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Subject[PATCH 5.4 91/92] binder: Prevent repeated use of ->mmap() via NULL mapping
Date
From: Jann Horn <jannh@google.com>

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 880211667b20 ("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: 880211667b20 ("binder: remove kernel vm_area for buffer space")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191018205631.248274-2-jannh@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/android/binder_alloc.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -681,17 +681,17 @@ int binder_alloc_mmap_handler(struct bin
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 @@ err_alloc_buf_struct_failed:
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,

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