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

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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: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Kees Cook 2016-05-18 06:37:47 -07:00 committed by Jonathan Corbet
parent 6d5244c72e
commit c9de4a82c8
1 changed files with 18 additions and 10 deletions
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Documentation/security/self-protection.txt
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@ -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