Re: [PATCH 5.2 36/37] vhost: block speculation of translated descriptors

[Stefan Lippers-Hollmann]

Hi

On 2019-09-13, Greg Kroah-Hartman wrote:
> From: Michael S. Tsirkin <mst@xxxxxxxxxx>
>
> commit a89db445fbd7f1f8457b03759aa7343fa530ef6b upstream.
>
> iovec addresses coming from vhost are assumed to be
> pre-validated, but in fact can be speculated to a value
> out of range.
>
> Userspace address are later validated with array_index_nospec so we can
> be sure kernel info does not leak through these addresses, but vhost
> must also not leak userspace info outside the allowed memory table to
> guests.
>
> Following the defence in depth principle, make sure
> the address is not validated out of node range.
[...]

This fails to compile as part of 5.2.15-rc1 on i386 (amd64 is fine), using
gcc 9.2.1. Reverting just this patch results in a successful build again.

> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1965,8 +1965,10 @@ static int translate_desc(struct vhost_v
>  		_iov = iov + ret;
>  		size = node->size - addr + node->start;
>  		_iov->iov_len = min((u64)len - s, size);
> -		_iov->iov_base = (void __user *)(unsigned long)
> -			(node->userspace_addr + addr - node->start);
> +		_iov->iov_base = (void __user *)
> +			((unsigned long)node->userspace_addr +
> +			 array_index_nospec((unsigned long)(addr - node->start),
> +					    node->size));
>  		s += size;
>  		addr += size;
>  		++ret;
>
>

  CC [M]  drivers/vhost/vhost.o
In file included from /build/linux-5.2/include/linux/export.h:45,
                 from /build/linux-5.2/include/linux/linkage.h:7,
                 from /build/linux-5.2/include/linux/kernel.h:8,
                 from /build/linux-5.2/include/linux/list.h:9,
                 from /build/linux-5.2/include/linux/wait.h:7,
                 from /build/linux-5.2/include/linux/eventfd.h:13,
                 from /build/linux-5.2/drivers/vhost/vhost.c:13:
/build/linux-5.2/drivers/vhost/vhost.c: In function 'translate_desc':
/build/linux-5.2/include/linux/compiler.h:345:38: error: call to '__compiletime_assert_1970' declared with attribute error: BUILD_BUG_ON failed: sizeof(_s) > sizeof(long)
  345 |  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
      |                                      ^
/build/linux-5.2/include/linux/compiler.h:326:4: note: in definition of macro '__compiletime_assert'
  326 |    prefix ## suffix();    \
      |    ^~~~~~
/build/linux-5.2/include/linux/compiler.h:345:2: note: in expansion of macro '_compiletime_assert'
  345 |  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
      |  ^~~~~~~~~~~~~~~~~~~
/build/linux-5.2/include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
/build/linux-5.2/include/linux/build_bug.h:50:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
   50 |  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |  ^~~~~~~~~~~~~~~~
/build/linux-5.2/include/linux/nospec.h:56:2: note: in expansion of macro 'BUILD_BUG_ON'
   56 |  BUILD_BUG_ON(sizeof(_s) > sizeof(long));   \
      |  ^~~~~~~~~~~~
/build/linux-5.2/drivers/vhost/vhost.c:1970:5: note: in expansion of macro 'array_index_nospec'
 1970 |     array_index_nospec((unsigned long)(addr - node->start),
      |     ^~~~~~~~~~~~~~~~~~
make[3]: *** [/build/linux-5.2/scripts/Makefile.build:285: drivers/vhost/vhost.o] Error 1
make[2]: *** [/build/linux-5.2/scripts/Makefile.build:489: drivers/vhost] Error 2
make[1]: *** [/build/linux-5.2/Makefile:1072: drivers] Error 2
make: *** [/build/linux-5.2/Makefile:179: sub-make] Error 2

Regards
	Stefan Lippers-Hollmann