Re: [PATCH 1/2] binder: Don't modify VMA bounds in ->mmap handler

From: "Christian Brauner" <christian.brauner@ubuntu.com>
To: "Jann Horn" <jannh@google.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Arve Hjønnevåg" <arve@android.com>,
	"Todd Kjos" <tkjos@android.com>,
	"Martijn Coenen" <maco@android.com>,
	"Joel Fernandes" <joel@joelfernandes.org>,
	"Christian Brauner" <christian@brauner.io>,
	jannh@google.com
Cc: <devel@driverdev.osuosl.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2] binder: Don't modify VMA bounds in ->mmap handler
Date: Wed, 16 Oct 2019 17:46:31 +0200	[thread overview]
Message-ID: <BXR2DIW8IZSX.16Y0Y9PLOTGTS@wittgenstein> (raw)
In-Reply-To: <20191016150119.154756-1-jannh@google.com>

On Wed Oct 16, 2019 at 5:01 PM Jann Horn wrote:
> binder_mmap() tries to prevent the creation of overly big binder mappings
> by silently truncating the size of the VMA to 4MiB. However, this violates
> the API contract of mmap(). If userspace attempts to create a large binder
> VMA, and later attempts to unmap that VMA, it will call munmap() on a range
> beyond the end of the VMA, which may have been allocated to another VMA in
> the meantime. This can lead to userspace memory corruption.
> 
> The following sequence of calls leads to a segfault without this commit:
> 
> int main(void) {
>   int binder_fd = open("/dev/binder", O_RDWR);
>   if (binder_fd == -1) err(1, "open binder");
>   void *binder_mapping = mmap(NULL, 0x800000UL, PROT_READ, MAP_SHARED,
>                               binder_fd, 0);
>   if (binder_mapping == MAP_FAILED) err(1, "mmap binder");
>   void *data_mapping = mmap(NULL, 0x400000UL, PROT_READ|PROT_WRITE,
>                             MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
>   if (data_mapping == MAP_FAILED) err(1, "mmap data");
>   munmap(binder_mapping, 0x800000UL);
>   *(char*)data_mapping = 1;
>   return 0;
> }
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>

Hm, aerc kept crashing for me so I'm not sure whether or not prior
messages made it so sorry if this arrives multiple times.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>