linux-kernel - [PATCH 22/82] x86/sgx: Refactor intentional wrap-around calculation

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Message-Id: <20240123002814.1396804-22-keescook@chromium.org>
Date: Mon, 22 Jan 2024 16:26:57 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Jarkko Sakkinen <jarkko@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-sgx@...r.kernel.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 22/82] x86/sgx: 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
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: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: linux-sgx@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 arch/x86/kernel/cpu/sgx/ioctl.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
index b65ab214bdf5..4b8f6c9f8ef5 100644
--- a/arch/x86/kernel/cpu/sgx/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/ioctl.c
@@ -350,16 +350,18 @@ static int sgx_validate_offset_length(struct sgx_encl *encl,
 				      unsigned long offset,
 				      unsigned long length)
 {
+	unsigned long sum;
+
 	if (!IS_ALIGNED(offset, PAGE_SIZE))
 		return -EINVAL;
 
 	if (!length || !IS_ALIGNED(length, PAGE_SIZE))
 		return -EINVAL;
 
-	if (offset + length < offset)
+	if (check_add_overflow(offset, length, &sum))
 		return -EINVAL;
 
-	if (offset + length - PAGE_SIZE >= encl->size)
+	if (sum - PAGE_SIZE >= encl->size)
 		return -EINVAL;
 
 	return 0;
-- 
2.34.1