Re: [PATCH] btrfs: Don't free tree_root when exiting btrfs_ioctl_get_subvol_info()

Marcos Paulo de Souza
2020-02-13T17:40:36Z

On Thu, 2020-02-13 at 21:01 +0800, Qu Wenruo wrote:
> [BUG]
> When calling BTRF_IOC_GET_SUBVOL_INFO ioctl, we can easily hit the
> following backtrace:
> BUG: kernel NULL pointer dereference, address: 0000000000000024
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: 0000 [#1] SMP PTI
> CPU: 0 PID: 27421 Comm: python3 Not tainted 5.6.0-rc1+ #539
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-
> 1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
> RIP: 0010:btrfs_root_node+0x7/0x30 [btrfs]
> Call Trace:
> btrfs_read_lock_root_node+0x1f/0x40 [btrfs]
> btrfs_search_slot+0x60f/0xa40 [btrfs]
> btrfs_ioctl+0x11f7/0x30b0 [btrfs]
> ksys_ioctl+0x82/0xc0
> __x64_sys_ioctl+0x11/0x20
> do_syscall_64+0x43/0x130
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x7fcb78d43387
> ---[ end trace 1c21a7c6c0523b8c ]---
>
> [CAUSE]
> We're abusing local @root, it's originally a subvolume root, but in
> root backref search, it's re-assigned to tree_root.
>
> Then we call "btrfs_put_root(root);" when exiting.
> If that @root is reassgined to tree-root, we freed the most important
> tree, and cause use-after-free.
>
> [FIX]
> Don't re-assgiend @root, use fs_info->tree_root directly.
>
> Reported-by: Marcos Paulo de Souza <mpdesouza@suse.de>
> Fixes: 8c319b625e0a ("btrfs: hold a ref on the root in
> btrfs_ioctl_get_subvol_info")
> [To David: please fold the fix into that commit]
> Signed-off-by: Qu Wenruo <wqu@suse.com>

This patch fixed the issue, thanks Qu.

Tested-by: Marcos Paulo de Souza <mpdesouza@suse.com>

> ---
> fs/btrfs/ioctl.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index e6b7cf45a066..43195970f70c 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -2696,17 +2696,16 @@ static int btrfs_ioctl_get_subvol_info(struct
> file *file, void __user *argp)
> subvol_info->rtime.nsec = btrfs_stack_timespec_nsec(&root_item-
> >rtime);
>
> if (key.objectid != BTRFS_FS_TREE_OBJECTID) {
> - /* Search root tree for ROOT_BACKREF of this subvolume
> */
> - root = fs_info->tree_root;
> -
> key.type = BTRFS_ROOT_BACKREF_KEY;
> key.offset = 0;
> - ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> +
> + /* Search root tree for ROOT_BACKREF of this subvolume
> */
> + ret = btrfs_search_slot(NULL, fs_info->tree_root, &key,
> path, 0, 0);
> if (ret < 0) {
> goto out;
> } else if (path->slots[0] >=
> btrfs_header_nritems(path->nodes[0])) {
> - ret = btrfs_next_leaf(root, path);
> + ret = btrfs_next_leaf(fs_info->tree_root,
> path);
> if (ret < 0) {
> goto out;
> } else if (ret > 0) {