"bad pmd" errors + oops with KPTI on 4.14.11 after loading X.509 certs

Benjamin Gilbert
2018-01-03T08:36:27Z

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Hi all,

In our regression tests on kernel 4.14.11, we're occasionally seeing a run
of "bad pmd" messages during boot, followed by a "BUG: unable to handle
kernel paging request". This happens on no more than a couple percent of
boots, but we've seen it on AWS HVM, GCE, Oracle Cloud VMs, and local QEMU
instances. It always happens immediately after "Loading compiled-in X.509
certificates". I can't reproduce it on 4.14.10, nor, so far, on 4.14.11
with pti=off. Here's a sample backtrace:

[ 4.762964] Loading compiled-in X.509 certificates
[ 4.765620] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee000(800000007d6000e3)
[ 4.769099] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee008(800000007d8000e3)
[ 4.772479] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee010(800000007da000e3)
[ 4.775919] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee018(800000007dc000e3)
[ 4.779251] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee020(800000007de000e3)
[ 4.782558] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee028(800000007e0000e3)
[ 4.794160] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee030(800000007e2000e3)
[ 4.797525] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee038(800000007e4000e3)
[ 4.800776] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee040(800000007e6000e3)
[ 4.804100] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee048(800000007e8000e3)
[ 4.807437] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee050(800000007ea000e3)
[ 4.810729] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee058(800000007ec000e3)
[ 4.813989] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee060(800000007ee000e3)
[ 4.817294] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee068(800000007f0000e3)
[ 4.820713] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee070(800000007f2000e3)
[ 4.823943] ../source/mm/pgtable-generic.c:40: bad pmd
ffff8b39bf7ee078(800000007f4000e3)
[ 4.827311] BUG: unable to handle kernel paging request at
fffffe27c1fdfba0
[ 4.830109] IP: free_page_and_swap_cache+0x6/0xa0
[ 4.831999] PGD 7f7ef067 P4D 7f7ef067 PUD 0
[ 4.833779] Oops: 0000 [#1] SMP PTI
[ 4.835197] Modules linked in:
[ 4.836450] CPU: 0 PID: 45 Comm: modprobe Not tainted 4.14.11-coreos #1
[ 4.839009] Hardware name: Xen HVM domU, BIOS 4.2.amazon 08/24/2006
[ 4.841551] task: ffff8b39b5a71e40 task.stack: ffffb92580558000
[ 4.844062] RIP: 0010:free_page_and_swap_cache+0x6/0xa0
[ 4.846238] RSP: 0018:ffffb9258055bc98 EFLAGS: 00010297
[ 4.848300] RAX: 0000000000000000 RBX: fffffe27c0001000 RCX:
ffff8b39bf7ef4f8
[ 4.851184] RDX: 000000000007f7ee RSI: fffffe27c1fdfb80 RDI:
fffffe27c1fdfb80
[ 4.854090] RBP: ffff8b39bf7ee000 R08: 0000000000000000 R09:
0000000000000162
[ 4.856946] R10: ffffffffffffff90 R11: 0000000000000161 R12:
fffffe27ffe00000
[ 4.859777] R13: ffff8b39bf7ef000 R14: fffffe2800000000 R15:
ffffb9258055bd60
[ 4.862602] FS: 0000000000000000(0000) GS:ffff8b39bd200000(0000)
knlGS:0000000000000000
[ 4.865860] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 4.868175] CR2: fffffe27c1fdfba0 CR3: 000000002d00a001 CR4:
00000000001606f0
[ 4.871162] Call Trace:
[ 4.872188] free_pgd_range+0x3a5/0x5b0
[ 4.873781] free_ldt_pgtables.part.2+0x60/0xa0
[ 4.875679] ? arch_tlb_finish_mmu+0x42/0x70
[ 4.877476] ? tlb_finish_mmu+0x1f/0x30
[ 4.878999] exit_mmap+0x5b/0x1a0
[ 4.880327] ? dput+0xb8/0x1e0
[ 4.881575] ? hrtimer_try_to_cancel+0x25/0x110
[ 4.883388] mmput+0x52/0x110
[ 4.884620] do_exit+0x330/0xb10
[ 4.886044] ? task_work_run+0x6b/0xa0
[ 4.887544] do_group_exit+0x3c/0xa0
[ 4.889012] SyS_exit_group+0x10/0x10
[ 4.890473] entry_SYSCALL_64_fastpath+0x1a/0x7d
[ 4.892364] RIP: 0033:0x7f4a41d4ded9
[ 4.893812] RSP: 002b:00007ffe25d85708 EFLAGS: 00000246 ORIG_RAX:
00000000000000e7
[ 4.896974] RAX: ffffffffffffffda RBX: 00005601b3c9e2e0 RCX:
00007f4a41d4ded9
[ 4.899830] RDX: 0000000000000000 RSI: 0000000000000001 RDI:
0000000000000001
[ 4.902647] RBP: 00005601b3c9d0e8 R08: 000000000000003c R09:
00000000000000e7
[ 4.905743] R10: ffffffffffffff90 R11: 0000000000000246 R12:
00005601b3c9d090
[ 4.908659] R13: 0000000000000004 R14: 0000000000000001 R15:
00007ffe25d85828
[ 4.911495] Code: e0 01 48 83 f8 01 19 c0 25 01 fe ff ff 05 00 02 00 00
3e 29 43 1c 5b 5d 41 5c c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53
<48> 8b 57 20 48 89 fb 48 8d 42 ff 83 e2 01 48 0f 44 c7 48 8b 48
[ 4.919014] RIP: free_page_and_swap_cache+0x6/0xa0 RSP: ffffb9258055bc98
[ 4.921801] CR2: fffffe27c1fdfba0
[ 4.923232] ---[ end trace e79ccb938bf80a4e ]---
[ 4.925166] Kernel panic - not syncing: Fatal exception
[ 4.927390] Kernel Offset: 0x1c000000 from 0xffffffff81000000
(relocation range: 0xffffffff80000000-0xffffffffbfffffff)

Traces were obtained via virtual serial port. The backtrace varies a bit,
as does the comm.

The kernel config and a collection of backtraces are attached. Our diff on
top of vanilla 4.14.11 (unchanged from 4.14.10, and containing nothing
especially relevant):

https://github.com/coreos/linux/compare/v4.14.11...coreos:v4.14.11-coreos

I'm happy to try test builds, etc. For ease of reproduction if needed, an
affected OS image:

https://storage.googleapis.com/builds.developer.core-os.net/boards/amd64-usr/1632.0.0%2Bjenkins2-master%2Blocal-999/coreos_production_qemu_image.img.bz2

and a wrapper script to start it with QEMU:

https://storage.googleapis.com/builds.developer.core-os.net/boards/amd64-usr/1632.0.0%2Bjenkins2-master%2Blocal-999/coreos_production_qemu.sh

Get in with "ssh -p 2222 core@localhost". Corresponding debug symbols:

https://storage.googleapis.com/builds.developer.core-os.net/boards/amd64-usr/1632.0.0%2Bjenkins2-master%2Blocal-999/pkgs/sys-kernel/coreos-kernel-4.14.11.tbz2
https://storage.googleapis.com/builds.developer.core-os.net/boards/amd64-usr/1632.0.0%2Bjenkins2-master%2Blocal-999/pkgs/sys-kernel/coreos-modules-4.14.11.tbz2

--Benjamin Gilbert

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