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

David Sterba
2020-02-14T13:38:33Z

On Thu, Feb 13, 2020 at 09:01:57PM +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]

Folded, thanks for the report and fix.