** Changed in: linux (Ubuntu Bionic)
Status: In Progress => Fix Committed
--
You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1837810
Title:
KVM: Fix zero_page reference counter overflow when using KSM on KVM
compute host
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Bionic:
Fix Committed
Status in linux source package in Focal:
Fix Committed
Bug description:
BugLink: https://bugs.launchpad.net/bugs/1837810
[Impact]
We are seeing a problem on OpenStack compute nodes, and KVM hosts,
where a kernel oops is generated, and all running KVM machines are
placed into the pause state.
This is caused by the kernel's reserved zero_page reference counter
overflowing from a positive number to a negative number, and hitting a
(WARN_ON_ONCE(page_ref_count(page) <= 0)) condition in try_get_page().
This only happens if the machine has Kernel Samepage Mapping (KSM)
enabled, with "use_zero_pages" turned on. Each time a new VM starts
and the kernel does a KSM merge run during a EPT violation, the
reference counter for the zero_page is incremented in try_async_pf()
and never decremented. Eventually, the reference counter will
overflow, causing the KVM subsystem to fail.
Syslog:
error : qemuMonitorJSONCheckError:392 : internal error: unable to execute
QEMU command 'cont': Resetting the Virtual Machine is required
QEMU Logs:
error: kvm run failed Bad address
EAX=000afe00 EBX=0000000b ECX=00000080 EDX=00000cfe
ESI=0003fe00 EDI=000afe00 EBP=00000007 ESP=00006d74
EIP=000ee344 EFL=00010002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0010 00000000 ffffffff 00c09300 DPL=0 DS [-WA]
CS =0008 00000000 ffffffff 00c09b00 DPL=0 CS32 [-RA]
SS =0010 00000000 ffffffff 00c09300 DPL=0 DS [-WA]
DS =0010 00000000 ffffffff 00c09300 DPL=0 DS [-WA]
FS =0010 00000000 ffffffff 00c09300 DPL=0 DS [-WA]
GS =0010 00000000 ffffffff 00c09300 DPL=0 DS [-WA]
LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
GDT= 000f7040 00000037
IDT= 000f707e 00000000
CR0=00000011 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000
DR3=0000000000000000
DR6=00000000ffff0ff0 DR7=0000000000000400
EFER=0000000000000000
Code=c3 57 56 b8 00 fe 0a 00 be 00 fe 03 00 b9 80 00 00 00 89 c7 <f3> a5 a1
00 80 03 00 8b 15 04 80 03 00 a3 00 80 0a 00 89 15 04 80 0a 00 b8 ae e2 00 00 31
Kernel Oops:
[ 167.695986] WARNING: CPU: 1 PID: 3016 at
/build/linux-hwe-FEhT7y/linux-hwe-4.15.0/include/linux/mm.h:852
follow_page_pte+0x6f4/0x710
[ 167.696023] CPU: 1 PID: 3016 Comm: CPU 0/KVM Tainted: G OE
4.15.0-106-generic #107~16.04.1-Ubuntu
[ 167.696023] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.13.0-1ubuntu1 04/01/2014
[ 167.696025] RIP: 0010:follow_page_pte+0x6f4/0x710
[ 167.696026] RSP: 0018:ffffa81802023908 EFLAGS: 00010286
[ 167.696027] RAX: ffffed8786e33a80 RBX: ffffed878c6d21b0 RCX:
0000000080000000
[ 167.696027] RDX: 0000000000000000 RSI: 00003ffffffff000 RDI:
80000001b8cea225
[ 167.696028] RBP: ffffa81802023970 R08: 80000001b8cea225 R09:
ffff90c4d55fa340
