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

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Message-Id: <20240123002814.1396804-33-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:27:08 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	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: [PATCH 33/82] mm/vmalloc: Refactor intentional wrap-around calculation

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