Re: [scsi] 6aded12b10: kernel_BUG_at_mm/usercopy.c

[Kees Cook]

On Wed, Mar 23, 2022 at 08:14:10AM +0100, Christoph Hellwig wrote:
> The actual warning is;
> 
> [   34.496096][  T331] usercopy: Kernel memory overwrite attempt detected to spans multiple pages (off set 0, size 6)!
> 
> This is for the cmnd field in struct scsi_cmnd, which is allocated by
> the block layer as part of the request allocator.  So with a specific
> packing it can legitimately span pages.
> 
> Kees: how can we annotate that this is ok?

The main problem is that CONFIG_HARDENED_USERCOPY_PAGESPAN=y is broken
(and nothing should be setting it).

This series removes it:
https://lore.kernel.org/linux-hardening/20220110231530.665970-1-willy@xxxxxxxxxxxxx/

Matthew, what's the status of that series? Will it make the current
merge window?

As for the SCSI changes, I'm a bit worried about type confusion, as I
don't see anything actually validating types/sizes when converting:

static inline void *blk_mq_rq_to_pdu(struct request *rq)
{
        return rq + 1;
}

But I guess that ship has sailed. :P

Regardless, I'm concerned that disabling PAGESPAN will just uncover
further checks, though. Where is allocation happening? The check is here:

static int scsi_fill_sghdr_rq(struct scsi_device *sdev, struct request *rq,
                struct sg_io_hdr *hdr, fmode_t mode)
{
        struct scsi_cmnd *scmd = blk_mq_rq_to_pdu(rq);

        if (hdr->cmd_len < 6)
                return -EMSGSIZE;
        if (copy_from_user(scmd->cmnd, hdr->cmdp, hdr->cmd_len))
                return -EFAULT;
	...
}

I don't see any earlier marking for this copy_from_user(), so I assume
the old allocation was a plain kmalloc().

For comparision, a related marking can be seen for a copy_to_user() case
in commit 0afe76e88c57 ("scsi: Define usercopy region in scsi_sense_cache
slab cache")

I *think* the allocation is happening in scsi_ioctl_reset()? But that's
a plain kmalloc(), so I'm not sure why PAGESPAN would have tripped...
are there other allocation paths?

-- 
Kees Cook