CVE request: Linux: ZERO_SIZE_PTR dereference for long symlinks in Be FS

[Timo Warns <warns () pre-sense de>]

The Linux kernel contains a vulnerability in the driver for Be file
systems that may lead to a kernel oops via a corrupted Be file system.

In fs/befs/linuxvfs.c, befs_follow_link() reads a length attribute for
a long symlink from a data stream of a Be file system.

    befs_data_stream *data = &befs_ino->i_data.ds;
    befs_off_t len = data->size;

The data->size / len value is not validated and can be 0 on a corrupted
file system.

befs_follow_link() allocates some memory based on len. Effectively,
kmalloc returns ZERO_SIZE_PTR in this case.

        link = kmalloc(len, GFP_NOFS);

Subsequently, an assignment dereferences ZERO_SIZE_PTR causing a kernel
oops:

                        link[len - 1] = '\0';

A patch is available at
http://git.kernel.org/linus/338d0f0a6fbc82407864606f5b64b75aeb3c70f2