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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 23, 2022 at 08:40:30AM -0700, Kees Cook wrote:
> 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@infradead.org/
> 
> Matthew, what's the status of that series? Will it make the current
> merge window?

I thought you were going to merge it!  I haven't put it in any of my
public trees.

> 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
>