Commit 78467257 authored Jun 05, 2019 by Todd Kjos Committed by Todd Kjos Jun 11, 2019

BACKPORT: binder: fix race between munmap() and direct reclaim



commit 5cec2d2e5839f9c0fec319c523a911e0a7fd299f upstream.

An munmap() on a binder device causes binder_vma_close() to be called
which clears the alloc->vma pointer.

If direct reclaim causes binder_alloc_free_page() to be called, there
is a race where alloc->vma is read into a local vma pointer and then
used later after the mm->mmap_sem is acquired. This can result in
calling zap_page_range() with an invalid vma which manifests as a
use-after-free in zap_page_range().

The fix is to check alloc->vma after acquiring the mmap_sem (which we
were acquiring anyway) and skip zap_page_range() if it has changed
to NULL.

Bug: 120025196
Change-Id: I2f3284d294326ec7736303374769640a1e028783
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Todd Kjos <tkjos@google.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: stable <stable@vger.kernel.org> # 4.14
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent bd858d73

drivers/android/binder_alloc.c

+9 −11

Original line number	Diff line number	Diff line
		@@ -924,15 +924,14 @@ enum lru_status binder_alloc_free_page(struct list_head *item,

		index = page - alloc->pages;
		page_addr = (uintptr_t)alloc->buffer + index * PAGE_SIZE;
		vma = alloc->vma;
		if (vma) {

		mm = alloc->vma_vm_mm;
		/* Same as mmget_not_zero() in later kernel versions */
		if (!atomic_inc_not_zero(&alloc->vma_vm_mm->mm_users))
		goto err_mmget;
		mm = alloc->vma_vm_mm;
		if (!down_write_trylock(&mm->mmap_sem))
		goto err_down_write_mmap_sem_failed;
		}
		vma = alloc->vma;

		list_lru_isolate(lru, item);
		spin_unlock(lock);
		@@ -946,10 +945,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
		PAGE_SIZE, NULL);

		trace_binder_unmap_user_end(alloc, index);

		}
		up_write(&mm->mmap_sem);
		mmput(mm);
		}

		trace_binder_unmap_kernel_start(alloc, index);