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binder: Prevent repeated use of ->mmap() via NULL mapping

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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 880211667b ("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: 880211667b ("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>
This commit is contained in:
Jann Horn 2019-10-18 22:56:30 +02:00 committed by Greg Kroah-Hartman
parent 8eb52a1ee3
commit a7a74d7ff5
1 changed files with 7 additions and 6 deletions
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13
drivers/android/binder_alloc.c
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@ -680,17 +680,17 @@ 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 = (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);
mutex_unlock(&binder_alloc_mmap_lock);
alloc->buffer = (void __user *)vma->vm_start;
alloc->pages = kcalloc(alloc->buffer_size / PAGE_SIZE,
sizeof(alloc->pages[0]),
GFP_KERNEL);
@ -721,8 +721,9 @@ 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,