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Message-ID: <202003041103.A5842AD@keescook>
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
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