Message ID | e639c7a057653c1947b3a4acf2fba6c7798000b5.1605690144.git.johannes.thumshirn@wdc.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | [v3] btrfs: don't access possibly stale fs_info data for printing duplicate device | expand |
On 18/11/20 5:03 pm, Johannes Thumshirn wrote: > Syzbot reported a possible use-after-free when printing a duplicate device > warning device_list_add(). > > At this point it can happen that a btrfs_device::fs_info is not correctly > setup yet, so we're accessing stale data, when printing the warning > message using the btrfs_printk() wrappers. > > The syzkaller reproducer for this use-after-free crafts a filesystem image > and loop mounts it twice in a loop. The mount will fail as the crafted > image has an invalid chunk tree. When this happens btrfs_mount_root() will > call deactivate_locked_super(), which then cleans up fs_info and > fs_info::sb. If a second thread now adds the same block-device to the > file-system, it will get detected as a duplicate device and > device_list_add() will reject the duplicate and print a warning. But as > the fs_info pointer passed in is non-NULL this will result in a > use-after-free. > > Instead of printing possibly uninitialized or already freed memory in > btrfs_printk(), explicitly pass in a NULL fs_info so the printing of the > device name will be skipped altogether. > > Link: https://lore.kernel.org/linux-btrfs/000000000000c9e14b05afcc41ba@google.com > Reported-by: syzbot+582e66e5edf36a22c7b0@syzkaller.appspotmail.com > Reviewed-by: Nikolay Borisov <nborisov@suse.com> > Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> > > --- > Changes to v2: > - Add comment why we're passing NULL to btrfs_warn_in_rcu() > - Clarify commit message > > Changes to v1: > - Use btrfs_warn_in_rcu(NULL,) instead of pr_warn() > --- > fs/btrfs/volumes.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index bb1aa96e1233..3f2af8106d5b 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -940,7 +940,13 @@ static noinline struct btrfs_device *device_list_add(const char *path, > if (device->bdev != path_bdev) { > bdput(path_bdev); > mutex_unlock(&fs_devices->device_list_mutex); > - btrfs_warn_in_rcu(device->fs_info, > + /* > + * device->fs_info may not be reliable here, so > + * pass in a NULL fs_info. This avoids a > + * possible use-after-free when the fs_info and > + * fs_info->sb are already torn down. > + */ > + btrfs_warn_in_rcu(NULL, > "duplicate device %s devid %llu generation %llu scanned by %s (%d)", > path, devid, found_transid, > current->comm, > Few lines below, there is btrfs_info_in_rcu() and, we won't have reliable device->fs_info even there. But we have seen those info quite a lot of times. And so far, it's been ok. So Reviewed-by: Anand Jain <anand.jain@oracle.com>
On Wed, Nov 18, 2020 at 05:28:57PM +0800, Anand Jain wrote: > On 18/11/20 5:03 pm, Johannes Thumshirn wrote: > > --- a/fs/btrfs/volumes.c > > +++ b/fs/btrfs/volumes.c > > @@ -940,7 +940,13 @@ static noinline struct btrfs_device *device_list_add(const char *path, > > if (device->bdev != path_bdev) { > > bdput(path_bdev); > > mutex_unlock(&fs_devices->device_list_mutex); > > - btrfs_warn_in_rcu(device->fs_info, > > + /* > > + * device->fs_info may not be reliable here, so > > + * pass in a NULL fs_info. This avoids a > > + * possible use-after-free when the fs_info and > > + * fs_info->sb are already torn down. > > + */ > > + btrfs_warn_in_rcu(NULL, > > "duplicate device %s devid %llu generation %llu scanned by %s (%d)", > > path, devid, found_transid, > > current->comm, > > > > Few lines below, there is btrfs_info_in_rcu() and, we won't have > reliable device->fs_info even there. But we have seen those info > quite a lot of times. And so far, it's been ok. So Right, this also looks like potentially unsafe use but given that the 2nd print is still under device_list_mutex it think it will be harder to hit.
On Wed, Nov 18, 2020 at 06:03:26PM +0900, Johannes Thumshirn wrote: > Syzbot reported a possible use-after-free when printing a duplicate device > warning device_list_add(). > > At this point it can happen that a btrfs_device::fs_info is not correctly > setup yet, so we're accessing stale data, when printing the warning > message using the btrfs_printk() wrappers. > > The syzkaller reproducer for this use-after-free crafts a filesystem image > and loop mounts it twice in a loop. The mount will fail as the crafted > image has an invalid chunk tree. When this happens btrfs_mount_root() will > call deactivate_locked_super(), which then cleans up fs_info and > fs_info::sb. If a second thread now adds the same block-device to the > file-system, it will get detected as a duplicate device and > device_list_add() will reject the duplicate and print a warning. But as > the fs_info pointer passed in is non-NULL this will result in a > use-after-free. > > Instead of printing possibly uninitialized or already freed memory in > btrfs_printk(), explicitly pass in a NULL fs_info so the printing of the > device name will be skipped altogether. > > Link: https://lore.kernel.org/linux-btrfs/000000000000c9e14b05afcc41ba@google.com I've added the stacktrace from the link. > Reported-by: syzbot+582e66e5edf36a22c7b0@syzkaller.appspotmail.com > Reviewed-by: Nikolay Borisov <nborisov@suse.com> > Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Added to misc-next, thanks.
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index bb1aa96e1233..3f2af8106d5b 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -940,7 +940,13 @@ static noinline struct btrfs_device *device_list_add(const char *path, if (device->bdev != path_bdev) { bdput(path_bdev); mutex_unlock(&fs_devices->device_list_mutex); - btrfs_warn_in_rcu(device->fs_info, + /* + * device->fs_info may not be reliable here, so + * pass in a NULL fs_info. This avoids a + * possible use-after-free when the fs_info and + * fs_info->sb are already torn down. + */ + btrfs_warn_in_rcu(NULL, "duplicate device %s devid %llu generation %llu scanned by %s (%d)", path, devid, found_transid, current->comm,