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>
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