[PATCH] ocxl: Fix concurrent AFU open and device removal

[Greg Kurz]

On Mon, 24 Jun 2019 17:39:26 +0200
Frederic Barrat <fbarrat at linux.ibm.com> wrote:

> Le 24/06/2019 à 17:24, Greg Kurz a écrit :
> > On Mon, 24 Jun 2019 16:41:48 +0200
> > Frederic Barrat <fbarrat at linux.ibm.com> wrote:
> >   
> >> If an ocxl device is unbound through sysfs at the same time its AFU is
> >> being opened by a user process, the open code may dereference freed
> >> stuctures, which can lead to kernel oops messages. You'd have to hit a
> >> tiny time window, but it's possible. It's fairly easy to test by
> >> making the time window bigger artificially.
> >>
> >> Fix it with a combination of 2 changes:
> >> - when an AFU device is found in the IDR by looking for the device
> >> minor number, we should hold a reference on the device until after the
> >> context is allocated. A reference on the AFU structure is kept when
> >> the context is allocated, so we can release the reference on the
> >> device after the context allocation.
> >> - with the fix above, there's still another even tinier window,
> >> between the time the AFU device is found in the IDR and the reference
> >> on the device is taken. We can fix this one by removing the IDR entry
> >> earlier, when the device setup is removed, instead of waiting for the
> >> 'release' device callback. With proper locking around the IDR.
> >>
> >> Fixes: 75ca758adbaf ("ocxl: Create a clear delineation between ocxl backend & frontend")
> >> Signed-off-by: Frederic Barrat <fbarrat at linux.ibm.com>
> >> ---
> >> mpe: this fixes a commit merged in v5.2-rc1. It's late, and I don't think it's that important. If it's for the next merge window, I would add:
> >> Cc: stable at vger.kernel.org      # v5.2
> >>
> >>
> >> drivers/misc/ocxl/file.c | 23 +++++++++++------------
> >>   1 file changed, 11 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/drivers/misc/ocxl/file.c b/drivers/misc/ocxl/file.c
> >> index 2870c25da166..4d1b44de1492 100644
> >> --- a/drivers/misc/ocxl/file.c
> >> +++ b/drivers/misc/ocxl/file.c
> >> @@ -18,18 +18,15 @@ static struct class *ocxl_class;
> >>   static struct mutex minors_idr_lock;
> >>   static struct idr minors_idr;
> >>   
> >> -static struct ocxl_file_info *find_file_info(dev_t devno)
> >> +static struct ocxl_file_info *find_and_get_file_info(dev_t devno)
> >>   {
> >>   	struct ocxl_file_info *info;
> >>   
> >> -	/*
> >> -	 * We don't declare an RCU critical section here, as our AFU
> >> -	 * is protected by a reference counter on the device. By the time the
> >> -	 * info reference is removed from the idr, the ref count of
> >> -	 * the device is already at 0, so no user API will access that AFU and
> >> -	 * this function can't return it.
> >> -	 */
> >> +	mutex_lock(&minors_idr_lock);
> >>   	info = idr_find(&minors_idr, MINOR(devno));
> >> +	if (info)
> >> +		get_device(&info->dev);
> >> +	mutex_unlock(&minors_idr_lock);
> >>   	return info;
> >>   }
> >>   
> >> @@ -58,14 +55,16 @@ static int afu_open(struct inode *inode, struct file *file)
> >>   
> >>   	pr_debug("%s for device %x\n", __func__, inode->i_rdev);
> >>   
> >> -	info = find_file_info(inode->i_rdev);
> >> +	info = find_and_get_file_info(inode->i_rdev);
> >>   	if (!info)
> >>   		return -ENODEV;
> >>   
> >>   	rc = ocxl_context_alloc(&ctx, info->afu, inode->i_mapping);
> >> -	if (rc)
> >> +	if (rc) {
> >> +		put_device(&info->dev);  
> > 
> > You could have a single call site for put_device() since it's
> > needed for both branches. No big deal.  
> 
> 
> Agreed. Will fix if I end up respinning, but won't if it's the only 
> complaint :-)
> 
> 
> 
> >>   		return rc;
> >> -
> >> +	}
> >> +	put_device(&info->dev);
> >>   	file->private_data = ctx;
> >>   	return 0;
> >>   }
> >> @@ -487,7 +486,6 @@ static void info_release(struct device *dev)
> >>   {
> >>   	struct ocxl_file_info *info = container_of(dev, struct ocxl_file_info, dev);
> >>   
> >> -	free_minor(info);
> >>   	ocxl_afu_put(info->afu);
> >>   	kfree(info);
> >>   }
> >> @@ -577,6 +575,7 @@ void ocxl_file_unregister_afu(struct ocxl_afu *afu)
> >>   
> >>   	ocxl_file_make_invisible(info);
> >>   	ocxl_sysfs_unregister_afu(info);
> >> +	free_minor(info);  
> > 
> > Since the IDR entry is added by ocxl_file_register_afu(), it seems to make
> > sense to undo that in ocxl_file_unregister_afu(). Out of curiosity, was there
> > any historical reason to do this in info_release() in the first place ?  
> 
> 
> Yeah, it makes a lot of sense to remove the IDR entry in 
> ocxl_file_unregister_afu(), that's where we undo the device. I wish I 
> had noticed during the code reviews.
> I don't think there was any good reason to have it in info_release() in 
> the first place. I remember the code went through many iterations to get 
> the reference counting on the AFU structure and device done correctly, 
> but we let that one slip.
> 
> I now think the pre-5.2 ocxl code was also exposed to the 2nd window 
> mentioned in the commit log (but the first window is new with the 
> refactoring introduced in 5.2-rc1).
> 

This calls for two separate patches then IMHO.

>    Fred
> 
> 
> 
> > 
> > Reviewed-by: Greg Kurz <groug at kaod.org>
> >   
> >>   	device_unregister(&info->dev);
> >>   }
> >>     
> >   
>