docs: self-protection: rename "leak" to "exposure"

The meaning of "leak" can be both "untracked resource allocation" and
"memory content disclosure". This document's use was entirely of the
latter meaning, so avoid the confusion by using the Common Weakness
Enumeration name for this: Information Exposure (CWE-200). Additionally
adds a section on structure randomization.
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/security/self-protection.txt

@@ -183,8 +183,9 @@ provide meaningful defenses.
 ### Canaries, blinding, and other secrets
 
 It should be noted that things like the stack canary discussed earlier
-are technically statistical defenses, since they rely on a (leakable)
-secret value.
+are technically statistical defenses, since they rely on a secret value,
+and such values may become discoverable through an information exposure
+flaw.
 
 Blinding literal values for things like JITs, where the executable
 contents may be partially under the control of userspace, need a similar
@@ -199,8 +200,8 @@ working?) in order to maximize their success.
 Since the location of kernel memory is almost always instrumental in
 mounting a successful attack, making the location non-deterministic
 raises the difficulty of an exploit. (Note that this in turn makes
-the value of leaks higher, since they may be used to discover desired
-memory locations.)
+the value of information exposures higher, since they may be used to
+discover desired memory locations.)
 
 #### Text and module base
 
@@ -222,14 +223,21 @@ become more difficult to locate.
 Much of the kernel's dynamic memory (e.g. kmalloc, vmalloc, etc) ends up
 being relatively deterministic in layout due to the order of early-boot
 initializations. If the base address of these areas is not the same
-between boots, targeting them is frustrated, requiring a leak specific
-to the region.
+between boots, targeting them is frustrated, requiring an information
+exposure specific to the region.
+
+#### Structure layout
+
+By performing a per-build randomization of the layout of sensitive
+structures, attacks must either be tuned to known kernel builds or expose
+enough kernel memory to determine structure layouts before manipulating
+them.
 
 
-## Preventing Leaks
+## Preventing Information Exposures
 
 Since the locations of sensitive structures are the primary target for
-attacks, it is important to defend against leaks of both kernel memory
+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).
 
@@ -250,8 +258,8 @@ sure structure holes are cleared.
 When releasing memory, it is best to poison the contents (clear stack on
 syscall return, wipe heap memory on a free), to avoid reuse attacks that
 rely on the old contents of memory. This frustrates many uninitialized
-variable attacks, stack info leaks, heap info leaks, and use-after-free
-attacks.
+variable attacks, stack content exposures, heap content exposures, and
+use-after-free attacks.
 
 ### Destination tracking