On Wed, 2021-07-14 at 10:46 +0200, Christian König wrote:
> Am 14.07.21 um 09:11 schrieb guangming.cao@xxxxxxxxxxxx:
> > From: Guangming Cao <Guangming.Cao@xxxxxxxxxxxx>
> >
> > 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.
> >
> > 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@xxxxxxxxxxxx>
> > ---
> > 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);
> > }
> >
> > /**
>
>
