Re: [PATCH] sg: Fix a double-fetch bug in drivers/scsi/sg.c
From: Jiri Slaby
Date: Wed Jun 05 2019 - 02:04:53 EST
On 23. 05. 19, 4:38, Gen Zhang wrote:
> In sg_write(), the opcode of the command is fetched the first time from
> the userspace by __get_user(). Then the whole command, the opcode
> included, is fetched again from userspace by __copy_from_user().
> However, a malicious user can change the opcode between the two fetches.
> This can cause inconsistent data and potential errors as cmnd is used in
> the following codes.
> Thus we should check opcode between the two fetches to prevent this.
> Signed-off-by: Gen Zhang <blackgod016574@xxxxxxxxx>
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index d3f1531..a2971b8 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -694,6 +694,8 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
> hp->flags = input_size; /* structure abuse ... */
> hp->pack_id = old_hdr.pack_id;
> hp->usr_ptr = NULL;
> + if (opcode != cmnd)
> + return -EINVAL;
Isn't it too early to check cmnd which is copied only here:
> if (__copy_from_user(cmnd, buf, cmd_size))
> return -EFAULT;