Re: [PATCH-next] nbd: fix incomplete validation of ioctl arg

[Josef Bacik]

On Mon, Feb 06, 2023 at 10:58:05PM +0800, Zhong Jinghua wrote:
> We tested and found an alarm caused by nbd_ioctl arg without verification.
> The UBSAN warning calltrace like below:
> 
> UBSAN: Undefined behaviour in fs/buffer.c:1709:35
> signed integer overflow:
> -9223372036854775808 - 1 cannot be represented in type 'long long int'
> CPU: 3 PID: 2523 Comm: syz-executor.0 Not tainted 4.19.90 #1
> Hardware name: linux,dummy-virt (DT)
> Call trace:
>  dump_backtrace+0x0/0x3f0 arch/arm64/kernel/time.c:78
>  show_stack+0x28/0x38 arch/arm64/kernel/traps.c:158
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x170/0x1dc lib/dump_stack.c:118
>  ubsan_epilogue+0x18/0xb4 lib/ubsan.c:161
>  handle_overflow+0x188/0x1dc lib/ubsan.c:192
>  __ubsan_handle_sub_overflow+0x34/0x44 lib/ubsan.c:206
>  __block_write_full_page+0x94c/0xa20 fs/buffer.c:1709
>  block_write_full_page+0x1f0/0x280 fs/buffer.c:2934
>  blkdev_writepage+0x34/0x40 fs/block_dev.c:607
>  __writepage+0x68/0xe8 mm/page-writeback.c:2305
>  write_cache_pages+0x44c/0xc70 mm/page-writeback.c:2240
>  generic_writepages+0xdc/0x148 mm/page-writeback.c:2329
>  blkdev_writepages+0x2c/0x38 fs/block_dev.c:2114
>  do_writepages+0xd4/0x250 mm/page-writeback.c:2344
> 
> The reason for triggering this warning is __block_write_full_page()
> -> i_size_read(inode) - 1 overflow.
> inode->i_size is assigned in __nbd_ioctl() -> nbd_set_size() -> bytesize.
> We think it is necessary to limit the size of arg to prevent errors.
> 
> Moreover, __nbd_ioctl() -> nbd_add_socket(), arg will be cast to int.
> Assuming the value of arg is 0x80000000000000001) (on a 64-bit machine),
> it will become 1 after the coercion, which will return unexpected results.
> 
> Fix it by adding checks to prevent passing in too large numbers.
> 
> Signed-off-by: Zhong Jinghua <zhongjinghua@xxxxxxxxxx>

Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>

Thanks,

Josef