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Message-ID: <20130313083554.GA20033@gremlin.ru>
Date: Wed, 13 Mar 2013 12:35:54 +0400
From: gremlin@...mlin.ru
To: oss-security@...ts.openwall.com
Subject: Linux kernel + devtmpfs automount == insecure /dev/{,u}random mode
linux/drivers/char/mem.c contains the following code:
static const struct memdev {
const char *name;
umode_t mode;
const struct file_operations *fops;
struct backing_dev_info *dev_info;
} devlist[] = {
// ...
[8] = { "random", 0666, &random_fops, NULL },
[9] = { "urandom", 0666, &urandom_fops, NULL },
// ...
};
This allows writing to these devices by an unprivileged user
resulting in re-initializing the entropy pool (as described
in `man 4 random`) and thus making the data predictable.
Just boot the kernel with "init=/bin/sh" parameter and issue
the `ls -l /dev/*random` command - you'll see something like:
crw-rw-rw- 1 root root 1, 8 Mar 13 08:30 /dev/random
crw-rw-rw- 1 root root 1, 9 Mar 13 08:30 /dev/urandom
The obvious fix is to create these devices with mode 0644,
so only root will be able to re-initialize the entropy pool.
Possibly, this even deserves a CVE to be assigned...
--
Alexey V. Vissarionov aka Gremlin from Kremlin <gremlin ПРИ gremlin ТЧК ru>
GPG key ID: 0xEF3B1FA8, keyserver: hkp://subkeys.pgp.net
GPG key fingerprint: 8832 FE9F A791 F796 8AC9 6E4E 909D AC45 EF3B 1FA8
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