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

[Kees Cook <keescook@xxxxxxxxxxxx>]

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@xxxxxxxxxxxx>
---
 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