Re: [PATCH] btrfs: set tree_root->node = NULL on error

[Nikolay Borisov <nborisov@suse.com>]

On 30.06.20 г. 21:53 ч., Josef Bacik wrote:
> Eric reported an issue where mounting -o recovery with a fuzzed fs
> resulted in a kernel panic.  This is because we tried to free the tree
> node, except it was an error from the read.  Fix this by properly
> resetting the tree_root->node == NULL in this case.  The panic was the
> following
> 
> BTRFS warning (device loop0): failed to read tree root
> BUG: kernel NULL pointer dereference, address: 000000000000001f
> RIP: 0010:free_extent_buffer+0xe/0x90 [btrfs]
> Call Trace:
>  free_root_extent_buffers.part.0+0x11/0x30 [btrfs]
>  free_root_pointers+0x1a/0xa2 [btrfs]
>  open_ctree+0x1776/0x18a5 [btrfs]
>  btrfs_mount_root.cold+0x13/0xfa [btrfs]
>  ? selinux_fs_context_parse_param+0x37/0x80
>  legacy_get_tree+0x27/0x40
>  vfs_get_tree+0x25/0xb0
>  fc_mount+0xe/0x30
>  vfs_kern_mount.part.0+0x71/0x90
>  btrfs_mount+0x147/0x3e0 [btrfs]
>  ? cred_has_capability+0x7c/0x120
>  ? legacy_get_tree+0x27/0x40
>  legacy_get_tree+0x27/0x40
>  vfs_get_tree+0x25/0xb0
>  do_mount+0x735/0xa40
>  __x64_sys_mount+0x8e/0xd0
>  do_syscall_64+0x4d/0x90
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x7f5302f851be
> 
> Fixes: b8522a1e5f42 ("btrfs: Factor out tree roots initialization during mount")
> Reported-by: Eric Sandeen <sandeen@redhat.com>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>  fs/btrfs/disk-io.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 7c07578866f3..c27022f13150 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2592,10 +2592,12 @@ static int __cold init_tree_roots(struct btrfs_fs_info *fs_info)
>  		    !extent_buffer_uptodate(tree_root->node)) {
>  			handle_error = true;
>  
> -			if (IS_ERR(tree_root->node))
> +			if (IS_ERR(tree_root->node)) {
>  				ret = PTR_ERR(tree_root->node);
> -			else if (!extent_buffer_uptodate(tree_root->node))
> +				tree_root->node = NULL;
> +			} else if (!extent_buffer_uptodate(tree_root->node)) {
>  				ret = -EUCLEAN;
> +			}

So this is problematic only if we fail on the last iteration of the loop
as this results in init_tree_roots returning err value with
tree_root->node = -ERR. Subsequently the caller does: fail_tree_roots
which calls free_root_pointers on the bogus value. IMO this piece of
information should go into the changelog in some form or another.


>  
>  			btrfs_warn(fs_info, "failed to read tree root");
>  			continue;
>