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Commit 015c08bc authored by Jann Horn's avatar Jann Horn Committed by Greg Kroah-Hartman
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binder: Prevent repeated use of ->mmap() via NULL mapping 



commit a7a74d7ff55a0c657bc46238b050460b9eacea95 upstream.

binder_alloc_mmap_handler() attempts to detect the use of ->mmap() on a
binder_proc whose binder_alloc has already been initialized by checking
whether alloc->buffer is non-zero.

Before commit 88021166 ("binder: remove kernel vm_area for buffer
space"), alloc->buffer was a kernel mapping address, which is always
non-zero, but since that commit, it is a userspace mapping address.

A sufficiently privileged user can map /dev/binder at NULL, tricking
binder_alloc_mmap_handler() into assuming that the binder_proc has not been
mapped yet. This leads to memory unsafety.
Luckily, no context on Android has such privileges, and on a typical Linux
desktop system, you need to be root to do that.

Fix it by using the mapping size instead of the mapping address to
distinguish the mapped case. A valid VMA can't have size zero.

Fixes: 88021166 ("binder: remove kernel vm_area for buffer space")
Cc: stable@vger.kernel.org
Signed-off-by: default avatarJann Horn <jannh@google.com>
Acked-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191018205631.248274-2-jannh@google.com


Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 6e0efd9e

drivers/android/binder_alloc.c

+6 −5
Original line number Diff line number Diff line
@@ -681,17 +681,17 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc, 
	struct binder_buffer *buffer;



	mutex_lock(&binder_alloc_mmap_lock);

	if (alloc->buffer) {

	if (alloc->buffer_size) {

		ret = -EBUSY;

		failure_string = "already mapped";

		goto err_already_mapped;

	}

	alloc->buffer_size = min_t(unsigned long, vma->vm_end - vma->vm_start,

				   SZ_4M);

	mutex_unlock(&binder_alloc_mmap_lock);



	alloc->buffer = (void __user *)vma->vm_start;

	mutex_unlock(&binder_alloc_mmap_lock);



	alloc->buffer_size = min_t(unsigned long, vma->vm_end - vma->vm_start,

				   SZ_4M);

	alloc->pages = kcalloc(alloc->buffer_size / PAGE_SIZE,

			       sizeof(alloc->pages[0]),

			       GFP_KERNEL);
@@ -722,8 +722,9 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc, 
	kfree(alloc->pages);

	alloc->pages = NULL;

err_alloc_pages_failed:

	mutex_lock(&binder_alloc_mmap_lock);

	alloc->buffer = NULL;

	mutex_lock(&binder_alloc_mmap_lock);

	alloc->buffer_size = 0;

err_already_mapped:

	mutex_unlock(&binder_alloc_mmap_lock);

	binder_alloc_debug(BINDER_DEBUG_USER_ERROR,