Re: [PATCH v2 1/3] udmabuf: fix racy memfd sealing check

On Wed, Dec 4, 2024 at 11:27 AM Jann Horn jannh@google.com wrote:

The current check_memfd_seals() is racy: Since we first do check_memfd_seals() and then udmabuf_pin_folios() without holding any relevant lock across both, F_SEAL_WRITE can be set in between. This is problematic because we can end up holding pins to pages in a write-sealed memfd.

Fix it using the inode lock, that's probably the easiest way. In the future, we might want to consider moving this logic into memfd, especially if anyone else wants to use memfd_pin_folios().

I am curious, why is it not possible to have a reproducer for this issue, is it not reproducible and is theoretical?

thanks,

- Joel

Reported-by: Julian Orth ju.orth@gmail.com Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106 Closes: https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTgpD+StkV9... Fixes: fbb0de795078 ("Add udmabuf misc device") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn jannh@google.com

---

drivers/dma-buf/udmabuf.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c index 8ce1f074c2d32a0a9f59ff7184359e37d56548c6..c1d8c2766d6d36fc5fe1b3d73057f6e01ec6678f 100644 --- a/drivers/dma-buf/udmabuf.c +++ b/drivers/dma-buf/udmabuf.c @@ -436,14 +436,19 @@ static long udmabuf_create(struct miscdevice *device, goto err; }

•           /*
  

•            * Take the inode lock to protect against concurrent
  

•            * memfd_add_seals(), which takes this lock in write mode.
  

•            */
  

•           inode_lock_shared(file_inode(memfd));
            ret = check_memfd_seals(memfd);
  

•           if (ret < 0) {
  

•                   fput(memfd);
  

•                   goto err;
  

•           }
  

•           if (ret)
  

•                   goto out_unlock;
  
            ret = udmabuf_pin_folios(ubuf, memfd, list[i].offset,
                                     list[i].size, folios);
  

+out_unlock:

•           inode_unlock_shared(file_inode(memfd));
            fput(memfd);
            if (ret)
                    goto err;
  
  

-- 2.47.0.338.g60cca15819-goog