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 <me@tobin.cc> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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@ -270,6 +270,21 @@ attacks, it is important to defend against exposure of both kernel memory
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addresses and kernel memory contents (since they may contain kernel
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addresses or other sensitive things like canary values).
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Kernel addresses
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----------------
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Printing kernel addresses to userspace leaks sensitive information about
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the kernel memory layout. Care should be exercised when using any printk
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specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb]
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in certain circumstances [*]). Any file written to using one of these
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specifiers should be readable only by privileged processes.
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Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1
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addresses printed with the specifier %p are hashed before printing.
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[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is
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printed. If KALLSYMS is not enabled the raw address is printed.
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Unique identifiers
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------------------
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