Re: [PATCH] iommufd: Add top-level bounds check on kernel buffer size

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Jan 27, 2023 at 04:57:26PM -0800, Kees Cook wrote:
> On Fri, Jan 27, 2023 at 08:47:34PM -0400, Jason Gunthorpe wrote:
> > On Fri, Jan 27, 2023 at 02:38:17PM -0800, Kees Cook wrote:
> > > While the op->size assignments are already bounds-checked at static
> > > initializer time, these limits aren't aggregated and tracked when doing
> > > later variable range checking under -Warray-bounds. Help the compiler
> > > see that we know what we're talking about, and we'll never ask to
> > > write more that sizeof(ucmd.cmd) bytes during the memset() inside
> > > copy_struct_from_user(). Seen under GCC 13:
> > > 
> > > In function 'copy_struct_from_user',
> > >     inlined from 'iommufd_fops_ioctl' at ../drivers/iommu/iommufd/main.c:333:8:
> > > ../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' with type 'union ucmd_buffer' [-Warray-bounds=]
> > >    59 | #define __underlying_memset     __builtin_memset
> > 
> > This seems strange to me
> > 
> > I thought the way gcc handled this was if it knew the value must be in
> > a certain range then it would check it
> > 
> > If it couldn't figure out any ranges it would not make a warning.
> > 
> > So why did it decide "rest" was in that really weird range?
> 
> It's because it got bounds-checked at the lower end (for the minimum
> size test).

Where? There is no sizeof(ucmd.ubuffer) in this code.

There are no statically computable constants at all.

The minimum size test loads from a struct:

	if (ucmd.user_size < op->min_size)
		return -EINVAL;

So, either gcc can't see through that and thus has no idea what the
bound check is

Or, gcc has figured out that struct iommufd_ioctl_op::min_size has a
finite set of values

If the latter, why doesn't it also know that iommufd_ioctl_op::size
has finite set too?

Combined with the weird report that the upper end of that range is -2
(not UINT_MAX), something very strange is going on inside gcc.

Jason