Patch "klist: fix starting point removed bug in klist iterators" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    klist: fix starting point removed bug in klist iterators

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     klist-fix-starting-point-removed-bug-in-klist-iterators.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 00cd29b799e3449f0c68b1cc77cd4a5f95b42d17 Mon Sep 17 00:00:00 2001
From: James Bottomley <James.Bottomley@HansenPartnership.com>
Date: Wed, 13 Jan 2016 08:10:31 -0800
Subject: klist: fix starting point removed bug in klist iterators

From: James Bottomley <James.Bottomley@HansenPartnership.com>

commit 00cd29b799e3449f0c68b1cc77cd4a5f95b42d17 upstream.

The starting node for a klist iteration is often passed in from
somewhere way above the klist infrastructure, meaning there's no
guarantee the node is still on the list.  We've seen this in SCSI where
we use bus_find_device() to iterate through a list of devices.  In the
face of heavy hotplug activity, the last device returned by
bus_find_device() can be removed before the next call.  This leads to

Dec  3 13:22:02 localhost kernel: WARNING: CPU: 2 PID: 28073 at include/linux/kref.h:47 klist_iter_init_node+0x3d/0x50()
Dec  3 13:22:02 localhost kernel: Modules linked in: scsi_debug x86_pkg_temp_thermal kvm_intel kvm irqbypass crc32c_intel joydev iTCO_wdt dcdbas ipmi_devintf acpi_power_meter iTCO_vendor_support ipmi_si imsghandler pcspkr wmi acpi_cpufreq tpm_tis tpm shpchp lpc_ich mfd_core nfsd nfs_acl lockd grace sunrpc tg3 ptp pps_core
Dec  3 13:22:02 localhost kernel: CPU: 2 PID: 28073 Comm: cat Not tainted 4.4.0-rc1+ #2
Dec  3 13:22:02 localhost kernel: Hardware name: Dell Inc. PowerEdge R320/08VT7V, BIOS 2.0.22 11/19/2013
Dec  3 13:22:02 localhost kernel: ffffffff81a20e77 ffff880613acfd18 ffffffff81321eef 0000000000000000
Dec  3 13:22:02 localhost kernel: ffff880613acfd50 ffffffff8107ca52 ffff88061176b198 0000000000000000
Dec  3 13:22:02 localhost kernel: ffffffff814542b0 ffff880610cfb100 ffff88061176b198 ffff880613acfd60
Dec  3 13:22:02 localhost kernel: Call Trace:
Dec  3 13:22:02 localhost kernel: [<ffffffff81321eef>] dump_stack+0x44/0x55
Dec  3 13:22:02 localhost kernel: [<ffffffff8107ca52>] warn_slowpath_common+0x82/0xc0
Dec  3 13:22:02 localhost kernel: [<ffffffff814542b0>] ? proc_scsi_show+0x20/0x20
Dec  3 13:22:02 localhost kernel: [<ffffffff8107cb4a>] warn_slowpath_null+0x1a/0x20
Dec  3 13:22:02 localhost kernel: [<ffffffff8167225d>] klist_iter_init_node+0x3d/0x50
Dec  3 13:22:02 localhost kernel: [<ffffffff81421d41>] bus_find_device+0x51/0xb0
Dec  3 13:22:02 localhost kernel: [<ffffffff814545ad>] scsi_seq_next+0x2d/0x40
[...]

And an eventual crash. It can actually occur in any hotplug system
which has a device finder and a starting device.

We can fix this globally by making sure the starting node for
klist_iter_init_node() is actually a member of the list before using it
(and by starting from the beginning if it isn't).

Reported-by: Ewan D. Milne <emilne@redhat.com>
Tested-by: Ewan D. Milne <emilne@redhat.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 lib/klist.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/lib/klist.c
+++ b/lib/klist.c
@@ -282,9 +282,9 @@ void klist_iter_init_node(struct klist *
 			  struct klist_node *n)
 {
 	i->i_klist = k;
-	i->i_cur = n;
-	if (n)
-		kref_get(&n->n_ref);
+	i->i_cur = NULL;
+	if (n && kref_get_unless_zero(&n->n_ref))
+		i->i_cur = n;
 }
 EXPORT_SYMBOL_GPL(klist_iter_init_node);
 


Patches currently in stable-queue which might be from James.Bottomley@HansenPartnership.com are

queue-4.4/scsi-fix-soft-lockup-in-scsi_remove_target-on-module-removal.patch
queue-4.4/drivers-scsi-sg.c-mark-vma-as-vm_io-to-prevent-migration.patch
queue-4.4/klist-fix-starting-point-removed-bug-in-klist-iterators.patch
queue-4.4/scsi-fix-crashes-in-sd-and-sr-runtime-pm.patch