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

[Christian Brauner]

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@xxxxxxxxxxxxxxx
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>

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@xxxxxxxxxx>