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SubjectRe: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race
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On 08/29/2016 09:55 PM, Tejun Heo wrote:
> Hello,
>
> On Sat, Aug 27, 2016 at 11:30:22AM +0200, Vegard Nossum wrote:
>>> Don't know what's the right fix, but I posted a slightly different one
>>> for the same crash some months ago:
>>> https://patchwork.kernel.org/patch/8556941/
>>>
>>
>> Ah, I'm sorry, I didn't see that.
>>
>> Your patch is 100% identical to my first attempt at a fix and I can
>> confirm that it also fixes the problem for me.
>>
>> 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?

Hrmph, I tried this patch first:

diff --git a/fs/block_dev.c b/fs/block_dev.c
index e17bdbd..489473d 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1884,6 +1884,7 @@ void iterate_bdevs(void (*func)(struct
block_device *, void *), void *arg)
spin_lock(&blockdev_superblock->s_inode_list_lock);
list_for_each_entry(inode, &blockdev_superblock->s_inodes,
i_sb_list) {
struct address_space *mapping = inode->i_mapping;
+ struct block_device *bdev;

spin_lock(&inode->i_lock);
if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW) ||
@@ -1905,7 +1906,11 @@ void iterate_bdevs(void (*func)(struct
block_device *, void *), void *arg)
iput(old_inode);
old_inode = inode;

- func(I_BDEV(inode), arg);
+ bdev = bd_acquire(inode);
+ if (bdev) {
+ func(bdev, arg);
+ bdput(bdev);
+ }
spin_lock(&blockdev_superblock->s_inode_list_lock);
}
That didn't work at all. I guess bd_acquire() would just do a bdgrab()
and not touch ->bd_holders, whereas blkdev_get() would increment
bd_holders and therefore prevent __blkdev_put() from freeing the block
device? Too confusing...

I'll give your suggestion a try.


Vegard

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