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

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Date: Tue, 30 Jan 2024 13:57:12 -0800
From: Kees Cook <keescook@...omium.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Lorenzo Stoakes <lstoakes@...il.com>, linux-hardening@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 33/82] mm/vmalloc: Refactor intentional wrap-around
 calculation

On Tue, Jan 30, 2024 at 08:54:00PM +0100, Uladzislau Rezki wrote:
> 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@...ux-foundation.org>
> > > Cc: Uladzislau Rezki <urezki@...il.com>
> > > Cc: Christoph Hellwig <hch@...radead.org>
> > > Cc: Lorenzo Stoakes <lstoakes@...il.com>
> > > Cc: linux-mm@...ck.org
> > > Signed-off-by: Kees Cook <keescook@...omium.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@...il.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"?

Sure, I can use that. Other folks in other patches have suggested "end",
so maybe nva_end or nva_end_addr ?

-Kees

-- 
Kees Cook