Re: request_queue use-after-free - inode_detach_wb()

From: Ilya Dryomov
Date: Thu Nov 19 2015 - 15:56:28 EST

On Wed, Nov 18, 2015 at 4:48 PM, Ilya Dryomov <idryomov@xxxxxxxxx> wrote:
> On Wed, Nov 18, 2015 at 4:30 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
>> Hello, Ilya.
>> On Wed, Nov 18, 2015 at 04:12:07PM +0100, Ilya Dryomov wrote:
>>> > It's stinky that the bdi is going away while the inode is still there.
>>> > Yeah, blkdev inodes are special and created early but I think it makes
>>> > sense to keep the underlying structures (queue and bdi) around while
>>> > bdev is associated with it. Would simply moving put_disk() after
>>> > bdput() work?
>>> I'd think so. struct block_device is essentially a "block device"
>>> pseudo-filesystem inode, and as such, may not be around during the
>>> entire lifetime of gendisk / queue. It may be kicked out of the inode
>>> cache as soon as the device is closed, so it makes sense to put it
>>> before putting gendisk / queue, which will outlive it.
>>> However, I'm confused by this comment
>>> /*
>>> * ->release can cause the queue to disappear, so flush all
>>> * dirty data before.
>>> */
>>> bdev_write_inode(bdev);
>>> It's not true, at least since your 523e1d399ce0 ("block: make gendisk
>>> hold a reference to its queue"), right? (It used to say "->release can
>>> cause the old bdi to disappear, so must switch it out first" and was
>>> changed by Christoph in the middle of his backing_dev_info series.)
>> Right, it started with each layer going away separately, which tends
>> to get tricky with hotunplug, and we've been gradually moving towards
>> a model where the entire stack stays till the last ref is gone, so
>> yeah the comment isn't true anymore.
> OK, I'll try to work up a patch to do bdput before put_disk and also
> drop this comment.

Doing bdput before put_disk in fs/block_dev.c helps, but isn't enough.
There is nothing guaranteeing that our bdput in __blkdev_put() is the
last one. One particular issue is the linkage between /dev inodes and
bdev internal inodes. /dev inodes hold bdev inodes, so:

186 static void __fput(struct file *file)
187 {
188 struct dentry *dentry = file->f_path.dentry;
189 struct vfsmount *mnt = file->f_path.mnt;
190 struct inode *inode = file->f_inode;
192 might_sleep();
194 fsnotify_close(file);
195 /*
196 * The function eventpoll_release() should be the first called
197 * in the file cleanup chain.
198 */
199 eventpoll_release(file);
200 locks_remove_file(file);
202 if (unlikely(file->f_flags & FASYNC)) {
203 if (file->f_op->fasync)
204 file->f_op->fasync(-1, file, 0);
205 }
206 ima_file_free(file);
207 if (file->f_op->release)
208 file->f_op->release(inode, file);

This translates to blkdev_put(). Suppose in response to this release
block device driver dropped gendisk, queue, etc. Then we, still in
blkdev_put(), did our bdput and put_disk. The queue is now gone, but
there's still a ref on the bdev inode - from the /dev inode. When the
latter gets evicted thanks to dput below, we end up in bd_forget(),
which finishes up with iput(bdev->bd_inode)...

209 security_file_free(file);
210 if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
211 !(file->f_mode & FMODE_PATH))) {
212 cdev_put(inode->i_cdev);
213 }
214 fops_put(file->f_op);
215 put_pid(file->;
216 if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
217 i_readcount_dec(inode);
218 if (file->f_mode & FMODE_WRITER) {
219 put_write_access(inode);
220 __mnt_drop_write(mnt);
221 }
222 file->f_path.dentry = NULL;
223 file->f_path.mnt = NULL;
224 file->f_inode = NULL;
225 file_free(file);
226 dput(dentry);
227 mntput(mnt);
228 }

On Wed, Nov 18, 2015 at 4:56 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Wed, Nov 18, 2015 at 04:48:06PM +0100, Ilya Dryomov wrote:
>> Just to be clear, the bdi/wb vs inode lifetime rules are that inodes
>> should always be within bdi/wb? There's been a lot of churn in this
> Yes, that's where *I* think we should be headed. Stuff in lower
> layers should stick around while upper layer things are around

I think the fundamental problem is the embedding of bdi in the queue.
The lifetime rules (or, rather, expectations) for the two seem to be
completely different and, while used together, they belong to different
subsystems. Even if we find a way to fix this particular race, there
is a good chance someone will reintroduce it in the future, perhaps in
a more subtle way.


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