Re: [PATCH] usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()

[Alan Stern]

On Mon, Aug 17, 2026 at 04:38:39PM +0530, Jeffin Philip wrote:
> In fsg_common_set_num_buffers(), n can be 0 as conversion to u8
> using kstrtou8() in fsg_opts_num_buffers_store() can return
> values from _0_ to 255. When passing 0 as "n" value to kzalloc_objs,
> it can return a ZERO_SIZE_PTR, which passes the null check for
> buffhds. This leads to a null pointer dereference later in bh->next
> in the do while loop. Fix this by adding a check for n = 0 case and
> returning -EINVAL if n is 0.
> 
> Reported-by: syzbot+791be35f1fbcc85d06d7@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=791be35f1fbcc85d06d7
> Fixes: fe5a6c48fd95 ("usb: gadget: storage: get rid of fsg_num_buffers_validate()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jeffin Philip <jeffinphilip14@xxxxxxxxx>

In fe5a6c48fd95 ("usb: gadget: storage: get rid of 
fsg_num_buffers_validate()"), the code that was changed originally 
required the number to lie between 2 and 32.  Even 1 was not acceptable.

Alan Stern

> ---
>  drivers/usb/gadget/function/f_mass_storage.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/usb/gadget/function/f_mass_storage.c b/drivers/usb/gadget/function/f_mass_storage.c
> index a50743caf083..640d3bcb7bf0 100644
> --- a/drivers/usb/gadget/function/f_mass_storage.c
> +++ b/drivers/usb/gadget/function/f_mass_storage.c
> @@ -2747,6 +2747,9 @@ int fsg_common_set_num_buffers(struct fsg_common *common, unsigned int n)
>  	struct fsg_buffhd *bh, *buffhds;
>  	int i;
>  
> +	if (!n)
> +		return -EINVAL;
> +
>  	buffhds = kzalloc_objs(*buffhds, n);
>  	if (!buffhds)
>  		return -ENOMEM;
> -- 
> 2.55.0
> 
>