fd(struct dma_buf *dmabuf, int flags)" just get
> > an
> > unused fd, via call "get_unused_fd_flags(flags)", and call
> > "fd_install(fd, dmabuf->file)", it will let associated "struct
> > file*"
> > in task's fdt->fd[fd] points to this dma_buf.file, not increase the
> > file count of dma_buf.file.
> > I think this is confusing, I can get more than 1 fds via
> > dma_buf_fd,
> > but they don't need to close it because they don't increase file
> > count.
> > 
> > However, dma_buf_put() can decrease file count at kernel side
> > directly.
> > If somebody write a ko to put file count of dma_buf.file many
> > times, it
> > will cause buffer freed earlier than except. At last on Android, I
> > think this is a little bit dangerous.
> 
> dma_buf_fd() takes the dma_buf pointer and converts it into a fd. So
> the 
> reference is consumed.
> 
> That's why users of this interface make sure to get a separate 
> reference, see drm_gem_prime_handle_to_fd() for example:
> 
> ...
> out_have_handle:
>      ret = dma_buf_fd(dmabuf, flags);
>      /*
>       * We must _not_ remove the buffer from the handle cache since
> the 
> newly
>       * created dma buf is already linked in the global obj->dma_buf 
> pointer,
>       * and that is invariant as long as a userspace gem handle
> exists.
>       * Closing the handle will clean out the cache anyway, so we
> don't 
> leak.
>       */
>      if (ret < 0) {
>          goto fail_put_dmabuf;
>      } else {
>          *prime_fd = ret;
>          ret = 0;
>      }
> 
>      goto out;
> 
> fail_put_dmabuf:
>      dma_buf_put(dmabuf);
> out:
> ...
> 
> You could submit a patch to improve the documentation and explicitly 
> note on dma_buf_fd() that the reference is consumed, but all of this
> is 
> working perfectly fine.
> 
> Regards,
> Christian.
> 

Thanks for your reply!

Yes, drm works fine because it fully understand what dma-buf api will
do. Improve the documentation is really good idea to prevent this case.

But, what I can't understand is, for kernel api exported to
corresponding users, we don't need to ensure all api is safe?

And for general cases, dma-buf framework also need to prevent this
case, isn't it, it will make dma-buf framework more strong?


