linux-kernel - [PATCH] docs: deprecated.rst: Add %p to the list

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Date:   Wed, 4 Mar 2020 11:13:28 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Jonathan Corbet <corbet@....net>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] docs: deprecated.rst: Add %p to the list

Once in a while %p usage comes up, and I've needed to have a reference
to point people to. Add %p details to deprecated.rst.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 Documentation/process/deprecated.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index f9f196d3a69b..a4db119f4e09 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -109,6 +109,23 @@ the given limit of bytes to copy. This is inefficient and can lead to
 linear read overflows if a source string is not NUL-terminated. The
 safe replacement is :c:func:`strscpy`.
 
+%p format specifier
+-------------------
+Using %p in format strings leads to a huge number of address exposures.
+Instead of leaving these to be exploitable, "%p" should not be used in
+the kernel. If used currently, it is a hashed value, rendering it
+unusable for addressing. Paraphrasing Linus's current `guideance <https://lore.kernel.org/lkml/CA+55aFwQEd_d40g4mUCSsVRZzrFPUJt74vc6PPpb675hYNXcKw@mail.gmail.com/>`_:
+
+- Just use %p and get the hashed value.
+- If the hashed value is pointless, ask yourself whether the pointer
+  itself is important. Maybe it should be removed entirely?
+- As a last option, if you really think the true pointer value is
+  important, why is some system state or user privilege level considered
+  "special"? If it is well justified (in comments and commit log), maybe
+  you can use %px along with making sure you have sensible permissions.
+
+A system-wide toggle will `not be accepted <https://lore.kernel.org/lkml/CA+55aFwieC1-nAs+NFq9RTwaR8ef9hWa4MjNBWL41F-8wM49eA@mail.gmail.com/>`_.
+
 Variable Length Arrays (VLAs)
 -----------------------------
 Using stack VLAs produces much worse machine code than statically
-- 
2.20.1


-- 
Kees Cook