overflow: Implement size_t saturating arithmetic helpers

In order to perform more open-coded replacements of common allocation
size arithmetic, the kernel needs saturating (SIZE_MAX) helpers for
multiplication, addition, and subtraction. For example, it is common in
allocators, especially on realloc, to add to an existing size:

    p = krealloc(map->patch,
                 sizeof(struct reg_sequence) * (map->patch_regs + num_regs),
                 GFP_KERNEL);

There is no existing saturating replacement for this calculation, and
just leaving the addition open coded inside array_size() could
potentially overflow as well. For example, an overflow in an expression
for a size_t argument might wrap to zero:

    array_size(anything, something_at_size_max + 1) == 0

Introduce size_mul(), size_add(), and size_sub() helpers that
implicitly promote arguments to size_t and saturated calculations for
use in allocations. With these helpers it is also possible to redefine
array_size(), array3_size(), flex_array_size(), and struct_size() in
terms of the new helpers.

As with the check_*_overflow() helpers, the new helpers use __must_check,
though what is really desired is a way to make sure that assignment is
only to a size_t lvalue. Without this, it's still possible to introduce
overflow/underflow via type conversion (i.e. from size_t to int).
Enforcing this will currently need to be left to static analysis or
future use of -Wconversion.

Additionally update the overflow unit tests to force runtime evaluation
for the pathological cases.

Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Len Baker <len.baker@gmx.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 69 insertions, 41 deletions

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 4669632bd72b..59d7228104d0 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -118,81 +118,94 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 }))
 
 /**
- * array_size() - Calculate size of 2-dimensional array.
- *
- * @a: dimension one
- * @b: dimension two
+ * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
  *
- * Calculates size of 2-dimensional array: @a * @b.
+ * @factor1: first factor
+ * @factor2: second factor
  *
- * Returns: number of bytes needed to represent the array or SIZE_MAX on
- * overflow.
+ * Returns: calculate @factor1 * @factor2, both promoted to size_t,
+ * with any overflow causing the return value to be SIZE_MAX. The
+ * lvalue must be size_t to avoid implicit type conversion.
  */
-static inline __must_check size_t array_size(size_t a, size_t b)
+static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
 {
 	size_t bytes;
 
-	if (check_mul_overflow(a, b, &bytes))
+	if (check_mul_overflow(factor1, factor2, &bytes))
 		return SIZE_MAX;
 
 	return bytes;
 }
 
 /**
- * array3_size() - Calculate size of 3-dimensional array.
+ * size_add() - Calculate size_t addition with saturation at SIZE_MAX
  *
- * @a: dimension one
- * @b: dimension two
- * @c: dimension three
- *
- * Calculates size of 3-dimensional array: @a * @b * @c.
+ * @addend1: first addend
+ * @addend2: second addend
  *
- * Returns: number of bytes needed to represent the array or SIZE_MAX on
- * overflow.
+ * Returns: calculate @addend1 + @addend2, both promoted to size_t,
+ * with any overflow causing the return value to be SIZE_MAX. The
+ * lvalue must be size_t to avoid implicit type conversion.
  */
-static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
+static inline size_t __must_check size_add(size_t addend1, size_t addend2)
 {
 	size_t bytes;
 
-	if (check_mul_overflow(a, b, &bytes))
-		return SIZE_MAX;
-	if (check_mul_overflow(bytes, c, &bytes))
+	if (check_add_overflow(addend1, addend2, &bytes))
 		return SIZE_MAX;
 
 	return bytes;
 }
 
-/*
- * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for
- * struct_size() below.
+/**
+ * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
+ *
+ * @minuend: value to subtract from
+ * @subtrahend: value to subtract from @minuend
+ *
+ * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
+ * with any overflow causing the return value to be SIZE_MAX. For
+ * composition with the size_add() and size_mul() helpers, neither
+ * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
+ * The lvalue must be size_t to avoid implicit type conversion.
  */
-static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
+static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 {
 	size_t bytes;
 
-	if (check_mul_overflow(a, b, &bytes))
-		return SIZE_MAX;
-	if (check_add_overflow(bytes, c, &bytes))
+	if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
+	    check_sub_overflow(minuend, subtrahend, &bytes))
 		return SIZE_MAX;
 
 	return bytes;
 }
 
 /**
- * struct_size() - Calculate size of structure with trailing array.
- * @p: Pointer to the structure.
- * @member: Name of the array member.
- * @count: Number of elements in the array.
+ * array_size() - Calculate size of 2-dimensional array.
  *
- * Calculates size of memory needed for structure @p followed by an
- * array of @count number of @member elements.
+ * @a: dimension one
+ * @b: dimension two
  *
- * Return: number of bytes needed or SIZE_MAX on overflow.
+ * Calculates size of 2-dimensional array: @a * @b.
+ *
+ * Returns: number of bytes needed to represent the array or SIZE_MAX on
+ * overflow.
  */
-#define struct_size(p, member, count)					\
-	__ab_c_size(count,						\
-		    sizeof(*(p)->member) + __must_be_array((p)->member),\
-		    sizeof(*(p)))
+#define array_size(a, b)	size_mul(a, b)
+
+/**
+ * array3_size() - Calculate size of 3-dimensional array.
+ *
+ * @a: dimension one
+ * @b: dimension two
+ * @c: dimension three
+ *
+ * Calculates size of 3-dimensional array: @a * @b * @c.
+ *
+ * Returns: number of bytes needed to represent the array or SIZE_MAX on
+ * overflow.
+ */
+#define array3_size(a, b, c)	size_mul(size_mul(a, b), c)
 
 /**
  * flex_array_size() - Calculate size of a flexible array member
@@ -208,7 +221,22 @@ static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
  * Return: number of bytes needed or SIZE_MAX on overflow.
  */
 #define flex_array_size(p, member, count)				\
-	array_size(count,						\
-		    sizeof(*(p)->member) + __must_be_array((p)->member))
+	size_mul(count,							\
+		 sizeof(*(p)->member) + __must_be_array((p)->member))
+
+/**
+ * struct_size() - Calculate size of structure with trailing flexible array.
+ *
+ * @p: Pointer to the structure.
+ * @member: Name of the array member.
+ * @count: Number of elements in the array.
+ *
+ * Calculates size of memory needed for structure @p followed by an
+ * array of @count number of @member elements.
+ *
+ * Return: number of bytes needed or SIZE_MAX on overflow.
+ */
+#define struct_size(p, member, count)					\
+	size_add(sizeof(*(p)), flex_array_size(p, member, count))
 
 #endif /* __LINUX_OVERFLOW_H */