[PATCH 31/82] ARC: dw2 unwind: Refactor intentional wrap-around calculation

[Kees Cook]

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 pointer 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: Vineet Gupta <vgupta@xxxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Song Liu <song@xxxxxxxxxx>
Cc: Yihao Han <hanyihao@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "dean.yang_cp" <yangdianqing@xxxxxxxxxx>
Cc: Jinchao Wang <wjc@xxxxxxxxxx>
Cc: linux-snps-arc@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/arc/kernel/unwind.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arc/kernel/unwind.c b/arch/arc/kernel/unwind.c
index 9270d0a713c3..8924fa2a8f29 100644
--- a/arch/arc/kernel/unwind.c
+++ b/arch/arc/kernel/unwind.c
@@ -612,6 +612,7 @@ static signed fde_pointer_type(const u32 *cie)
 		const char *aug;
 		const u8 *end = (const u8 *)(cie + 1) + *cie;
 		uleb128_t len;
+		const u8 *sum;
 
 		/* check if augmentation size is first (and thus present) */
 		if (*ptr != 'z')
@@ -630,10 +631,10 @@ static signed fde_pointer_type(const u32 *cie)
 		version <= 1 ? (void) ++ptr : (void)get_uleb128(&ptr, end);
 		len = get_uleb128(&ptr, end);	/* augmentation length */
 
-		if (ptr + len < ptr || ptr + len > end)
+		if (check_add_overflow(ptr, len, &sum) || sum > end)
 			return -1;
 
-		end = ptr + len;
+		end = sum;
 		while (*++aug) {
 			if (ptr >= end)
 				return -1;
-- 
2.34.1