linux-kernel - Re: [PATCH] scsi: sg: fix a missing-check bug

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Date:   Mon, 7 May 2018 01:13:03 -0400
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     Wenwen Wang <wang6495@....edu>
Cc:     Kangjie Lu <kjlu@....edu>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "open list:SCSI SG DRIVER" <linux-scsi@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] scsi: sg: fix a missing-check bug

On 2018-05-05 11:21 PM, Wenwen Wang wrote:
> In sg_write(), the opcode of the command is firstly copied from the
> userspace pointer 'buf' and saved to the kernel variable 'opcode', using
> the __get_user() function. The size of the command, i.e., 'cmd_size' is
> then calculated based on the 'opcode'. After that, the whole command,
> including the opcode, is copied again from 'buf' using the
> __copy_from_user() function and saved to 'cmnd'. Finally, the function
>   sg_common_write() is invoked to process 'cmnd'. Given that the 'buf'
> pointer resides in userspace, a malicious userspace process can race to
> change the opcode of the command between the two copies. That means, the
> opcode indicated by the variable 'opcode' could be different from the
> opcode in 'cmnd'. This can cause inconsistent data in 'cmnd' and
> potential logical errors in the function sg_common_write(), as it needs to
> work on 'cmnd'.
> 
> This patch reuses the opcode obtained in the first copy and only copies the
> remaining part of the command from userspace.
> 
> Signed-off-by: Wenwen Wang <wang6495@....edu>
> ---
>   drivers/scsi/sg.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index c198b963..0ad8106 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -657,7 +657,8 @@ 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 (__copy_from_user(cmnd, buf, cmd_size))
> +	cmnd[0] = opcode;
> +	if (__copy_from_user(cmnd + 1, buf + 1, cmd_size - 1))
>   		return -EFAULT;
>   	/*
>   	 * SG_DXFER_TO_FROM_DEV is functionally equivalent to SG_DXFER_FROM_DEV,
> 

That is in the deprecated "v2" part of the sg driver (for around 15 years).
There are lots more interesting races with that interface than that one
described above. My guess is that all system calls would be susceptible
to playing around with a buffer being passed to or from the OS by a thread
other than the one doing the system call, during that call. Surely no Unix
like OS gives any security guarantees to a thread being attacked by a
malevolent thread in the same process!

My question is did this actually cause to program to fail; or is it something
that a sanity checker flagged?

Also wouldn't it be better just to return an error such as EINVAL if
opcode != command[0]  ?

Doug Gilbert