BRs!
Guangming
> > 
> > > > Solution:
> > > >       Add a kernel count for dma_buf, and make sure the file
> > > > count
> > > >           of dma_buf.file hold by kernel is 1.
> > > > 
> > > > Notes: For this solution, kref couldn't work because kernel ref
> > > >          maybe added from 0, but kref don't allow it.
> > > > 
> > > > Signed-off-by: Guangming Cao <Guangming.Cao@mediatek.com>
> > > > ---
> > > >    drivers/dma-buf/dma-buf.c | 23 +++++++++++++++++++----
> > > >    include/linux/dma-buf.h   |  6 ++++--
> > > >    2 files changed, 23 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-
> > > > buf.c
> > > > index 511fe0d217a0..04ee92aac8b9 100644
> > > > --- a/drivers/dma-buf/dma-buf.c
> > > > +++ b/drivers/dma-buf/dma-buf.c
> > > > @@ -62,6 +62,7 @@ static void dma_buf_release(struct dentry
> > > > *dentry)
> > > >      if (unlikely(!dmabuf))
> > > >              return;
> > > >    
> > > > +   WARN_ON(atomic64_read(&dmabuf->kernel_ref));
> > > >      BUG_ON(dmabuf->vmapping_counter);
> > > >    
> > > >      /*
> > > > @@ -555,6 +556,7 @@ struct dma_buf *dma_buf_export(const struct
> > > > dma_buf_export_info *exp_info)
> > > >              goto err_module;
> > > >      }
> > > >    
> > > > +   atomic64_set(&dmabuf->kernel_ref, 1);
> > > >      dmabuf->priv = exp_info->priv;
> > > >      dmabuf->ops = exp_info->ops;
> > > >      dmabuf->size = exp_info->size;
> > > > @@ -617,6 +619,9 @@ int dma_buf_fd(struct dma_buf *dmabuf, int
> > > > flags)
> > > >    
> > > >      fd_install(fd, dmabuf->file);
> > > >    
> > > > +   /* Add file cnt for each new fd */
> > > > +   get_file(dmabuf->file);
> > > > +
> > > >      return fd;
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(dma_buf_fd);
> > > > @@ -626,12 +631,13 @@ EXPORT_SYMBOL_GPL(dma_buf_fd);
> > > >     * @fd:   [in]    fd associated with the struct dma_buf to
> > > > be
> > > > returned
> > > >     *
> > > >     * On success, returns the struct dma_buf associated with an
> > > > fd;
> > > > uses
> > > > - * file's refcounting done by fget to increase refcount.
> > > > returns
> > > > ERR_PTR
> > > > - * otherwise.
> > > > + * dmabuf's ref refcounting done by kref_get to increase
> > > > refcount.
> > > > + * Returns ERR_PTR otherwise.
> > > >     */
> > > >    struct dma_buf *dma_buf_get(int fd)
> > > >    {
> > > >      struct file *file;
> > > > +   struct dma_buf *dmabuf;
> > > >    
> > > >      file = fget(fd);
> > > >    
> > > > @@ -643,7 +649,12 @@ struct dma_buf *dma_buf_get(int fd)
> > > >              return ERR_PTR(-EINVAL);
> > > >      }
> > > >    
> > > > -   return file->private_data;
> > > > +   dmabuf = file->private_data;
> > > > +   /* replace file count increase as ref increase for kernel
> > > > user
> > > > */
> > > > +   get_dma_buf(dmabuf);
> > > > +   fput(file);
> > > > +
> > > > +   return dmabuf;
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(dma_buf_get);
> > > >    
> > > > @@ -662,7 +673,11 @@ void dma_buf_put(struct dma_buf *dmabuf)
> > > >      if (WARN_ON(!dmabuf || !dmabuf->file))
> > > >              return;
> > > >    
> > > > -   fput(dmabuf->file);
> > > > +   if (WARN_ON(!atomic64_read(&dmabuf->kernel_ref)))
> > > > +           return;
> > > > +
> > > > +   if (!atomic64_dec_return(&dmabuf->kernel_ref))
> > > > +           fput(dmabuf->file);
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(dma_buf_put);
> > > >    
> > > > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > > > index efdc56b9d95f..bc790cb028eb 100644
> > > > --- a/include/linux/dma-buf.h
> > > > +++ b/include/linux/dma-buf.h
> > > > @@ -308,6 +308,7 @@ struct dma_buf_ops {
> > > >    struct dma_buf {
> > > >      size_t size;
> > > >      struct file *file;
> > > > +   atomic64_t kernel_ref;
> > > >      struct list_head attachments;
> > > >      const struct dma_buf_ops *ops;
> > > >      struct mutex lock;
> > > > @@ -436,7 +437,7 @@ struct dma_buf_export_info {
> > > >                                       .owner = THIS_MODULE }
> > > >    
> > > >    /**
> > > > - * get_dma_buf - convenience wrapper for get_file.
> > > > + * get_dma_buf - increase a kernel ref of dma-buf
> > > >     * @dmabuf:       [in]    pointer to dma_buf
> > > >     *
> > > >     * Increments the reference count on the dma-buf, needed in
> > > > case
> > > > of drivers
> > > > @@ -446,7 +447,8 @@ struct dma_buf_export_info {
> > > >     */
> > > >    static inline void get_dma_buf(struct dma_buf *dmabuf)
> > > >    {
> > > > -   get_file(dmabuf->file);
> > > > +   if (atomic64_inc_return(&dmabuf->kernel_ref) == 1)
> > > > +           get_file(dmabuf->file);
> > > >    }
> > > >    
> > > >    /**
> 
>