[PATCH v4 3/3] doc: add documentation on printing kernel addresses
From: Tobin C. Harding
Date: Tue Dec 19 2017 - 16:18:18 EST
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 <me@xxxxxxxx>
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
@@ -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).
+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.