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Date:   Tue, 31 Oct 2023 08:39:48 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Yu Wang <quic_yyuwang@quicinc.com>
Cc:     johannes@sipsolutions.net, rafael@kernel.org,
        linux-kernel@vger.kernel.org, kernel@quicinc.com
Subject: Re: [PATCH] Devcoredump: fix use-after-free issue when releasing
 devcd device
Message-ID: <2023103126-matchless-overview-3558@gregkh>
References: <20231027055521.2679-1-quic_yyuwang@quicinc.com>
 <2023102757-spree-unruly-dcd6@gregkh>
 <df7d1881-2e8f-43e5-a589-cacfb26138ab@quicinc.com>
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In-Reply-To: <df7d1881-2e8f-43e5-a589-cacfb26138ab@quicinc.com>
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On Tue, Oct 31, 2023 at 03:15:12PM +0800, Yu Wang wrote:
> 
> 
> On 10/27/2023 7:12 PM, Greg KH wrote:
> > On Thu, Oct 26, 2023 at 10:55:21PM -0700, Yu Wang wrote:
> >> With sample code as below, it may hit use-after-free issue when
> >> releasing devcd device.
> >>
> >>     struct my_coredump_state {
> >>         struct completion dump_done;
> >>         ...
> >>     };
> >>
> >>     static void my_coredump_free(void *data)
> >>     {
> >>         struct my_coredump_state *dump_state = data;
> >>         ...
> >>         complete(&dump_state->dump_done);
> >>     }
> >>
> >>     static void my_dev_release(struct device *dev)
> >>     {
> >>         kfree(dev);
> >>     }
> >>
> >>     static void my_coredump()
> >>     {
> >>         struct my_coredump_state dump_state;
> >>         struct device *new_device =
> >>             kzalloc(sizeof(*new_device), GFP_KERNEL);
> >>
> >>         ...
> >>         new_device->release = my_dev_release;
> >>         device_initialize(new_device);
> >>         ...
> >>         device_add(new_device);
> >>         ...
> >>         init_completion(&dump_state.dump_done);
> >>         dev_coredumpm(new_device, NULL, &dump_state, datalen, GFP_KERNEL,
> >>                       my_coredump_read, my_coredump_free);
> >>         wait_for_completion(&dump_state.dump_done);
> >>         device_del(new_device);
> >>         put_device(new_device);
> >>     }
> >>
> >> In devcoredump framework, devcd_dev_release() will be called when
> >> releasing the devcd device, it will call the free() callback first
> >> and try to delete the symlink in sysfs directory of the failing device.
> >> Eventhough it has checked 'devcd->failing_dev->kobj.sd' before that,
> >> there is no mechanism to ensure it's still available when accessing
> >> it in kernfs_find_ns(), refer to the diagram as below:
> >>
> >>     Thread A was waiting for 'dump_state.dump_done' at #A-1-2 after
> >>     calling dev_coredumpm().
> >>     When thread B calling devcd->free() at #B-2-1, it wakes up
> >>     thread A from point #A-1-2, which will call device_del() to
> >>     delete the device.
> >>     If #B-2-2 comes before #A-3-1, but #B-4 comes after #A-4, it
> >>     will hit use-after-free issue when trying to access
> >>     'devcd->failing_dev->kobj.sd'.
> >>
> >>     #A-1-1: dev_coredumpm()
> >>       #A-1-2: wait_for_completion(&dump_state.dump_done)
> >>       #A-1-3: device_del()
> >>         #A-2: kobject_del()
> >>           #A-3-1: sysfs_remove_dir() --> set kobj->sd=NULL
> >>           #A-3-2: kernfs_put()
> >>             #A-4: kmem_cache_free() --> free kobj->sd
> >>
> >>     #B-1: devcd_dev_release()
> >>       #B-2-1: devcd->free(devcd->data)
> >>       #B-2-2: check devcd->failing_dev->kobj.sd
> >>       #B-2-3: sysfs_delete_link()
> >>         #B-3: kernfs_remove_by_name_ns()
> >>           #B-4: kernfs_find_ns() --> access devcd->failing_dev->kobj.sd
> >>
> >> To fix this issue, put operations on devcd->failing_dev before
> >> calling the free() callback in devcd_dev_release().
> >>
> >> Signed-off-by: Yu Wang <quic_yyuwang@quicinc.com>
> >> ---
> >>  drivers/base/devcoredump.c | 5 ++---
> >>  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > Also, what commit id does this fix?
> 
> Thanks for your comment :)
> Do you mean the commit which introduced this issue? It's from initial version of devcoredump.c.

Ok, but then what in-kernel code has the above pattern to cause this
"problem"?  Why not fix that up?

thanks,

greg k-h