[PATCH v2 2/2] tools/nolibc: check for overflow in malloc()

[Danish Khateeb <danishkhateeb03@gmail.com>]

malloc() adds the size of its header to the requested size without
checking for overflow. For a size within the header size of SIZE_MAX,
the sum wraps around to a few bytes, and malloc() returns a single page
instead of NULL.

calloc() checks its multiplication for overflow, but then passes the
product to malloc(), so calloc(SIZE_MAX, 1) returns a single page too.
So does realloc(ptr, SIZE_MAX).

Such a size is usually a bug in the caller, for instance a length of -1
used as a size_t, and returning a page instead of NULL can turn it into
a heap overflow.

Fail with ENOMEM when adding the header overflows, as glibc does for
sizes this large.

Fixes: 0e0ff638400b ("tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`")
Assisted-by: LLM
Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
---
 tools/include/nolibc/stdlib.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/include/nolibc/stdlib.h b/tools/include/nolibc/stdlib.h
index dc554faff22a..cea8290ac0b9 100644
--- a/tools/include/nolibc/stdlib.h
+++ b/tools/include/nolibc/stdlib.h
@@ -130,7 +130,11 @@ void *malloc(size_t len)
 {
 	struct nolibc_heap *heap;
 
-	len  = sizeof(*heap) + len;
+	if (__builtin_expect(__builtin_add_overflow(len, sizeof(*heap), &len), 0)) {
+		SET_ERRNO(ENOMEM);
+		return NULL;
+	}
+
 	heap = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE,
 		    -1, 0);
 	if (__builtin_expect(heap == MAP_FAILED, 0))
-- 
2.55.0