Re: [PATCH] f2fs: fix shift-out-of-bounds in f2fs_fill_super

[Chao Yu]

On 2023/1/29 15:55, Yangtao Li wrote:
> syzbot reported a bug which could cause shift-out-of-bounds issue:
>
> UBSAN: shift-out-of-bounds in fs/f2fs/super.c:4184:41
> shift exponent 613 is too large for 64-bit type 'loff_t' (aka 'long long')
> CPU: 1 PID: 5467 Comm: syz-executor.0 Not tainted 6.2.0-rc5-syzkaller #0
> Hardware name: Google Compute Engine/Google Compute Engine,
> 	BIOS Google 01/12/2023
> Call Trace:
>   <TASK>
>   __dump_stack lib/dump_stack.c:88 [inline]
>   dump_stack_lvl+0x1b1/0x290 lib/dump_stack.c:106
>   ubsan_epilogue lib/ubsan.c:151 [inline]
>   __ubsan_handle_shift_out_of_bounds+0x33d/0x3a0 lib/ubsan.c:321
>   f2fs_fill_super+0x5518/0x6ee0 fs/f2fs/super.c:4184
>   mount_bdev+0x26c/0x3a0 fs/super.c:1359
>   legacy_get_tree+0xea/0x180 fs/fs_context.c:610
>   vfs_get_tree+0x88/0x270 fs/super.c:1489
>   do_new_mount+0x289/0xad0 fs/namespace.c:3145
>   do_mount fs/namespace.c:3488 [inline]
>   __do_sys_mount fs/namespace.c:3697 [inline]
>   __se_sys_mount+0x2d3/0x3c0 fs/namespace.c:3674
>   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>   do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
>   entry_SYSCALL_64_after_hwframe+0x63/0xcd
>   </TASK>
>
> Since currently only 4kb block size is supported by f2fs,
> let's use 4kb directly to avoid triggering UBSAN exception.

It doesn't fix the root cause...

How can we get invalid raw_super->log_blocksize value after below sanity check
in sanity_check_raw_super():

	/* Currently, support only 4KB block size */
	if (le32_to_cpu(raw_super->log_blocksize) != F2FS_BLKSIZE_BITS) {
		f2fs_info(sbi, "Invalid log_blocksize (%u), supports only %u",
			  le32_to_cpu(raw_super->log_blocksize),
			  F2FS_BLKSIZE_BITS);
		return -EFSCORRUPTED;
	}

Thanks,

> Reported-by: syzbot+fea4bcda5eb938ee88ed@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Yangtao Li <frank.li@xxxxxxxx>
> ---
>   fs/f2fs/super.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index d8a65645ee48..41c2bbd3e719 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -4174,8 +4174,8 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
>   	if (err)
>   		goto free_options;
>   
> -	sb->s_maxbytes = max_file_blocks(NULL) <<
> -				le32_to_cpu(raw_super->log_blocksize);
> +	/* Currently, support only 4KB block size */
> +	sb->s_maxbytes = max_file_blocks(NULL) << F2FS_BLKSIZE_BITS;
>   	sb->s_max_links = F2FS_LINK_MAX;
>   
>   	err = f2fs_setup_casefold(sbi);