[PATCH 26/82] buildid: 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: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 lib/buildid.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/buildid.c b/lib/buildid.c
index e3a7acdeef0e..d0a310cb9b57 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -54,12 +54,14 @@ static inline int parse_build_id(const void *page_addr,
 				 const void *note_start,
 				 Elf32_Word note_size)
 {
+	const void *sum;
+
 	/* check for overflow */
-	if (note_start < page_addr || note_start + note_size < note_start)
+	if (note_start < page_addr || check_add_overflow(note_start, note_size, &sum))
 		return -EINVAL;
 
 	/* only supports note that fits in the first page */
-	if (note_start + note_size > page_addr + PAGE_SIZE)
+	if (sum > page_addr + PAGE_SIZE)
 		return -EINVAL;
 
 	return parse_build_id_buf(build_id, size, note_start, note_size);
-- 
2.34.1