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[PATCH] docs: 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. We should update the
documentation appropriately.

Add documentation regarding printing kernel addresses.

Signed-off-by: Tobin C. Harding <me@xxxxxxxx>

Is there a proffered method for subscripts in sphinx kernel docs? Here
we use '[*]' 


 Documentation/security/self-protection.rst | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst
index 60c8bd8b77bf..e711280cfdd7 100644
--- a/Documentation/security/self-protection.rst
+++ b/Documentation/security/self-protection.rst
@@ -270,6 +270,20 @@ 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 symbol lookup fails, the raw address is currently printed.
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