Re: [PATCH] uaccess: Add minimum bounds check on kernel buffer size

[Arnd Bergmann]

On Fri, Feb 3, 2023, at 20:35, Kees Cook wrote:
> While there is logic about the difference between ksize and usize,
> copy_struct_from_user() didn't check the size of the destination buffer
> (when it was known) against ksize. Add this check so there is an upper
> bounds check on the possible memset() call, otherwise lower bounds
> checks made by callers will trigger bounds warnings under -Warray-bounds.
> 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
>       |                                 ^
> ../include/linux/fortify-string.h:453:9: note: in expansion of macro 
> '__underlying_memset'
>   453 |         __underlying_memset(p, c, __fortify_size); \
>       |         ^~~~~~~~~~~~~~~~~~~
> ../include/linux/fortify-string.h:461:25: note: in expansion of macro 
> '__fortify_memset_chk'
>   461 | #define memset(p, c, s) __fortify_memset_chk(p, c, s, \
>       |                         ^~~~~~~~~~~~~~~~~~~~
> ../include/linux/uaccess.h:334:17: note: in expansion of macro 'memset'
>   334 |                 memset(dst + size, 0, rest);
>       |                 ^~~~~~
> ../drivers/iommu/iommufd/main.c: In function 'iommufd_fops_ioctl':
> ../drivers/iommu/iommufd/main.c:311:27: note: 'buf' declared here
>   311 |         union ucmd_buffer buf;
>       |                           ^~~
>

Hi Kees,

I started building with gcc-13.0.1 myself but ran into a lot of
other -Warray-bounds warnings in randconfig builds, so I ended up
turning it off once more with CONFIG_CC_NO_ARRAY_BOUNDS in order
to keep building without warnings.

Is there anything else I need to do to get to the point of
just addressing actual issues instead of false positives?
Do you already have a patch series for fixing the others?

> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index afb18f198843..ab9728138ad6 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -329,6 +329,10 @@ copy_struct_from_user(void *dst, size_t ksize, 
> const void __user *src,
>  	size_t size = min(ksize, usize);
>  	size_t rest = max(ksize, usize) - size;
> 
> +	/* Double check if ksize is larger than a known object size. */
> +	if (WARN_ON_ONCE(ksize > __builtin_object_size(dst, 1)))
> +		return -E2BIG;
> +

WARN_ON_ONCE() may be a little expensive since that adds two
comparisons and a static variable to each copy, but it's probably
fine. 

      Arnd