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

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Message-Id: <BXR2DIW8IZSX.16Y0Y9PLOTGTS@wittgenstein>
Date:   Wed, 16 Oct 2019 17:46:31 +0200
From:   "Christian Brauner" <christian.brauner@...ntu.com>
To:     "Jann Horn" <jannh@...gle.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        "Todd Kjos" <tkjos@...roid.com>,
        "Martijn Coenen" <maco@...roid.com>,
        "Joel Fernandes" <joel@...lfernandes.org>,
        "Christian Brauner" <christian@...uner.io>, <jannh@...gle.com>
Cc:     <devel@...verdev.osuosl.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] binder: Don't modify VMA bounds in ->mmap handler

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@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.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@...ntu.com>