Re: [PATCH] usb: gadget: f_mass_storage: Fix potential integer overflow in check_command_size_in_blocks()

[Kees Cook <kees@kernel.org>]

On Wed, Feb 25, 2026 at 06:49:58AM -0500, pip-izony wrote:
> From: Seungjin Bae <eeodqql09@gmail.com>
> 
> The `check_command_size_in_blocks()` function calculates the data size
> in bytes by left shifting `common->data_size_from_cmnd` by the block
> size (`common->curlun->blkbits`). However, it does not validate whether
> this shift operation will cause an integer overflow.
> 
> Initially, the block size is set up in `fsg_lun_open()` , and the
> `common->data_size_from_cmnd` is set up in `do_scsi_command()`. During
> initialization, there is no integer overflow check for the interaction
> between two variables.
> 
> So if a malicious USB host sends a SCSI READ or WRITE command
> requesting a large amount of data (`common->data_size_from_cmnd`), the
> left shift operation can wrap around. This results in a truncated data
> size, which can bypass boundary checks and potentially lead to memory
> corruption or out-of-bounds accesses.
> 
> Fix this by ensuring that `common->data_size_from_cmnd` is not greater
> than `(U32_MAX >> common->curlun->blkbits)` before performing the shift.
> 
> Fixes: 144974e7f9e3 ("usb: gadget: mass_storage: support multi-luns with different logic block size")
> Signed-off-by: Seungjin Bae <eeodqql09@gmail.com>
> ---
>  drivers/usb/gadget/function/f_mass_storage.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
> index 6af96e2b44eb..12e7ef2e074e 100644
> --- a/drivers/usb/gadget/function/f_mass_storage.c
> +++ b/drivers/usb/gadget/function/f_mass_storage.c
> @@ -1853,8 +1853,18 @@ static int check_command_size_in_blocks(struct fsg_common *common,
>  		int cmnd_size, enum data_direction data_dir,
>  		unsigned int mask, int needs_medium, const char *name)
>  {
> -	if (common->curlun)
> -		common->data_size_from_cmnd <<= common->curlun->blkbits;
> +	unsigned int blkbits;
> +
> +	if (common->curlun) {
> +		blkbits = common->curlun->blkbits;
> +		if (cmnd_size > 10 &&
> +			common->data_size_from_cmnd > (U32_MAX >> blkbits)) {
> +			common->phase_error = 1;
> +			return -EINVAL;
> +		}
> +		common->data_size_from_cmnd <<= blkbits;
> +	}
> +

Can check_shl_overflow() be used here to make this easier?

>  	return check_command(common, cmnd_size, data_dir,
>  			mask, needs_medium, name);
>  }
> -- 
> 2.43.0

-- 
Kees Cook