[ 167.696028] R10: 0000000000000000 R11: 0000000000000000 R12:
ffffed8786e33a80
[ 167.696029] R13: 0000000000000326 R14: ffff90c4db94fc50 R15:
ffff90c4d55fa340
[ 167.696030] FS: 00007f6a7798c700(0000) GS:ffff90c4edc80000(0000)
knlGS:0000000000000000
[ 167.696030] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 167.696031] CR2: 0000000000000000 CR3: 0000000315580002 CR4:
0000000000162ee0
[ 167.696033] Call Trace:
[ 167.696047] follow_pmd_mask+0x273/0x630
[ 167.696049] follow_page_mask+0x178/0x230
[ 167.696051] __get_user_pages+0xb8/0x740
[ 167.696052] get_user_pages+0x42/0x50
[ 167.696068] __gfn_to_pfn_memslot+0x18b/0x3b0 [kvm]
[ 167.696079] ? mmu_set_spte+0x1dd/0x3a0 [kvm]
[ 167.696090] try_async_pf+0x66/0x220 [kvm]
[ 167.696101] tdp_page_fault+0x14b/0x2b0 [kvm]
[ 167.696104] ? vmexit_fill_RSB+0x10/0x40 [kvm_intel]
[ 167.696114] kvm_mmu_page_fault+0x62/0x180 [kvm]
[ 167.696117] handle_ept_violation+0xbc/0x160 [kvm_intel]
[ 167.696119] vmx_handle_exit+0xa5/0x580 [kvm_intel]
[ 167.696129] vcpu_enter_guest+0x414/0x1260 [kvm]
[ 167.696138] ? kvm_arch_vcpu_load+0x4d/0x280 [kvm]
[ 167.696148] kvm_arch_vcpu_ioctl_run+0xd9/0x3d0 [kvm]
[ 167.696157] ? kvm_arch_vcpu_ioctl_run+0xd9/0x3d0 [kvm]
[ 167.696165] kvm_vcpu_ioctl+0x33a/0x610 [kvm]
[ 167.696166] ? do_futex+0x129/0x590
[ 167.696171] ? __switch_to+0x34c/0x4e0
[ 167.696174] ? __switch_to_asm+0x35/0x70
[ 167.696176] do_vfs_ioctl+0xa4/0x600
[ 167.696177] SyS_ioctl+0x79/0x90
[ 167.696180] ? exit_to_usermode_loop+0xa5/0xd0
[ 167.696181] do_syscall_64+0x73/0x130
[ 167.696182] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 167.696184] RIP: 0033:0x7f6a80482007
[ 167.696184] RSP: 002b:00007f6a7798b8b8 EFLAGS: 00000246 ORIG_RAX:
0000000000000010
[ 167.696185] RAX: ffffffffffffffda RBX: 000000000000ae80 RCX:
00007f6a80482007
[ 167.696185] RDX: 0000000000000000 RSI: 000000000000ae80 RDI:
0000000000000016
[ 167.696186] RBP: 000055fe135f3240 R08: 000055fe118be530 R09:
0000000000000001
[ 167.696186] R10: 0000000000000000 R11: 0000000000000246 R12:
0000000000000000
[ 167.696187] R13: 00007f6a85852000 R14: 0000000000000000 R15:
000055fe135f3240
[ 167.696188] Code: 4d 63 e6 e9 f2 fc ff ff 4c 89 45 d0 48 8b 47 10 e8 22 f0
9e 00 4c 8b 45 d0 e9 89 fc ff ff 4c 89 e7 e8 81 3f fd ff e9 aa fc ff ff <0f> 0b
49 c7 c4 f4 ff ff ff e9 c1 fc ff ff 0f 1f 40 00 66 2e 0f
[ 167.696200] ---[ end trace 7573f6868ea8f069 ]---
[Fix]
This was fixed in 5.6-rc1 with the following commit:
commit 7df003c85218b5f5b10a7f6418208f31e813f38f
Author: Zhuang Yanying <[email protected]>
Date: Sat Oct 12 11:37:31 2019 +0800
Subject: KVM: fix overflow of zero page refcount with ksm running
Link:
https://github.com/torvalds/linux/commit/7df003c85218b5f5b10a7f6418208f31e813f38f
The fix adds a check to see if the Page Frame Number (pfn) is linked
to the zero page, and if it is, treats it as reserved. This has the
effect that put_page() is no longer called on the zero_page, and
reference counting is no longer needed.
