On 01/29/2018 03:01 PM, Nikolay Borisov wrote:
On 29.01.2018 04:38, Anand Jain wrote:
On 01/26/2018 09:20 PM, Nikolay Borisov wrote:
Commit 4fde46f0cc71 ("Btrfs: free the stale device") introduced
btrfs_free_stale_device which iterates the device lists for all
registered btrfs filesystems and deletes those devices which aren't
mounted. In a btrfs_devices structure has only 1 device attached to it
and it is unused then btrfs_free_stale_devices will proceed to also
free the btrfs_fs_devices struct itself. Currently this leads to a UAF
since list_for_each_entry will try to perform a check on the already-
freed memory to see if it has to terminated the loop.
The fix is to use 'break' when we know we are freeing the current
fs_devs.
No break is needed as we need to iterate all stale devices and delete
the found stale entry, so commit [1] used list_for_each_entry_safe()
and removed the break,
We only do the break if we know we have a single device in the current
fs_devs struct. And executing free_fs_devices would have already freed
the device + fs_devs.
[1]
commit 38cf665d338fca33af4b16f9ec7cad6637fc0fec
Author: Anand Jain <Anand.Jain@xxxxxxxxxx>
btrfs: make btrfs_free_stale_device() to iterate all stales
I am guessing UAF might be in[2], instead ?
[2]
free_fs_devices(fs_devs)
::
while (!list_empty(&fs_devices->devices)) {
device = list_entry(fs_devices->devices.next,
struct btrfs_device, dev_list);
It's not that, I thought so at first. But here using list_empty you are
awlays accessing the head the of the list, which is guaranteed to be
valid since you do the freeing of fs_devices outside of the while loop.
break when num_devices == 1 is fine;
Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
Thanks, Anand
Thanks, Anand
Fixes: 4fde46f0cc71 ("Btrfs: free the stale device")
Signed-off-by: Nikolay Borisov <nborisov@xxxxxxxx>
---
fs/btrfs/volumes.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f7147740b68e..c3ab55336ee0 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -645,6 +645,7 @@ static void btrfs_free_stale_devices(const char
*path,
btrfs_sysfs_remove_fsid(fs_devs);
list_del(&fs_devs->list);
free_fs_devices(fs_devs);
+ break;
} else {
fs_devs->num_devices--;
list_del(&dev->dev_list);
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