[PATCH 64/64] fortify: Add run-time WARN for cross-field memcpy()

Kees Cook
2021-07-27T21:01:52Z

This enables the run-time checking of dynamic memcpy() and memmove()
lengths, issuing a WARN when a write would exceed the size of the
target field.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/fortify-string.h | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index 4afd42079d3b..0d0acd959ba0 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -260,7 +260,7 @@ __FORTIFY_INLINE void fortify_memset_chk(__kernel_size_t size,
* V = vulnerable to run-time overflow
*
*/
-__FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size,
+__FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
const size_t p_size,
const size_t q_size,
const size_t p_size_field,
@@ -309,13 +309,25 @@ __FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size,
if ((p_size != (size_t)(-1) && p_size < size) ||
(q_size != (size_t)(-1) && q_size < size))
fortify_panic(func);
+
+ /*
+ * Warn when writing beyond destination field size. Since
+ * flexible-arrays are considered 0 bytes, we must ignore 0 sizes
+ * at runtime for now.
+ */
+ if (p_size_field && p_size != p_size_field && p_size_field < size)
+ return true;
+
+ return false;
}

#define __fortify_memcpy_chk(p, q, size, p_size, q_size, \
p_size_field, q_size_field, op) ({ \
size_t __fortify_size = (size_t)(size); \
- fortify_memcpy_chk(__fortify_size, p_size, q_size, \
- p_size_field, q_size_field, #op); \
+ WARN_ONCE(fortify_memcpy_chk(__fortify_size, p_size, q_size, \
+ p_size_field, q_size_field, #op), \
+ #op ": detected field-spanning write (size %zu) of single field (size %zu)\n", \
+ __fortify_size, p_size_field); \
__underlying_##op(p, q, __fortify_size); \
})

--
2.30.2