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
Cc: Luis Chamberlain
Cc: Song Liu
Cc: Yihao Han
Cc: Thomas Gleixner
Cc: "dean.yang_cp"
Cc: Jinchao Wang
Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Kees Cook
---
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
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