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Date: Tue, 19 Dec 2017 09:29:05 +1100
From: "Tobin C. Harding" <>
To: Jonathan Corbet <>
Cc: "Tobin C. Harding" <>,
	Randy Dunlap <>,,,
	Kees Cook <>,
	Alexander Popov <>,
	Joe Perches <>,
Subject: [PATCH v3 3/3] doc: add documentation on printing kernel addresses

Hashing addresses printed with printk specifier %p was implemented
recently. During development a number of issues were raised regarding
leaking kernel addresses to userspace. Other documentation was updated but
security/self-protection missed out.

Add self-protection documentation regarding printing kernel addresses.

Signed-off-by: Tobin C. Harding <>
 Documentation/security/self-protection.rst | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst
index 60c8bd8b77bf..0f53826c78b9 100644
--- a/Documentation/security/self-protection.rst
+++ b/Documentation/security/self-protection.rst
@@ -270,6 +270,21 @@ attacks, it is important to defend against exposure of both kernel memory
 addresses and kernel memory contents (since they may contain kernel
 addresses or other sensitive things like canary values).
+Kernel addresses
+Printing kernel addresses to userspace leaks sensitive information about
+the kernel memory layout. Care should be exercised when using any printk
+specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb]
+in certain circumstances [*]).  Any file written to using one of these
+specifiers should be readable only by privileged processes.
+Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1
+addresses printed with the specifier %p are hashed before printing.
+[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is
+printed. If KALLSYMS is not enabled the raw address is printed.
 Unique identifiers

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