[PATCH 3.4 03/23] shmem: fix faulting into a hole, not taking i_mutex

From: Greg Kroah-Hartman
Date: Sat Jul 26 2014 - 16:21:29 EST

3.4-stable review patch. If anyone has any objections, please let me know.


From: Hugh Dickins <hughd@xxxxxxxxxx>

commit 8e205f779d1443a94b5ae81aa359cb535dd3021e upstream.

Commit f00cdc6df7d7 ("shmem: fix faulting into a hole while it's
punched") was buggy: Sasha sent a lockdep report to remind us that
grabbing i_mutex in the fault path is a no-no (write syscall may already
hold i_mutex while faulting user buffer).

We tried a completely different approach (see following patch) but that
proved inadequate: good enough for a rational workload, but not good
enough against trinity - which forks off so many mappings of the object
that contention on i_mmap_mutex while hole-puncher holds i_mutex builds
into serious starvation when concurrent faults force the puncher to fall
back to single-page unmap_mapping_range() searches of the i_mmap tree.

So return to the original umbrella approach, but keep away from i_mutex
this time. We really don't want to bloat every shmem inode with a new
mutex or completion, just to protect this unlikely case from trinity.
So extend the original with wait_queue_head on stack at the hole-punch
end, and wait_queue item on the stack at the fault end.

This involves further use of i_lock to guard against the races: lockdep
has been happy so far, and I see fs/inode.c:unlock_new_inode() holds
i_lock around wake_up_bit(), which is comparable to what we do here.
i_lock is more convenient, but we could switch to shmem's info->lock.

This issue has been tagged with CVE-2014-4171, which will require commit
f00cdc6df7d7 and this and the following patch to be backported: we
suggest to 3.1+, though in fact the trinity forkbomb effect might go
back as far as 2.6.16, when madvise(,,MADV_REMOVE) came in - or might
not, since much has changed, with i_mmap_mutex a spinlock before 3.0.
Anyone running trinity on 3.0 and earlier? I don't think we need care.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Konstantin Khlebnikov <koct9i@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Lukas Czerner <lczerner@xxxxxxxxxx>
Cc: Dave Jones <davej@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

mm/shmem.c | 64 +++++++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 44 insertions(+), 20 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -82,6 +82,7 @@ static struct vfsmount *shm_mnt;
* a time): we would prefer not to enlarge the shmem inode just for that.
struct shmem_falloc {
+ wait_queue_head_t *waitq; /* faults into hole wait for punch to end */
pgoff_t start; /* start of range currently being fallocated */
pgoff_t next; /* the next page offset to be fallocated */
@@ -1074,37 +1075,57 @@ static int shmem_fault(struct vm_area_st
* Trinity finds that probing a hole which tmpfs is punching can
* prevent the hole-punch from ever completing: which in turn
* locks writers out with its hold on i_mutex. So refrain from
- * faulting pages into the hole while it's being punched, and
- * wait on i_mutex to be released if vmf->flags permits.
+ * faulting pages into the hole while it's being punched. Although
+ * shmem_truncate_range() does remove the additions, it may be unable to
+ * keep up, as each new page needs its own unmap_mapping_range() call,
+ * and the i_mmap tree grows ever slower to scan if new vmas are added.
+ *
+ * It does not matter if we sometimes reach this check just before the
+ * hole-punch begins, so that one fault then races with the punch:
+ * we just need to make racing faults a rare case.
+ *
+ * The implementation below would be much simpler if we just used a
+ * standard mutex or completion: but we cannot take i_mutex in fault,
+ * and bloating every shmem inode for this unlikely case would be sad.
if (unlikely(inode->i_private)) {
struct shmem_falloc *shmem_falloc;

shmem_falloc = inode->i_private;
- if (!shmem_falloc ||
- vmf->pgoff < shmem_falloc->start ||
- vmf->pgoff >= shmem_falloc->next)
- shmem_falloc = NULL;
- spin_unlock(&inode->i_lock);
- /*
- * i_lock has protected us from taking shmem_falloc seriously
- * once return from vmtruncate_range() went back up that stack.
- * i_lock does not serialize with i_mutex at all, but it does
- * not matter if sometimes we wait unnecessarily, or sometimes
- * miss out on waiting: we just need to make those cases rare.
- */
- if (shmem_falloc) {
+ if (shmem_falloc &&
+ vmf->pgoff >= shmem_falloc->start &&
+ vmf->pgoff < shmem_falloc->next) {
+ wait_queue_head_t *shmem_falloc_waitq;
+ DEFINE_WAIT(shmem_fault_wait);
if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
+ /* It's polite to up mmap_sem if we can */
- mutex_lock(&inode->i_mutex);
- mutex_unlock(&inode->i_mutex);
- return VM_FAULT_RETRY;
- /* cond_resched? Leave that to GUP or return to user */
+ shmem_falloc_waitq = shmem_falloc->waitq;
+ prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
+ spin_unlock(&inode->i_lock);
+ schedule();
+ /*
+ * shmem_falloc_waitq points into the vmtruncate_range()
+ * stack of the hole-punching task: shmem_falloc_waitq
+ * is usually invalid by the time we reach here, but
+ * finish_wait() does not dereference it in that case;
+ * though i_lock needed lest racing with wake_up_all().
+ */
+ spin_lock(&inode->i_lock);
+ finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
+ spin_unlock(&inode->i_lock);
+ return ret;
+ spin_unlock(&inode->i_lock);

error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);
@@ -1135,7 +1156,9 @@ int vmtruncate_range(struct inode *inode
struct address_space *mapping = inode->i_mapping;
loff_t unmap_start = round_up(lstart, PAGE_SIZE);
loff_t unmap_end = round_down(1 + lend, PAGE_SIZE) - 1;
+ DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);

+ shmem_falloc.waitq = &shmem_falloc_waitq;
shmem_falloc.start = unmap_start >> PAGE_SHIFT;
shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
@@ -1150,6 +1173,7 @@ int vmtruncate_range(struct inode *inode

inode->i_private = NULL;
+ wake_up_all(&shmem_falloc_waitq);

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