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tmpfs mempolicy: fix /proc/mounts corrupting memory 

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 e17f74af35 "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>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2013-01-02 02:01:33 -08:00 committed by Linus Torvalds
parent 128dd1759d
commit f2a07f40db
1 changed files with 26 additions and 38 deletions
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64
mm/mempolicy.c
View File

@ -2595,8 +2595,7 @@ 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.
*/
static const char * const policy_modes[] =
{
@ -2610,28 +2609,21 @@ 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, '=');
@ -2719,24 +2711,23 @@ int mpol_parse_str(char *str, struct mempolicy **mpol, int no_context)
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:
@ -2756,13 +2747,13 @@ 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;
@ -2788,7 +2779,7 @@ int mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol, int no_context)
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;
@ -2796,10 +2787,7 @@ int mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol, int no_context)
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;
default: