[ 017/123] tmpfs mempolicy: fix /proc/mounts corrupting memory

Greg Kroah-Hartman
2013-01-09T21:53:10Z

From: Hugh Dickins <hughd@google.com>

commit f2a07f40dbc603c15f8b06e6ec7f768af67b424f upstream.

Recently I suggested using "mount -o remount,mpol=local /tmp" in NUMA
mempolicy testing. Very nasty. Reading /proc/mounts, /proc/pid/mounts
or /proc/pid/mountinfo may then corrupt one bit of kernel memory, often
in a page table (causing "Bad swap" or "Bad page map" warning or "Bad
pagetable" oops), sometimes in a vm_area_struct or rbnode or somewhere
worse. "mpol=prefer" and "mpol=prefer:Node" are equally toxic.

Recent NUMA enhancements are not to blame: this dates back to 2.6.35,
when commit e17f74af351c "mempolicy: don't call mpol_set_nodemask() when
no_context" skipped mpol_parse_str()'s call to mpol_set_nodemask(),
which used to initialize v.preferred_node, or set MPOL_F_LOCAL in flags.
With slab poisoning, you can then rely on mpol_to_str() to set the bit
for node 0x6b6b, probably in the next page above the caller's stack.

mpol_parse_str() is only called from shmem_parse_options(): no_context
is always true, so call it unused for now, and remove !no_context code.
Set v.nodes or v.preferred_node or MPOL_F_LOCAL as mpol_to_str() might
expect. Then mpol_to_str() can ignore its no_context argument also,
the mpol being appropriately initialized whether contextualized or not.
Rename its no_context unused too, and let subsequent patch remove them
(that's not needed for stable backporting, which would involve rejects).

I don't understand why MPOL_LOCAL is described as a pseudo-policy:
it's a reasonable policy which suffers from a confusing implementation
in terms of MPOL_PREFERRED with MPOL_F_LOCAL. I believe this would be
much more robust if MPOL_LOCAL were recognized in switch statements
throughout, MPOL_F_LOCAL deleted, and MPOL_PREFERRED use the (possibly
empty) nodes mask like everyone else, instead of its preferred_node
variant (I presume an optimization from the days before MPOL_LOCAL).
But that would take me too long to get right and fully tested.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/mempolicy.c | 64 +++++++++++++++++++++++----------------------------------
1 file changed, 26 insertions(+), 38 deletions(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2372,8 +2372,7 @@ void numa_default_policy(void)
*/

/*
- * "local" is pseudo-policy: MPOL_PREFERRED with MPOL_F_LOCAL flag
- * Used only for mpol_parse_str() and mpol_to_str()
+ * "local" is implemented internally by MPOL_PREFERRED with MPOL_F_LOCAL flag.
*/
#define MPOL_LOCAL MPOL_MAX
static const char * const policy_modes[] =
@@ -2388,28 +2387,21 @@ static const char * const policy_modes[]

#ifdef CONFIG_TMPFS
/**
- * mpol_parse_str - parse string to mempolicy
+ * mpol_parse_str - parse string to mempolicy, for tmpfs mpol mount option.
* @str: string containing mempolicy to parse
* @mpol: pointer to struct mempolicy pointer, returned on success.
- * @no_context: flag whether to "contextualize" the mempolicy
+ * @unused: redundant argument, to be removed later.
*
* Format of input:
* <mode>[=<flags>][:<nodelist>]
*
- * if @no_context is true, save the input nodemask in w.user_nodemask in
- * the returned mempolicy. This will be used to "clone" the mempolicy in
- * a specific context [cpuset] at a later time. Used to parse tmpfs mpol
- * mount option. Note that if 'static' or 'relative' mode flags were
- * specified, the input nodemask will already have been saved. Saving
- * it again is redundant, but safe.
- *
* On success, returns 0, else 1
*/
-int mpol_parse_str(char *str, struct mempolicy **mpol, int no_context)
+int mpol_parse_str(char *str, struct mempolicy **mpol, int unused)
{
struct mempolicy *new = NULL;
unsigned short mode;
- unsigned short uninitialized_var(mode_flags);
+ unsigned short mode_flags;
nodemask_t nodes;
char *nodelist = strchr(str, ':');
char *flags = strchr(str, '=');
@@ -2497,24 +2489,23 @@ int mpol_parse_str(char *str, struct mem
if (IS_ERR(new))
goto out;

- if (no_context) {
- /* save for contextualization */
- new->w.user_nodemask = nodes;
- } else {
- int ret;
- NODEMASK_SCRATCH(scratch);
- if (scratch) {
- task_lock(current);
- ret = mpol_set_nodemask(new, &nodes, scratch);
- task_unlock(current);
- } else
- ret = -ENOMEM;
- NODEMASK_SCRATCH_FREE(scratch);
- if (ret) {
- mpol_put(new);
- goto out;
- }
- }
+ /*
+ * Save nodes for mpol_to_str() to show the tmpfs mount options
+ * for /proc/mounts, /proc/pid/mounts and /proc/pid/mountinfo.
+ */
+ if (mode != MPOL_PREFERRED)
+ new->v.nodes = nodes;
+ else if (nodelist)
+ new->v.preferred_node = first_node(nodes);
+ else
+ new->flags |= MPOL_F_LOCAL;
+
+ /*
+ * Save nodes for contextualization: this will be used to "clone"
+ * the mempolicy in a specific context [cpuset] at a later time.
+ */
+ new->w.user_nodemask = nodes;
+
err = 0;

out:
@@ -2534,13 +2525,13 @@ out:
* @buffer: to contain formatted mempolicy string
* @maxlen: length of @buffer
* @pol: pointer to mempolicy to be formatted
- * @no_context: "context free" mempolicy - use nodemask in w.user_nodemask
+ * @unused: redundant argument, to be removed later.
*
* Convert a mempolicy into a string.
* Returns the number of characters in buffer (if positive)
* or an error (negative)
*/
-int mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol, int no_context)
+int mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol, int unused)
{
char *p = buffer;
int l;
@@ -2566,7 +2557,7 @@ int mpol_to_str(char *buffer, int maxlen
case MPOL_PREFERRED:
nodes_clear(nodes);
if (flags & MPOL_F_LOCAL)
- mode = MPOL_LOCAL; /* pseudo-policy */
+ mode = MPOL_LOCAL;
else
node_set(pol->v.preferred_node, nodes);
break;
@@ -2574,10 +2565,7 @@ int mpol_to_str(char *buffer, int maxlen
case MPOL_BIND:
/* Fall through */
case MPOL_INTERLEAVE:
- if (no_context)
- nodes = pol->w.user_nodemask;
- else
- nodes = pol->v.nodes;
+ nodes = pol->v.nodes;
break;