This is a clean cherry pick to Bionic and Focal kernels.
[Testcase]
Create a new KVM host, and make sure it has plenty of ram. 16gb should
be okay.
Install KVM packages:
$ sudo apt install -y qemu-kvm libvirt-bin qemu-utils genisoimage
virtinst
Enable Kernel Samepage Mapping, and use_zero_pages:
$ echo 10000 | sudo tee /sys/kernel/mm/ksm/pages_to_scan
$ echo 1 | sudo tee /sys/kernel/mm/ksm/run
$ echo 1 | sudo tee /sys/kernel/mm/ksm/use_zero_pages
I wrote a script which creates and destroys xenial KVM VMs in a infinite loop:
https://paste.ubuntu.com/p/CvRTsDkdC7/
Save the script to disk, and execute it:
$ chmod +x ksm_refcnt_overflow.sh
$ ./ksm_refcnt_overflow.sh
Each time a VM is created and destroyed the reference counter will
increase.
I wrote a kernel module which exposes a /proc interface, which we can
use to look at the value of the zero_page reference counter. It works
by taking the memory allocated for the zero page: empty_zero_page,
which is defined in arch/x86/include/asm/pgtable.h, running
virt_to_page() to get the page struct, which we can then dereference
to get _refcount;
https://paste.ubuntu.com/p/MJMN8jMVds/
Save the module to disk, create its Makefile from the included
documentation, and build it:
$ make
$ sudo insmod zero_page_refcount.ko
From there, we can examine the reference counter with:
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x687 or 1671
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x846 or 2118
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x9f8 or 2552
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0xcb2 or 3250
We see it steadily increase. Instead of waiting months for it to
overflow, I implemented a /proc entry to set it to near overflow. You
can use it with:
$ cat /proc/zero_page_refcount_set
Zero Page Refcount set to 0x1FFFFFFFFF000
After that, wait a few seconds and the reference counter will
overflow:
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x7fffff16 or 2147483414
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x80000000 or -2147483648
All VMs will become paused:
$ virsh list
Id Name State
----------------------------------------------------
1 instance-0 paused
2 instance-1 paused
QEMU will error out, and the kernel will oops with the messages in the
impact section.
I built a test kernel, which is available here:
https://launchpad.net/~mruffell/+archive/ubuntu/sf290373-test
If you install the test kernel and try reproduce, you will notice the
reference counter is never incremented past 1:
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x1 or 1
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x1 or 1
$ cat /proc/zero_page_refcount
Zero Page Refcount: 0x1 or 1
This resolves the problem.
[Regression Potential]
While the change itself seems simple, it changes how the kernel treats
the zero_page. The zero_page is important, since it is just a page
full of 0's. Each time memory is allocated which is all 0s, the kernel
sets it to use the zero_page to save memory. When an application
writes to the buffer, a EPT violation happens, and the kernel does a
COW to new pages to hold the data.
The change is limited to how the KVM subsystem handles the zero_page.
This will not break the entire kernel if a regression occurs, only
KVM.
If a regression were to occur, users could turn off KSM and disable
KSM use_zero_pages until a fix is ready, as this particular use of
zero_pages is limited to KSM.
The fix landed in upstream 5.6, and has not been backported to stable
kernels.
I have read a bit of the paging code, especially around where the
zero_page is used, and where its reference counters were being
incorrectly incremented.
I think the fix is correct, and I believe it won't cause any
regressions.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1837810/+subscriptions
--
Mailing list: https://launchpad.net/~kernel-packages
Post to : [email protected]
Unsubscribe : https://launchpad.net/~kernel-packages
More help : https://help.launchpad.net/ListHelp
