Re: [PATCH 27/82] m68k: Refactor intentional wrap-around calculation

[Liam R. Howlett]

* Kees Cook <keescook@xxxxxxxxxxxx> [240122 19:36]:
> In an effort to separate intentional arithmetic wrap-around from
> unexpected wrap-around, we need to refactor places that depend on this
> kind of math. One of the most common code patterns of this is:
> 
> 	VAR + value < VAR
> 
> Notably, this is considered "undefined behavior" for signed and pointer
> types, which the kernel works around by using the -fno-strict-overflow
> option in the build[1] (which used to just be -fwrapv). Regardless, we
> want to get the kernel source to the position where we can meaningfully
> instrument arithmetic wrap-around conditions and catch them when they
> are unexpected, regardless of whether they are signed[2], unsigned[3],
> or pointer[4] types.
> 
> Refactor open-coded unsigned wrap-around addition test to use
> check_add_overflow(), retaining the result for later usage (which removes
> the redundant open-coded addition). This paves the way to enabling the
> unsigned wrap-around sanitizer[2] in the future.
> 
> Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
> Link: https://github.com/KSPP/linux/issues/26 [2]
> Link: https://github.com/KSPP/linux/issues/27 [3]
> Link: https://github.com/KSPP/linux/issues/344 [4]
> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Liam Howlett <liam.howlett@xxxxxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/m68k/kernel/sys_m68k.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/m68k/kernel/sys_m68k.c b/arch/m68k/kernel/sys_m68k.c
> index 1af5e6082467..b2b9248f2566 100644
> --- a/arch/m68k/kernel/sys_m68k.c
> +++ b/arch/m68k/kernel/sys_m68k.c
> @@ -391,10 +391,11 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
>  
>  		mmap_read_lock(current->mm);
>  	} else {
> +		unsigned long sum;
>  		struct vm_area_struct *vma;
>  
>  		/* Check for overflow.  */

With your nice self-documenting code, you can probably drop that
comment.

With or without the change,

Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>

> -		if (addr + len < addr)
> +		if (check_add_overflow(addr, len, &sum))
>  			goto out;
>  
>  		/*
> @@ -403,7 +404,7 @@ sys_cacheflush (unsigned long addr, int scope, int cache, unsigned long len)
>  		 */
>  		mmap_read_lock(current->mm);
>  		vma = vma_lookup(current->mm, addr);
> -		if (!vma || addr + len > vma->vm_end)
> +		if (!vma || sum > vma->vm_end)
>  			goto out_unlock;
>  	}
>  
> -- 
> 2.34.1
>