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 <a...@linux-foundation.org>
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
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
