Re: [PATCH v2] dma-buf: fix use-after-free in dmabuffs_dname

[Greg KH]

On Wed, May 13, 2020 at 05:40:26PM +0530, Charan Teja Kalla wrote:
> 
> Thank you Greg for the comments.
> On 5/12/2020 2:22 PM, Greg KH wrote:
> > On Fri, May 08, 2020 at 12:11:03PM +0530, Charan Teja Reddy wrote:
> >> The following race occurs while accessing the dmabuf object exported as
> >> file:
> >> P1				P2
> >> dma_buf_release()          dmabuffs_dname()
> >> 			   [say lsof reading /proc/<P1 pid>/fd/<num>]
> >>
> >> 			   read dmabuf stored in dentry->d_fsdata
> >> Free the dmabuf object
> >> 			   Start accessing the dmabuf structure
> >>
> >> In the above description, the dmabuf object freed in P1 is being
> >> accessed from P2 which is resulting into the use-after-free. Below is
> >> the dump stack reported.
> >>
> >> We are reading the dmabuf object stored in the dentry->d_fsdata but
> >> there is no binding between the dentry and the dmabuf which means that
> >> the dmabuf can be freed while it is being read from ->d_fsdata and
> >> inuse. Reviews on the patch V1 says that protecting the dmabuf inuse
> >> with an extra refcount is not a viable solution as the exported dmabuf
> >> is already under file's refcount and keeping the multiple refcounts on
> >> the same object coordinated is not possible.
> >>
> >> As we are reading the dmabuf in ->d_fsdata just to get the user passed
> >> name, we can directly store the name in d_fsdata thus can avoid the
> >> reading of dmabuf altogether.
> >>
> >> Call Trace:
> >>  kasan_report+0x12/0x20
> >>  __asan_report_load8_noabort+0x14/0x20
> >>  dmabuffs_dname+0x4f4/0x560
> >>  tomoyo_realpath_from_path+0x165/0x660
> >>  tomoyo_get_realpath
> >>  tomoyo_check_open_permission+0x2a3/0x3e0
> >>  tomoyo_file_open
> >>  tomoyo_file_open+0xa9/0xd0
> >>  security_file_open+0x71/0x300
> >>  do_dentry_open+0x37a/0x1380
> >>  vfs_open+0xa0/0xd0
> >>  path_openat+0x12ee/0x3490
> >>  do_filp_open+0x192/0x260
> >>  do_sys_openat2+0x5eb/0x7e0
> >>  do_sys_open+0xf2/0x180
> >>
> >> Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
> >> Reported-by: syzbot+3643a18836bce555bff6@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> Cc: <stable@xxxxxxxxxxxxxxx> [5.3+]
> >> Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
> >> ---
> >>
> >> Changes in v2: 
> >>
> >> - Pass the user passed name in ->d_fsdata instead of dmabuf
> >> - Improve the commit message
> >>
> >> Changes in v1: (https://patchwork.kernel.org/patch/11514063/)
> >>
> >>  drivers/dma-buf/dma-buf.c | 17 ++++++++++-------
> >>  1 file changed, 10 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> >> index 01ce125..0071f7d 100644
> >> --- a/drivers/dma-buf/dma-buf.c
> >> +++ b/drivers/dma-buf/dma-buf.c
> >> @@ -25,6 +25,7 @@
> >>  #include <linux/mm.h>
> >>  #include <linux/mount.h>
> >>  #include <linux/pseudo_fs.h>
> >> +#include <linux/dcache.h>
> >>  
> >>  #include <uapi/linux/dma-buf.h>
> >>  #include <uapi/linux/magic.h>
> >> @@ -40,15 +41,13 @@ struct dma_buf_list {
> >>  
> >>  static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
> >>  {
> >> -	struct dma_buf *dmabuf;
> >>  	char name[DMA_BUF_NAME_LEN];
> >>  	size_t ret = 0;
> >>  
> >> -	dmabuf = dentry->d_fsdata;
> >> -	dma_resv_lock(dmabuf->resv, NULL);
> >> -	if (dmabuf->name)
> >> -		ret = strlcpy(name, dmabuf->name, DMA_BUF_NAME_LEN);
> >> -	dma_resv_unlock(dmabuf->resv);
> >> +	spin_lock(&dentry->d_lock);
> > 
> > Are you sure this lock always protects d_fsdata?
> 
> I think yes. In the dma-buf.c, I have to make sure that d_fsdata should
> always be under d_lock thus it will be protected. (In this posted patch
> there is one place(in dma_buf_set_name) that is missed, will update this
> in V3).
> 
> > 
> >> +	if (dentry->d_fsdata)
> >> +		ret = strlcpy(name, dentry->d_fsdata, DMA_BUF_NAME_LEN);
> >> +	spin_unlock(&dentry->d_lock);
> >>  
> >>  	return dynamic_dname(dentry, buffer, buflen, "/%s:%s",
> >>  			     dentry->d_name.name, ret > 0 ? name : "");
> > 
> > If the above check fails the name will be what?  How could d_name.name
> > be valid but d_fsdata not be valid?
> 
> In case of check fails, empty string "" is appended to the name by the
> code, ret > 0 ? name : "", ret is initialized to zero. Thus the name
> string will be like "/dmabuf:".

