Re: [PATCH] squashfs: fix null-ptr-deref in squashfs_fill_super

[Phillip Lougher]

On 01/11/2022 07:33, Baokun Li wrote:
> When squashfs_read_table() returns an error or `sb->s_magic
> != SQUASHFS_MAGIC`, enters the error branch and calls
> msblk->thread_ops->destroy(msblk) to destroy msblk.
> However, msblk->thread_ops has not been initialized.
> Therefore, the following problem is triggered:
>
> ==================================================================
> BUG: KASAN: null-ptr-deref in squashfs_fill_super+0xe7a/0x13b0
> Read of size 8 at addr 0000000000000008 by task swapper/0/1
>
> CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc3-next-20221031 #367
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0x73/0x9f
>   print_report+0x743/0x759
>   kasan_report+0xc0/0x120
>   __asan_load8+0xd3/0x140
>   squashfs_fill_super+0xe7a/0x13b0
>   get_tree_bdev+0x27b/0x450
>   squashfs_get_tree+0x19/0x30
>   vfs_get_tree+0x49/0x150
>   path_mount+0xaae/0x1350
>   init_mount+0xad/0x100
>   do_mount_root+0xbc/0x1d0
>   mount_block_root+0x173/0x316
>   mount_root+0x223/0x236
>   prepare_namespace+0x1eb/0x237
>   kernel_init_freeable+0x528/0x576
>   kernel_init+0x29/0x250
>   ret_from_fork+0x1f/0x30
>   </TASK>
> ==================================================================
>
> To solve this issue, msblk->thread_ops is initialized immediately after
> msblk is assigned a value.
>
> Fixes: b0645770d3c7 ("squashfs: add the mount parameter theads=<single|multi|percpu>")
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>

Reviewed-by: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>

> --
>   fs/squashfs/super.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
> index 1e428ca9414e..7d5265a39d20 100644
> --- a/fs/squashfs/super.c
> +++ b/fs/squashfs/super.c
> @@ -197,6 +197,7 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
>   		return -ENOMEM;
>   	}
>   	msblk = sb->s_fs_info;
> +	msblk->thread_ops = opts->thread_ops;
>   
>   	msblk->panic_on_errors = (opts->errors == Opt_errors_panic);
>   
> @@ -231,7 +232,7 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
>   			       sb->s_bdev);
>   		goto failed_mount;
>   	}
> -	msblk->thread_ops = opts->thread_ops;
> +
>   	if (opts->thread_num == 0) {
>   		msblk->max_thread_num = msblk->thread_ops->max_decompressors();
>   	} else {