Re: [PATCH RESEND V12 7/8] fuse: Use daemon creds in passthrough mode

[Alessio Balsini <balsini@android.com>]

On Fri, Feb 05, 2021 at 05:23:56PM +0800, Peng Tao wrote:
> On Mon, Jan 25, 2021 at 11:31 PM Alessio Balsini <balsini@android.com> wrote:
> >
> > When using FUSE passthrough, read/write operations are directly
> > forwarded to the lower file system file through VFS, but there is no
> > guarantee that the process that is triggering the request has the right
> > permissions to access the lower file system. This would cause the
> > read/write access to fail.
> >
> > In passthrough file systems, where the FUSE daemon is responsible for
> > the enforcement of the lower file system access policies, often happens
> > that the process dealing with the FUSE file system doesn't have access
> > to the lower file system.
> > Being the FUSE daemon in charge of implementing the FUSE file
> > operations, that in the case of read/write operations usually simply
> > results in the copy of memory buffers from/to the lower file system
> > respectively, these operations are executed with the FUSE daemon
> > privileges.
> >
> > This patch adds a reference to the FUSE daemon credentials, referenced
> > at FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl() time so that they can be used
> > to temporarily raise the user credentials when accessing lower file
> > system files in passthrough.
> > The process accessing the FUSE file with passthrough enabled temporarily
> > receives the privileges of the FUSE daemon while performing read/write
> > operations. Similar behavior is implemented in overlayfs.
> > These privileges will be reverted as soon as the IO operation completes.
> > This feature does not provide any higher security privileges to those
> > processes accessing the FUSE file system with passthrough enabled. This
> > is because it is still the FUSE daemon responsible for enabling or not
> > the passthrough feature at file open time, and should enable the feature
> > only after appropriate access policy checks.
> >
> > Signed-off-by: Alessio Balsini <balsini@android.com>
> > ---
> >  fs/fuse/fuse_i.h      |  5 ++++-
> >  fs/fuse/passthrough.c | 11 +++++++++++
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > index c4730d893324..815af1845b16 100644
> > --- a/fs/fuse/fuse_i.h
> > +++ b/fs/fuse/fuse_i.h
> > @@ -182,10 +182,13 @@ struct fuse_release_args;
> >
> >  /**
> >   * Reference to lower filesystem file for read/write operations handled in
> > - * passthrough mode
> > + * passthrough mode.
> > + * This struct also tracks the credentials to be used for handling read/write
> > + * operations.
> >   */
> >  struct fuse_passthrough {
> >         struct file *filp;
> > +       struct cred *cred;
> >  };
> >
> >  /** FUSE specific file data */
> > diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
> > index c7fa1eeb7639..24866c5fe7e2 100644
> > --- a/fs/fuse/passthrough.c
> > +++ b/fs/fuse/passthrough.c
> > @@ -52,6 +52,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
> >                                    struct iov_iter *iter)
> >  {
> >         ssize_t ret;
> > +       const struct cred *old_cred;
> >         struct file *fuse_filp = iocb_fuse->ki_filp;
> >         struct fuse_file *ff = fuse_filp->private_data;
> >         struct file *passthrough_filp = ff->passthrough.filp;
> > @@ -59,6 +60,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
> >         if (!iov_iter_count(iter))
> >                 return 0;
> >
> > +       old_cred = override_creds(ff->passthrough.cred);
> >         if (is_sync_kiocb(iocb_fuse)) {
> >                 ret = vfs_iter_read(passthrough_filp, iter, &iocb_fuse->ki_pos,
> >                                     iocb_to_rw_flags(iocb_fuse->ki_flags,
> > @@ -77,6 +79,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
> >                 if (ret != -EIOCBQUEUED)
> >                         fuse_aio_cleanup_handler(aio_req);
> >         }
> > +       revert_creds(old_cred);
> cred should be reverted when kmalloc() fails above.
> 
> Cheers,
> Tao
> -- 
> Into Sth. Rich & Strange

Thanks Tao, definitely!

Please find the fixup at the bottom of this email.
I keep the WIP V13 here:

  https://github.com/balsini/linux/tree/fuse-passthrough-v13-v5.11-rc5

Thanks,
Alessio

---8<---
From 63797a2cc6b3946bce59989adcb8f39f70f27643 Mon Sep 17 00:00:00 2001
From: Alessio Balsini <balsini@android.com>
Date: Fri, 5 Feb 2021 10:58:49 +0000
Subject: [PATCH] fuse: Fix crediantials leak in passthrough read_iter

If the system doesn't have enough memory when fuse_passthrough_read_iter
is requested in asynchronous IO, an error is directly returned without
restoring the caller's credentials.
Fix by always ensuring credentials are restored.

Fixes: 20210125153057.3623715-8-balsini@android.com ("fuse: Use daemon creds in passthrough mode")
Signed-off-by: Alessio Balsini <balsini@android.com>
---
 fs/fuse/passthrough.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c
index 284979f87747..1df94c1d8a00 100644
--- a/fs/fuse/passthrough.c
+++ b/fs/fuse/passthrough.c
@@ -69,8 +69,10 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
                struct fuse_aio_req *aio_req;

                aio_req = kmalloc(sizeof(struct fuse_aio_req), GFP_KERNEL);
-               if (!aio_req)
-                       return -ENOMEM;
+               if (!aio_req) {
+                       ret = -ENOMEM;
+                       goto out;
+               }

                aio_req->iocb_fuse = iocb_fuse;
                kiocb_clone(&aio_req->iocb, iocb_fuse, passthrough_filp);
@@ -79,6 +81,7 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
                if (ret != -EIOCBQUEUED)
                        fuse_aio_cleanup_handler(aio_req);
        }
+out:
        revert_creds(old_cred);

        return ret;
--
2.30.0.365.g02bc693789-goog