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, 17 insertions, 3 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()
 ----------------------------------------------------------------------