So multiple objects can have the same "name" if this happens to multiple
ones at once?

> Regarding the validity of d_fsdata, we are setting the dmabuf's
> dentry->d_fsdata to NULL in the dma_buf_release() thus can go invalid if
> that dmabuf is in the free path.

Why are we allowing the name to be set if the dmabuf is on the free path
at all?  Shouldn't that be the real fix here?

> >> @@ -80,12 +79,16 @@ static int dma_buf_fs_init_context(struct fs_context *fc)
> >>  static int dma_buf_release(struct inode *inode, struct file *file)
> >>  {
> >>  	struct dma_buf *dmabuf;
> >> +	struct dentry *dentry = file->f_path.dentry;
> >>  
> >>  	if (!is_dma_buf_file(file))
> >>  		return -EINVAL;
> >>  
> >>  	dmabuf = file->private_data;
> >>  
> >> +	spin_lock(&dentry->d_lock);
> >> +	dentry->d_fsdata = NULL;
> >> +	spin_unlock(&dentry->d_lock);
> >>  	BUG_ON(dmabuf->vmapping_counter);
> >>  
> >>  	/*
> >> @@ -343,6 +346,7 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> >>  	}
> >>  	kfree(dmabuf->name);
> >>  	dmabuf->name = name;
> >> +	dmabuf->file->f_path.dentry->d_fsdata = name;
> > 
> > You are just changing the use of d_fsdata from being a pointer to the
> > dmabuf to being a pointer to the name string?  What's to keep that name
> > string around and not have the same reference counting issues that the
> > dmabuf structure itself has?  Who frees that string memory?
> > 
> 
> Yes, I am just storing the name string in the d_fsdata in place of
> dmabuf and this helps to get rid of any extra refcount requirement.
> Because the user passed name carried in the d_fsdata is copied to the
> local buffer in dmabuffs_dname under spin_lock(d_lock) and the same
> d_fsdata is set to NULL(under the d_lock only) when that dmabuf is in
> the release path. So, when d_fsdata is NULL, name string is not accessed
> from the dmabuffs_dname thus extra count is not required.
> 
> String memory, stored in the dmabuf->name, is released from the
> dma_buf_release(). Flow will be like, It fist sets d_fsdata=NULL and
> then free the dmabuf->name.
> 
> However from your comments I have realized that there is a race in this
> patch when using the name string between dma_buf_set_name() and
> dmabuffs_dname(). But, If the idea of passing the name string inplace of
> dmabuf in d_fsdata looks fine, I can update this next patch.

I'll leave that to the dmabuf authors/maintainers, but it feels odd to
me...

thanks,

greg k-h