[PATCH v2 2/2] overflow: Implement size_t saturating arithmetic helpers
From: Kees Cook
Date: Mon Jan 24 2022 - 22:24:50 EST
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.
Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
Cc: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Leon Romanovsky <leon@xxxxxxxxxx>
Cc: Keith Busch <kbusch@xxxxxxxxxx>
Cc: Len Baker <len.baker@xxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Documentation/process/deprecated.rst | 20 ++++-
include/linux/overflow.h | 110 +++++++++++++++++----------
lib/test_overflow.c | 98 ++++++++++++++++++++++++
3 files changed, 184 insertions(+), 44 deletions(-)
diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index 388cb19f5dbb..a6e36d9c3d14 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -71,6 +71,9 @@ Instead, the 2-factor form of the allocator should be used::
foo = kmalloc_array(count, size, GFP_KERNEL);
+Specifically, kmalloc() can be replaced with kmalloc_array(), and
+kzalloc() can be replaced with kcalloc().
+
If no 2-factor form is available, the saturate-on-overflow helpers should
be used::
@@ -91,9 +94,20 @@ Instead, use the helper::
array usage and switch to a `flexible array member
<#zero-length-and-one-element-arrays>`_ instead.
-See array_size(), array3_size(), and struct_size(),
-for more details as well as the related check_add_overflow() and
-check_mul_overflow() family of functions.
+For other calculations, please compose the use of the size_mul(),
+size_add(), and size_sub() helpers. For example, in the case of::
+
+ foo = krealloc(current_size + chunk_size * (count - 3), GFP_KERNEL);
+
+Instead, use the helpers::
+
+ foo = krealloc(size_add(current_size,
+ size_mul(chunk_size,
+ size_sub(count, 3))), GFP_KERNEL);
+
+For more details, also see array3_size() and flex_array_size(),
+as well as the related check_mul_overflow(), check_add_overflow(),
+check_sub_overflow(), and check_shl_overflow() family of functions.
simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
----------------------------------------------------------------------
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 */
diff --git a/lib/test_overflow.c b/lib/test_overflow.c
index cea37ae82615..712fb2351c27 100644
--- a/lib/test_overflow.c
+++ b/lib/test_overflow.c
@@ -594,12 +594,110 @@ static int __init test_overflow_allocation(void)
return err;
}
+struct __test_flex_array {
+ unsigned long flags;
+ size_t count;
+ unsigned long data[];
+};
+
+static int __init test_overflow_size_helpers(void)
+{
+ struct __test_flex_array *obj;
+ int count = 0;
+ int err = 0;
+ int var;
+
+#define check_one_size_helper(expected, func, args...) ({ \
+ bool __failure = false; \
+ size_t _r; \
+ \
+ _r = func(args); \
+ if (_r != (expected)) { \
+ pr_warn("expected " #func "(" #args ") " \
+ "to return %zu but got %zu instead\n", \
+ (size_t)(expected), _r); \
+ __failure = true; \
+ } \
+ count++; \
+ __failure; \
+})
+
+ var = 4;
+ err |= check_one_size_helper(20, size_mul, var++, 5);
+ err |= check_one_size_helper(20, size_mul, 4, var++);
+ err |= check_one_size_helper(0, size_mul, 0, 3);
+ err |= check_one_size_helper(0, size_mul, 3, 0);
+ err |= check_one_size_helper(6, size_mul, 2, 3);
+ err |= check_one_size_helper(SIZE_MAX, size_mul, SIZE_MAX, 1);
+ err |= check_one_size_helper(SIZE_MAX, size_mul, SIZE_MAX, 3);
+ err |= check_one_size_helper(SIZE_MAX, size_mul, SIZE_MAX, -3);
+
+ var = 4;
+ err |= check_one_size_helper(9, size_add, var++, 5);
+ err |= check_one_size_helper(9, size_add, 4, var++);
+ err |= check_one_size_helper(9, size_add, 9, 0);
+ err |= check_one_size_helper(9, size_add, 0, 9);
+ err |= check_one_size_helper(5, size_add, 2, 3);
+ err |= check_one_size_helper(SIZE_MAX, size_add, SIZE_MAX, 1);
+ err |= check_one_size_helper(SIZE_MAX, size_add, SIZE_MAX, 3);
+ err |= check_one_size_helper(SIZE_MAX, size_add, SIZE_MAX, -3);
+
+ var = 4;
+ err |= check_one_size_helper(1, size_sub, var--, 3);
+ err |= check_one_size_helper(1, size_sub, 4, var--);
+ err |= check_one_size_helper(1, size_sub, 3, 2);
+ err |= check_one_size_helper(9, size_sub, 9, 0);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, 9, -3);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, 0, 9);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, 2, 3);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, SIZE_MAX, 0);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, SIZE_MAX, 10);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, 0, SIZE_MAX);
+ err |= check_one_size_helper(SIZE_MAX, size_sub, 14, SIZE_MAX);
+ err |= check_one_size_helper(SIZE_MAX - 2, size_sub, SIZE_MAX - 1, 1);
+ err |= check_one_size_helper(SIZE_MAX - 4, size_sub, SIZE_MAX - 1, 3);
+ err |= check_one_size_helper(1, size_sub, SIZE_MAX - 1, -3);
+
+ var = 4;
+ err |= check_one_size_helper(4 * sizeof(*obj->data),
+ flex_array_size, obj, data, var++);
+ err |= check_one_size_helper(5 * sizeof(*obj->data),
+ flex_array_size, obj, data, var++);
+ err |= check_one_size_helper(0, flex_array_size, obj, data, 0);
+ err |= check_one_size_helper(sizeof(*obj->data),
+ flex_array_size, obj, data, 1);
+ err |= check_one_size_helper(7 * sizeof(*obj->data),
+ flex_array_size, obj, data, 7);
+ err |= check_one_size_helper(SIZE_MAX,
+ flex_array_size, obj, data, -1);
+ err |= check_one_size_helper(SIZE_MAX,
+ flex_array_size, obj, data, SIZE_MAX - 4);
+
+ var = 4;
+ err |= check_one_size_helper(sizeof(*obj) + (4 * sizeof(*obj->data)),
+ struct_size, obj, data, var++);
+ err |= check_one_size_helper(sizeof(*obj) + (5 * sizeof(*obj->data)),
+ struct_size, obj, data, var++);
+ err |= check_one_size_helper(sizeof(*obj), struct_size, obj, data, 0);
+ err |= check_one_size_helper(sizeof(*obj) + sizeof(*obj->data),
+ struct_size, obj, data, 1);
+ err |= check_one_size_helper(SIZE_MAX,
+ struct_size, obj, data, -3);
+ err |= check_one_size_helper(SIZE_MAX,
+ struct_size, obj, data, SIZE_MAX - 3);
+
+ pr_info("%d overflow size helper tests finished\n", count);
+
+ return err;
+}
+
static int __init test_module_init(void)
{
int err = 0;
err |= test_overflow_calculation();
err |= test_overflow_shift();
+ err |= test_overflow_size_helpers();
err |= test_overflow_allocation();
if (err) {
--
2.30.2