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Commit d2005e3f authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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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.

The patch adds the new trivial helper, mmget_not_zero(), it can have
more users.

Link: http://lkml.kernel.org/r/20160516172254.GA8595@redhat.com


Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent cd33a76b

fs/userfaultfd.c

+28 −13
Original line number Diff line number Diff line
@@ -137,7 +137,7 @@ 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 +434,9 @@ static int userfaultfd_release(struct inode *inode, struct file *file) 


	ACCESS_ONCE(ctx->released) = true;



	if (!mmget_not_zero(mm))

		goto wakeup;



	/*

	 * Flush page faults out of all CPUs. NOTE: all page faults

	 * must be retried without returning VM_FAULT_SIGBUS if
@@ -466,7 +469,8 @@ static int userfaultfd_release(struct inode *inode, struct file *file) 
		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

	}

	up_write(&mm->mmap_sem);



	mmput(mm);

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 +764,12 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, 
	start = uffdio_register.range.start;

	end = start + uffdio_register.range.len;



	ret = -ENOMEM;

	if (!mmget_not_zero(mm))

		goto out;



	down_write(&mm->mmap_sem);

	vma = find_vma_prev(mm, start, &prev);



	ret = -ENOMEM;

	if (!vma)

		goto out_unlock;
@@ -864,6 +870,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx, 
	} while (vma && vma->vm_start < end);

out_unlock:

	up_write(&mm->mmap_sem);

	mmput(mm);

	if (!ret) {

		/*

		 * Now that we scanned all vmas we can already tell
@@ -902,10 +909,12 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, 
	start = uffdio_unregister.start;

	end = start + uffdio_unregister.len;



	ret = -ENOMEM;

	if (!mmget_not_zero(mm))

		goto out;



	down_write(&mm->mmap_sem);

	vma = find_vma_prev(mm, start, &prev);



	ret = -ENOMEM;

	if (!vma)

		goto out_unlock;
@@ -998,6 +1007,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, 
	} while (vma && vma->vm_start < end);

out_unlock:

	up_write(&mm->mmap_sem);

	mmput(mm);

out:

	return ret;

}
@@ -1067,9 +1077,11 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx, 
		goto out;

	if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)

		goto out;



	if (mmget_not_zero(ctx->mm)) {

		ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,

				   uffdio_copy.len);

		mmput(ctx->mm);

	}

	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))

		return -EFAULT;

	if (ret < 0)
@@ -1110,8 +1122,11 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx, 
	if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)

		goto out;



	if (mmget_not_zero(ctx->mm)) {

		ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,

				     uffdio_zeropage.range.len);

		mmput(ctx->mm);

	}

	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))

		return -EFAULT;

	if (ret < 0)
@@ -1289,12 +1304,12 @@ 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:

include/linux/sched.h

+6 −1
Original line number Diff line number Diff line
@@ -2729,6 +2729,11 @@ static inline void mmdrop(struct mm_struct * mm) 
		__mmdrop(mm);

}



static inline bool mmget_not_zero(struct mm_struct *mm)

{

	return atomic_inc_not_zero(&mm->mm_users);

}



/* mmput gets rid of the mappings and all user-space */

extern void mmput(struct mm_struct *);

/* same as above but performs the slow path from the async kontext. Can