Commit 8e205f77 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

shmem: fix faulting into a hole, not taking i_mutex

Commit f00cdc6d ("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
f00cdc6d 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: default avatarHugh Dickins <>
Reported-by: default avatarSasha Levin <>
Tested-by: default avatarSasha Levin <>
Cc: Vlastimil Babka <>
Cc: Konstantin Khlebnikov <>
Cc: Johannes Weiner <>
Cc: Lukas Czerner <>
Cc: Dave Jones <>
Cc: <>	[3.1+]
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent c118678b
......@@ -85,7 +85,7 @@ static struct vfsmount *shm_mnt;
* a time): we would prefer not to enlarge the shmem inode just for that.
struct shmem_falloc {
int mode; /* FALLOC_FL mode currently operating */
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 */
pgoff_t nr_falloced; /* how many new pages have been fallocated */
......@@ -760,7 +760,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
shmem_falloc = inode->i_private;
if (shmem_falloc &&
!shmem_falloc->mode &&
!shmem_falloc->waitq &&
index >= shmem_falloc->start &&
index < shmem_falloc->next)
......@@ -1248,38 +1248,58 @@ static int shmem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
* 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_undo_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 ||
shmem_falloc->mode != FALLOC_FL_PUNCH_HOLE ||
vmf->pgoff < shmem_falloc->start ||
vmf->pgoff >= shmem_falloc->next)
shmem_falloc = NULL;
* i_lock has protected us from taking shmem_falloc seriously
* once return from shmem_fallocate() 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 &&
shmem_falloc->waitq &&
vmf->pgoff >= shmem_falloc->start &&
vmf->pgoff < shmem_falloc->next) {
wait_queue_head_t *shmem_falloc_waitq;
if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
/* It's polite to up mmap_sem if we can */
/* 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,
* shmem_falloc_waitq points into the shmem_fallocate()
* 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().
finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
return ret;
error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);
......@@ -1774,13 +1794,13 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
shmem_falloc.mode = mode & ~FALLOC_FL_KEEP_SIZE;
if (mode & FALLOC_FL_PUNCH_HOLE) {
struct address_space *mapping = file->f_mapping;
loff_t unmap_start = round_up(offset, PAGE_SIZE);
loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;
shmem_falloc.waitq = &shmem_falloc_waitq;
shmem_falloc.start = unmap_start >> PAGE_SHIFT; = (unmap_end + 1) >> PAGE_SHIFT;
......@@ -1792,8 +1812,13 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
1 + unmap_end - unmap_start, 0);
shmem_truncate_range(inode, offset, offset + len - 1);
/* No need to unmap again: hole-punching leaves COWed pages */
inode->i_private = NULL;
error = 0;
goto undone;
goto out;
/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
......@@ -1809,6 +1834,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
goto out;
shmem_falloc.waitq = NULL;
shmem_falloc.start = start; = start;
shmem_falloc.nr_falloced = 0;
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