[PATCH] firmware: Fix dereferencing NULL pointer of fw_priv->buf->fw_id

[Steven Rostedt <rostedt@goodmis.org>]

Commit e0fd9b1d9c44c0 ("firmware: use const for remaining firmware
names") converted struct firware_buf fw_id from a string within the
structure to a pointer to a constant string.

Unfortunately, this caused the following BUG:

 BUG: unable to handle kernel NULL pointer dereference at 00000000000000c0
 IP: [<ffffffff81823d20>] firmware_uevent+0x2f/0xaa
 PGD d325c067 PUD d3fc8067 PMD 0 
 Oops: 0000 [#1] SMP 
 Modules linked in: ip6t_REJECT nf_reject_ipv6 xt_tcpudp nf_conntrack_ipv6 nf_defrag_ipv6 xt_state ip6table_filter ip6_tables x_tables microcode r8169
 CPU: 0 PID: 1019 Comm: NetworkManager Not tainted 4.1.0-rc4-test+ #48
 Hardware name: MSI MS-7823/CSM-H87M-G43 (MS-7823), BIOS V1.6 02/22/2014
 task: ffff8800d35fc000 ti: ffff8800d363c000 task.ti: ffff8800d363c000
 RIP: 0010:[<ffffffff81823d20>]  [<ffffffff81823d20>] firmware_uevent+0x2f/0xaa
 RSP: 0018:ffff8800d363f648  EFLAGS: 00010202
 RAX: 0000000000000000 RBX: ffff880211d17008 RCX: 0000000000000003
 RDX: 0000000000000000 RSI: ffffffff828614b4 RDI: ffff880211d17008
 RBP: ffff8800d363f668 R08: 00000000fffffffd R09: 0000000000000202
 R10: ffffffffffffffff R11: 000000000000005f R12: 00000000fffffff4
 R13: ffff880212484010 R14: ffff880212484010 R15: ffffffff823b88e0
 FS:  00007f849a3107e0(0000) GS:ffff88021dc00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00000000000000c0 CR3: 00000000d31a4000 CR4: 00000000001406f0
 Stack:
  00000000fffffffd ffff880212484020 0000000000000000 ffff880211d17008
  ffff8800d363f6b8 ffffffff8180ecad ffff8800d363f6b8 ffffffff814f6a9e
  ffffffff82867bfb 0000000000000002 0000000000000000 ffff880211d17008
 Call Trace:
  [<ffffffff8180ecad>] dev_uevent+0x273/0x374
  [<ffffffff814f6a9e>] ? kobject_get_path+0xbe/0x126
  [<ffffffff814f82e4>] kobject_uevent_env+0x3af/0x80b
  [<ffffffff814f8752>] kobject_uevent+0x12/0x1b
  [<ffffffff8180cc36>] device_del+0x29f/0x2e6
  [<ffffffff81825cae>] _request_firmware+0xa2c/0xd03
  [<ffffffff8182602e>] request_firmware+0x15/0x1e
  [<ffffffffa000da96>] rtl_open+0x417/0xaf5 [r8169]
  [<ffffffff81ee0666>] __dev_open+0x110/0x18e
  [<ffffffff81ee03e2>] __dev_change_flags+0xec/0x1cd
  [<ffffffff81ee04ed>] dev_change_flags+0x2a/0x75
  [<ffffffff81ef2050>] do_setlink+0x3c2/0x9a6^M
  [<ffffffff810a106f>] ? mark_lock+0x34/0x363
  [<ffffffff81ef275d>] rtnl_setlink+0x129/0x13e
  [<ffffffff81f2dca4>] ? __netlink_ns_capable+0x4b/0x72
  [<ffffffff81ef4585>] rtnetlink_rcv_msg+0x241/0x257
  [<ffffffff81ef2ef9>] ? rtnl_lock+0x19/0x22
  [<ffffffff81ef4344>] ? __rtnl_unlock+0x20/0x20
  [<ffffffff81f30775>] netlink_rcv_skb+0x72/0xf0
  [<ffffffff81ef2f71>] rtnetlink_rcv+0x2f/0x44^M
  [<ffffffff81f303f8>] netlink_unicast+0x160/0x24f
  [<ffffffff81f30bcc>] netlink_sendmsg+0x3d9/0x42e
  [<ffffffff81ebb5f8>] sock_sendmsg_nosec+0x16/0x36
  [<ffffffff81ebdd33>] ___sys_sendmsg+0x19d/0x229
  [<ffffffff81ebb5f8>] ? sock_sendmsg_nosec+0x16/0x36
  [<ffffffff81ebdee8>] ? SYSC_sendto+0x129/0x146
  [<ffffffff8100adf7>] ? emulate_vsyscall+0x313/0x437
  [<ffffffff811947ea>] ? __fget_light+0x5c/0xbc
  [<ffffffff81ebe856>] __sys_sendmsg+0x52/0x7e
  [<ffffffff81ebe892>] SyS_sendmsg+0x10/0x19
  [<ffffffff82190617>] system_call_fastpath+0x12/0x6f

This was caused by firware_uevent() that dereferenced buf:

 fw_priv->buf->fw_id.

I guess when fw_id was part of buf, if buf was NULL, this pointer too
was NULL. But now that it is a pointer to a string and not just part of
buf, it is a dereference of NULL when buf is NULL causing the above
crash.

Add a check to see if buf is NULL before dereferencing it.

Fixes: e0fd9b1d9c44 ("firmware: use const for remaining firmware names")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---

Now it seems that I have workarounds to test against v4.2-rc1 after
fixing/finding 3 bugs!

diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
index 9c4288362a8e..071f22a630ce 100644
--- a/drivers/base/firmware_class.c
+++ b/drivers/base/firmware_class.c
@@ -567,7 +567,8 @@ static int firmware_uevent(struct device *dev, struct kobj_uevent_env *env)
 {
 	struct firmware_priv *fw_priv = to_firmware_priv(dev);
 
-	if (add_uevent_var(env, "FIRMWARE=%s", fw_priv->buf->fw_id))
+	if (add_uevent_var(env, "FIRMWARE=%s",
+			   fw_priv->buf ? fw_priv->buf->fw_id : NULL))
 		return -ENOMEM;
 	if (add_uevent_var(env, "TIMEOUT=%i", loading_timeout))
 		return -ENOMEM;