[Virtio-fs] [PATCH] Virtiofs: fix null pointer deference in directIO

[Liu Bo]

On Thu, Jul 18, 2019 at 02:18:39PM -0400, Vivek Goyal wrote:
> On Fri, Jul 12, 2019 at 06:35:42AM +0800, Liu Bo wrote:
> > Virtiofs has used memremap to create page structs for its managed memory
> > and set pagemap's type to MEMORY_DEVICE_FS_DAX.  Thus, the page from
> > virtiofs's managed memory is a ZONE_DEVICE page, if %devmap_managed_key
> > was enabled by pmem drivers, put_page() will run into
> > __put_devmap_managed_page().  However, as page_free ops is not defined, it
> > ends up with a NULL pointer dereference.
> > 
> > As virtiofs presents its managed memory range as a dax device, this patch
> > makes virtiofs mimic how nvdimm/pmem.c setups pagemap fsdax.
> > 
> > =================
> > 
> > [ 1223.732964] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
> > [ 1223.733043] PGD 800000026bd91067 P4D 800000026bd91067 PUD 27631e067 PMD 0
> > [ 1223.733085] Oops: 0010 [#1] SMP PTI
> > [ 1223.733114] CPU: 1 PID: 16505 Comm: build Not tainted 4.19.48-dff9509 #1
> > [ 1223.733157] RIP: 0010:          (null)
> > [ 1223.733179] Code: Bad RIP value.
> > ..
> > [ 1223.733671] Call Trace:
> > [ 1223.733704]  fuse_release_user_pages+0xad/0xb0
> > [ 1223.733751]  fuse_direct_io+0x3ec/0x6a0
> > [ 1223.733823]  fuse_dax_direct_write+0x3c/0x70
> > [ 1223.733863]  fuse_file_write_iter+0x2ea/0x4c0
> > [ 1223.733978]  __vfs_write+0xde/0x160
> > [ 1223.734009]  vfs_write+0xab/0x190
> > [ 1223.734061]  ksys_write+0x45/0xc0
> > [ 1223.734094]  do_syscall_64+0x6b/0x330
> > [ 1223.734214]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > [ 1223.734261] RIP: 0033:0x15536bcddfd0
> > 
> > The reproducer could be as simply as,
> > 
> > addr = mmap(..., fileA, ...);
> > fdB = open(fileB, O_DIRECT);
> > write(fdB, addr);
> > 
> > Signed-off-by: Liu Bo <bo.liu at linux.alibaba.com>
> 
> I am looking at this now and trying to understand it better. One problem
> I see is with page pinning. If a page is pinned, how do we make sure it
> is not evicted by reclaim logic. IIUC, right now memory range relciam
> logic is not aware about page pinning.
> 
> For instance, in above example, write(fdB, addr) will pin pages in fileA.
> While write is going on we need to make sure dax memory ranges covering
> those pages are not reclaimed.

Good question, it is indeed a problem, i_mmap_sem is not held by page
pinning so that dax range reclaim is free to go, nor page pinning in
within iomap_begin() and iomap_end() pair.

While that is a problem, this patch is fixing a different one because
of the lack of ->page_free definition.

thanks,
-liubo
> 
> /me needs to spend more time understanding this.
> 
> Vivek
> 
> > ---
> >  fs/fuse/virtio_fs.c | 26 +++++++++++++++++++++++++-
> >  1 file changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> > index 05ea28c..9b3f2e9 100644
> > --- a/fs/fuse/virtio_fs.c
> > +++ b/fs/fuse/virtio_fs.c
> > @@ -538,6 +538,28 @@ static void virtio_fs_cleanup_dax(void *data)
> >  	put_dax(fs->dax_dev);
> >  }
> >  
> > +static void virtio_fs_release_pgmap_ops(void *_pgmap)
> > +{
> > +	dev_pagemap_put_ops();
> > +}
> > +
> > +static void virtio_fs_fsdax_pagefree(struct page *page, void *data)
> > +{
> > +	wake_up_var(&page->_refcount);
> > +}
> > +
> > +static int virtio_fs_setup_pagemap_fsdax(struct device *dev,
> > +					 struct dev_pagemap *pgmap)
> > +{
> > +	dev_pagemap_get_ops();
> > +	if (devm_add_action_or_reset(dev, virtio_fs_release_pgmap_ops, pgmap))
> > +		return -ENOMEM;
> > +	pgmap->type = MEMORY_DEVICE_FS_DAX;
> > +	pgmap->page_free = virtio_fs_fsdax_pagefree;
> > +
> > +	return 0;
> > +}
> > +
> >  static int virtio_fs_setup_dax(struct virtio_device *vdev, struct virtio_fs *fs)
> >  {
> >  	struct virtio_shm_region cache_reg, journal_reg, vertab_reg;
> > @@ -600,7 +622,9 @@ static int virtio_fs_setup_dax(struct virtio_device *vdev, struct virtio_fs *fs)
> >  	pgmap->altmap_valid = false;
> >  	pgmap->ref = &mi->ref;
> >  	pgmap->kill = virtio_fs_percpu_kill;
> > -	pgmap->type = MEMORY_DEVICE_FS_DAX;
> > +
> > +	if (virtio_fs_setup_pagemap_fsdax(&vdev->dev, pgmap))
> > +		return -ENOMEM;
> >  
> >  	/* Ideally we would directly use the PCI BAR resource but
> >  	 * devm_memremap_pages() wants its own copy in pgmap.  So
> > -- 
> > 1.8.3.1
> > 
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