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SubjectRe: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race
On 08/29/2016 09:55 PM, Tejun Heo wrote:
> On Sat, Aug 27, 2016 at 11:30:22AM +0200, Vegard Nossum wrote:
>> If people who are more savvy in block/fs code could ack the locking bits
>> I think we should apply the patch ASAP because it's an easy local DOS if
>> you have (open/read) access to any block device.
> I think the right thing to do there is doing blkdev_get() /
> blkdev_put() around func() invocation in iterate_bdevs() rather than
> holding bd_mutex across the callback. Can you please verify whether
> that works?

Didn't work for me, I kept getting use-after-free in __blkdev_get() on
bdev->bd_invalidated after it calls bdev->bd_disk->fops->open(). I tried
a few related things without much luck.

The only thing that worked for me without holding the mutex across the
call was this:

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 08ae993..586d745 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1885,6 +1885,7 @@ void iterate_bdevs(void (*func)(struct
block_device *, void *), void *arg)
list_for_each_entry(inode, &blockdev_superblock->s_inodes, i_sb_list) {
struct address_space *mapping = inode->i_mapping;
+ struct block_device *bdev;

if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
@@ -1906,7 +1907,19 @@ void iterate_bdevs(void (*func)(struct
block_device *, void *), void *arg)
old_inode = inode;

- func(I_BDEV(inode), arg);
+ bdev = I_BDEV(inode);
+ mutex_lock(&bdev->bd_mutex);
+ bdev->bd_openers++;
+ bdev->bd_holders++;
+ mutex_unlock(&bdev->bd_mutex);
+ func(bdev, arg);
+ mutex_lock(&bdev->bd_mutex);
+ bdev->bd_openers--;
+ bdev->bd_holders--;
+ mutex_unlock(&bdev->bd_mutex);

I'm guessing that's too simple to work in general (especially when you
bring in partitions and stuff; I'm just opening /dev/sr0 in my reproducer).

It's been a long day, I'll have a look tomorrow and see if I didn't just
do something stupid.


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