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Date: Mon, 20 Jan 2020 16:47:12 -0000
From: "tip-bot2 for Xiaochen Shen" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Reinette Chatre <reinette.chatre@...el.com>,
Xiaochen Shen <xiaochen.shen@...el.com>,
Borislav Petkov <bp@...e.de>, Tony Luck <tony.luck@...el.com>,
Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org,
x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/urgent] x86/resctrl: Fix use-after-free due to inaccurate
refcount of rdtgroup
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 074fadee59ee7a9d2b216e9854bd4efb5dad679f
Gitweb: https://git.kernel.org/tip/074fadee59ee7a9d2b216e9854bd4efb5dad679f
Author: Xiaochen Shen <xiaochen.shen@...el.com>
AuthorDate: Thu, 09 Jan 2020 00:28:04 +08:00
Committer: Borislav Petkov <bp@...e.de>
CommitterDate: Mon, 20 Jan 2020 16:56:11 +01:00
x86/resctrl: Fix use-after-free due to inaccurate refcount of rdtgroup
There is a race condition in the following scenario which results in an
use-after-free issue when reading a monitoring file and deleting the
parent ctrl_mon group concurrently:
Thread 1 calls atomic_inc() to take refcount of rdtgrp and then calls
kernfs_break_active_protection() to drop the active reference of kernfs
node in rdtgroup_kn_lock_live().
In Thread 2, kernfs_remove() is a blocking routine. It waits on all sub
kernfs nodes to drop the active reference when removing all subtree
kernfs nodes recursively. Thread 2 could block on kernfs_remove() until
Thread 1 calls kernfs_break_active_protection(). Only after
kernfs_remove() completes the refcount of rdtgrp could be trusted.
Before Thread 1 calls atomic_inc() and kernfs_break_active_protection(),
Thread 2 could call kfree() when the refcount of rdtgrp (sentry) is 0
instead of 1 due to the race.
In Thread 1, in rdtgroup_kn_unlock(), referring to earlier rdtgrp memory
(rdtgrp->waitcount) which was already freed in Thread 2 results in
use-after-free issue.
Thread 1 (rdtgroup_mondata_show) Thread 2 (rdtgroup_rmdir)
-------------------------------- -------------------------
rdtgroup_kn_lock_live
/*
* kn active protection until
* kernfs_break_active_protection(kn)
*/
rdtgrp = kernfs_to_rdtgroup(kn)
rdtgroup_kn_lock_live
atomic_inc(&rdtgrp->waitcount)
mutex_lock
rdtgroup_rmdir_ctrl
free_all_child_rdtgrp
/*
* sentry->waitcount should be 1
* but is 0 now due to the race.
*/
kfree(sentry)*[1]
/*
* Only after kernfs_remove()
* completes, the refcount of
* rdtgrp could be trusted.
*/
atomic_inc(&rdtgrp->waitcount)
/* kn->active-- */
kernfs_break_active_protection(kn)
rdtgroup_ctrl_remove
rdtgrp->flags = RDT_DELETED
/*
* Blocking routine, wait for
* all sub kernfs nodes to drop
* active reference in
* kernfs_break_active_protection.
*/
kernfs_remove(rdtgrp->kn)
rdtgroup_kn_unlock
mutex_unlock
atomic_dec_and_test(
&rdtgrp->waitcount)
&& (flags & RDT_DELETED)
kernfs_unbreak_active_protection(kn)
kfree(rdtgrp)
mutex_lock
mon_event_read
rdtgroup_kn_unlock
mutex_unlock
/*
* Use-after-free: refer to earlier rdtgrp
* memory which was freed in [1].
*/
atomic_dec_and_test(&rdtgrp->waitcount)
&& (flags & RDT_DELETED)
/* kn->active++ */
kernfs_unbreak_active_protection(kn)
kfree(rdtgrp)
Fix it by moving free_all_child_rdtgrp() to after kernfs_remove() in
rdtgroup_rmdir_ctrl() to ensure it has the accurate refcount of rdtgrp.
Fixes: f3cbeacaa06e ("x86/intel_rdt/cqm: Add rmdir support")
Suggested-by: Reinette Chatre <reinette.chatre@...el.com>
Signed-off-by: Xiaochen Shen <xiaochen.shen@...el.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>
Reviewed-by: Tony Luck <tony.luck@...el.com>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/1578500886-21771-3-git-send-email-xiaochen.shen@intel.com
---
arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
index 23904ab..caab397 100644
--- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
+++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
@@ -2960,13 +2960,13 @@ static int rdtgroup_rmdir_ctrl(struct kernfs_node *kn, struct rdtgroup *rdtgrp,
closid_free(rdtgrp->closid);
free_rmid(rdtgrp->mon.rmid);
+ rdtgroup_ctrl_remove(kn, rdtgrp);
+
/*
* Free all the child monitor group rmids.
*/
free_all_child_rdtgrp(rdtgrp);
- rdtgroup_ctrl_remove(kn, rdtgrp);
-
return 0;
}
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