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Message-ID: <202003042301.F844A8C0EC@keescook>
Date: Wed, 4 Mar 2020 23:03:47 -0800
From: Kees Cook <keescook@...omium.org>
To: Jonathan Corbet <corbet@....net>
Cc: James Troup <james.troup@...onical.com>,
"Gustavo A. R. Silva" <gustavo@...eddedor.com>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2] 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>
---
v2: rewrite much of the text to be more clear (James Troup)
---
Documentation/process/deprecated.rst | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index f9f196d3a69b..8965446f0b71 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -109,6 +109,28 @@ 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
+-------------------
+Traditionally, using "%p" in format strings would lead to regular address
+exposure flaws in dmesg, proc, sysfs, etc. Instead of leaving these to
+be exploitable, all "%p" uses in the kernel are being printed as a hashed
+value, rendering them unusable for addressing. New uses of "%p" should not
+be added to the kernel. For text addresses, using "%pS" is likely better,
+as it produces the more useful symbol name instead. For nearly everything
+else, just do not add "%p" at all.
+
+Paraphrasing Linus's current `guidance <https://lore.kernel.org/lkml/CA+55aFwQEd_d40g4mUCSsVRZzrFPUJt74vc6PPpb675hYNXcKw@mail.gmail.com/>`_:
+
+- If the hashed "%p" value is pointless, ask yourself whether the pointer
+ itself is important. Maybe it should be removed entirely?
+- If you really think the true pointer value is important, why is some
+ system state or user privilege level considered "special"? If you think
+ you can justify it (in comments and commit log) well enough to stand
+ up to Linus's scrutiny, maybe you can use "%px", along with making sure
+ you have sensible permissions.
+
+And finally, know that a toggle for "%p" hashing 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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