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 <[email protected]> Signed-off-by: Jonathan Corbet <[email protected]>
kyagi · Dec 21, 2017 · 227d1a6 · 227d1a6
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commit 227d1a6
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diff --git 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
 ------------------