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Message-ID: <20160516152546.GA19129@redhat.com>
Date: Mon, 16 May 2016 17:25:46 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 1/1] userfaultfd: don't pin the user memory in
userfaultfd_file_create()
userfaultfd_file_create() increments mm->mm_users; this means that the memory
won't be unmapped/freed if mm owner exits/execs, and UFFDIO_COPY after that can
populate the orphaned mm more.
Change userfaultfd_file_create() and userfaultfd_ctx_put() to use mm->mm_count
to pin mm_struct. This means that atomic_inc_not_zero(mm->mm_users) is needed
when we are going to actually play with this memory. Except handle_userfault()
path doesn't need this, the caller must already have a reference.
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
fs/userfaultfd.c | 55 ++++++++++++++++++++++++++++++++++++++++++-------------
1 file changed, 42 insertions(+), 13 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 66cdb44..1a2f38a 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -70,6 +70,20 @@ struct userfaultfd_wake_range {
unsigned long len;
};
+/*
+ * mm_struct can't go away, but we need to verify that this memory is still
+ * alive and avoid the race with exit_mmap().
+ */
+static inline bool userfaultfd_get_mm(struct userfaultfd_ctx *ctx)
+{
+ return atomic_inc_not_zero(&ctx->mm->mm_users);
+}
+
+static inline void userfaultfd_put_mm(struct userfaultfd_ctx *ctx)
+{
+ mmput(ctx->mm);
+}
+
static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
int wake_flags, void *key)
{
@@ -137,7 +151,7 @@ static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
- mmput(ctx->mm);
+ mmdrop(ctx->mm);
kmem_cache_free(userfaultfd_ctx_cachep, ctx);
}
}
@@ -434,6 +448,9 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
ACCESS_ONCE(ctx->released) = true;
+ if (!userfaultfd_get_mm(ctx))
+ goto wakeup;
+
/*
* Flush page faults out of all CPUs. NOTE: all page faults
* must be retried without returning VM_FAULT_SIGBUS if
@@ -466,7 +483,8 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
}
up_write(&mm->mmap_sem);
-
+ userfaultfd_put_mm(ctx);
+wakeup:
/*
* After no new page faults can wait on this fault_*wqh, flush
* the last page faults that may have been already waiting on
@@ -760,10 +778,12 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
start = uffdio_register.range.start;
end = start + uffdio_register.range.len;
+ ret = -ENOMEM;
+ if (!userfaultfd_get_mm(ctx))
+ goto out;
+
down_write(&mm->mmap_sem);
vma = find_vma_prev(mm, start, &prev);
-
- ret = -ENOMEM;
if (!vma)
goto out_unlock;
@@ -864,6 +884,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
} while (vma && vma->vm_start < end);
out_unlock:
up_write(&mm->mmap_sem);
+ userfaultfd_put_mm(ctx);
if (!ret) {
/*
* Now that we scanned all vmas we can already tell
@@ -902,10 +923,12 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
start = uffdio_unregister.start;
end = start + uffdio_unregister.len;
+ ret = -ENOMEM;
+ if (!userfaultfd_get_mm(ctx))
+ goto out;
+
down_write(&mm->mmap_sem);
vma = find_vma_prev(mm, start, &prev);
-
- ret = -ENOMEM;
if (!vma)
goto out_unlock;
@@ -998,6 +1021,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
} while (vma && vma->vm_start < end);
out_unlock:
up_write(&mm->mmap_sem);
+ userfaultfd_put_mm(ctx);
out:
return ret;
}
@@ -1067,9 +1091,11 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
goto out;
if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
goto out;
-
- ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
- uffdio_copy.len);
+ if (userfaultfd_get_mm(ctx)) {
+ ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
+ uffdio_copy.len);
+ userfaultfd_put_mm(ctx);
+ }
if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
return -EFAULT;
if (ret < 0)
@@ -1110,8 +1136,11 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
goto out;
- ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
- uffdio_zeropage.range.len);
+ if (userfaultfd_get_mm(ctx)) {
+ ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
+ uffdio_zeropage.range.len);
+ userfaultfd_put_mm(ctx);
+ }
if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
return -EFAULT;
if (ret < 0)
@@ -1289,12 +1318,12 @@ static struct file *userfaultfd_file_create(int flags)
ctx->released = false;
ctx->mm = current->mm;
/* prevent the mm struct to be freed */
- atomic_inc(&ctx->mm->mm_users);
+ atomic_inc(&ctx->mm->mm_count);
file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
if (IS_ERR(file)) {
- mmput(ctx->mm);
+ mmdrop(ctx->mm);
kmem_cache_free(userfaultfd_ctx_cachep, ctx);
}
out:
--
2.5.0
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