Re: [PATCH 33/82] mm/vmalloc: Refactor intentional wrap-around calculation

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jan 30, 2024 at 06:55:57PM +0000, Lorenzo Stoakes wrote:
> On Mon, Jan 22, 2024 at 04:27:08PM -0800, Kees Cook wrote:
> > 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: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Uladzislau Rezki <urezki@gmail.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: linux-mm@kvack.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  mm/vmalloc.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index d12a17fc0c17..7932ac99e9d3 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1223,6 +1223,7 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
> >  	unsigned long align, unsigned long vstart)
> >  {
> >  	unsigned long nva_start_addr;
> > +	unsigned long sum;
> >
> >  	if (va->va_start > vstart)
> >  		nva_start_addr = ALIGN(va->va_start, align);
> > @@ -1230,11 +1231,11 @@ is_within_this_va(struct vmap_area *va, unsigned long size,
> >  		nva_start_addr = ALIGN(vstart, align);
> >
> >  	/* Can be overflowed due to big size or alignment. */
> > -	if (nva_start_addr + size < nva_start_addr ||
> > +	if (check_add_overflow(nva_start_addr, size, &sum) ||
> >  			nva_start_addr < vstart)
> >  		return false;
> >
> > -	return (nva_start_addr + size <= va->va_end);
> > +	return (sum <= va->va_end);
> >  }
> >
> >  /*
> > --
> > 2.34.1
> >
> 
> Looks good to me,
> 
> Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
>
Same here. One small nit though. The "sum" variable is not something
that it suits for. IMO, we should use a better name and replace it:

"nva_offset"?

--
Uladzislau Rezki