[Xen-devel] Dom0 Failing to Boot with Recent Linux Kernels (Spectre Mitigations)

[Brian Woods <brian.woods@xxxxxxx>]

I'm currently seeing an issue where when booting from a recent Linux
kernel without nospec_store_bypass_disable.  There's a NULL pointer
having to do with a lock.  I put some printks in and it seems that in
arch/x86/kernel/process.c
that speculative_store_bypass_ht_init isn't getting called which
initializes the spin lock.  Here's the serial output:

[    7.748191] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000008
[    7.748202] PGD 0 P4D 0
[    7.748208] Oops: 0002 [#1] SMP NOPTI
[    7.748212] Modules linked in: ip_tables x_tables autofs4 ext4 crc16 mbcache 
jbd2 fscrypto raid10 raid456 async_raid6_recov async_memcpy async_pq async_xore
[    7.748261] CPU: 4 PID: 321 Comm: (journald) Not tainted 4.17.2+ #1
[    7.748266] Hardware name: AMD Corporation Diesel Debug/Diesel Debug, BIOS 
TDD1007E 04/16/2018
[    7.748277] RIP: e030:_raw_spin_lock+0xc/0x20
[    7.748293] RSP: e02b:ffffc9004709feb8 EFLAGS: 00010046
[    7.748297] RAX: 0000000000000000 RBX: ffff880285715b30 RCX: ffffea0009ce30df
[    7.748302] RDX: 0000000000000001 RSI: 0000000000000008 RDI: 0000000000000008
[    7.748308] RBP: 0000000000000400 R08: aaaaaaaaaaaaaaaa R09: 0000000000000007
[    7.748313] R10: 0000000000000040 R11: ffff88027a9bc800 R12: 0206800000000000
[    7.748318] R13: 0000000000000000 R14: ffff880273aa0080 R15: 0000000000000000
[    7.748331] FS:  00007fc72e830940(0000) GS:ffff880285700000(0000) 
knlGS:0000000000000000
[    7.748336] CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[    7.748341] CR2: 0000000000000008 CR3: 0000000278c6c000 CR4: 0000000000040660
[    7.748347] Call Trace:
[    7.748354]  speculative_store_bypass_update+0x72/0x160
[    7.748361]  ssb_prctl_set+0x67/0xb0
[    7.748367]  do_seccomp+0x477/0x6c0
[    7.748385]  do_syscall_64+0x55/0x100
[    7.748390]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[    7.748395] RIP: 0033:0x7fc72ce13229
[    7.748399] RSP: 002b:00007ffda5166968 EFLAGS: 00000246 ORIG_RAX: 
000000000000013d
[    7.748405] RAX: ffffffffffffffda RBX: 000055745ea1dfe0 RCX: 00007fc72ce13229
[    7.748410] RDX: 000055745ea1dfe0 RSI: 0000000000000000 RDI: 0000000000000001
[    7.748415] RBP: 000055745ea5b740 R08: 000055745ea1dfe0 R09: 000000004000003e
[    7.748421] R10: 000000000000000d R11: 0000000000000246 R12: 00007ffda51669c0
[    7.748426] R13: 00007ffda51669b8 R14: 00007fc72e560c14 R15: 000000000000002a
[    7.748431] Code: ff 01 00 00 75 05 48 89 d8 5b c3 e8 1f 8f 9f ff 48 89 d8 
5b c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 31 c0 ba 01 00 00 00 <f0> 0f
[    7.748483] RIP: _raw_spin_lock+0xc/0x20 RSP: ffffc9004709feb8
[    7.748487] CR2: 0000000000000008
[    7.748492] ---[ end trace cf886bf535fde244 ]---


With nospec_store_bypass_disable, it boots fine etc.  It seems to works
fine (at least Dom0 can boot).

Linux Kernel -> 4.17.2
Xen -> current HEAD on master

Is this a known or expected problem?

-- 
Brian Woods

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