mm: clarify CONFIG_PAGE_POISONING and usage

The Kconfig text for CONFIG_PAGE_POISONING doesn't mention that it has to
be enabled explicitly.  This updates the documentation for that and adds a
note about CONFIG_PAGE_POISONING to the "page_poison" command line docs.
While here, change description of CONFIG_PAGE_POISONING_ZERO too, as it's
not "random" data, but rather the fixed debugging value that would be used
when not zeroing.  Additionally removes a stray "bool" in the Kconfig.

Link: http://lkml.kernel.org/r/20180725223832.GA43733@beast
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bffb0caa369305e221108ae7557605acf7809b89..970d837bd57fcaf73479e457279a0d300835619b 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3041,8 +3041,9 @@
                        on: enable the feature
 
        page_poison=    [KNL] Boot-time parameter changing the state of
-                       poisoning on the buddy allocator.
-                       off: turn off poisoning
+                       poisoning on the buddy allocator, available with
+                       CONFIG_PAGE_POISONING=y.
+                       off: turn off poisoning (default)
                        on: turn on poisoning
 
        panic=          [KNL] Kernel behaviour on panic: delay <timeout>

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index e5e606ee5f715e846a19ea9186391c47a2efddc0..9a7b8b049d04fb42ca02783eabf5e70ef9fe5495 100644 (file)
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -46,7 +46,8 @@ config PAGE_POISONING
          Fill the pages with poison patterns after free_pages() and verify
          the patterns before alloc_pages. The filling of the memory helps
          reduce the risk of information leaks from freed data. This does
-         have a potential performance impact.
+         have a potential performance impact if enabled with the
+         "page_poison=1" kernel boot option.
 
          Note that "poison" here is not the same thing as the "HWPoison"
          for CONFIG_MEMORY_FAILURE. This is software poisoning only.
@@ -65,7 +66,7 @@ config PAGE_POISONING_NO_SANITY
           say N.
 
 config PAGE_POISONING_ZERO
-       bool "Use zero for poisoning instead of random data"
+       bool "Use zero for poisoning instead of debugging value"
        depends on PAGE_POISONING
        ---help---
           Instead of using the existing poison value, fill the pages with
@@ -75,7 +76,6 @@ config PAGE_POISONING_ZERO
           allocation.
 
           If unsure, say N
-       bool
 
 config DEBUG_PAGE_REF
        bool "Enable tracepoint to track down page reference manipulation"