linux-hardening - [PATCH 78/82] mm/vmalloc: Refactor intentional wrap-around test

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Date: Mon, 22 Jan 2024 16:27:53 -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 78/82] mm/vmalloc: Refactor intentional wrap-around test

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 wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers 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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7932ac99e9d3..3d73f2ac6957 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3750,7 +3750,7 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 	addr = kasan_reset_tag(addr);

 	/* Don't allow overflow */
-	if ((unsigned long) addr + count < count)
+	if (add_would_overflow(count, (unsigned long)addr))
 		count = -(unsigned long) addr;

 	remains = count;
-- 
2.34.1