lkml.org 
[lkml]   [2022]   [May]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
Subject[PATCH v2 0/3] x86/crash: Fix double NMI shootdown bug
From
Fix a double NMI shootdown bug found and debugged by Guilherme, who did all
the hard work. NMI shootdown is a one-time thing; the handler leaves NMIs
blocked and enters halt. At best, a second (or third...) shootdown is an
expensive nop, at worst it can hang the kernel and prevent kexec'ing into
a new kernel, e.g. prior to the hardening of register_nmi_handler(), a
double shootdown resulted in a double list_add(), which is fatal when running
with CONFIG_BUG_ON_DATA_CORRUPTION=y.

With the "right" kexec/kdump configuration, emergency_vmx_disable_all() can
be reached after kdump_nmi_shootdown_cpus() (currently the only two users
of nmi_shootdown_cpus()).

To fix, move the disabling of virtualization into crash_nmi_callback(),
remove emergency_vmx_disable_all()'s callback, and do a shootdown for
emergency_vmx_disable_all() if and only if a shootdown hasn't yet occurred.
The only thing emergency_vmx_disable_all() cares about is disabling VMX/SVM
(obviously), and since I can't envision a use case for an NMI shootdown that
doesn't want to disable virtualization, doing that in the core handler means
emergency_vmx_disable_all() only needs to ensure _a_ shootdown occurs, it
doesn't care when that shootdown happened or what callback may have run.

Patch 2 is a related bug fix found while exploring ideas for patch 1.
Patch 3 is a cleanup to try to prevent future "fixed VMX but not SVM"
style bugs.

Guilherme and Vitaly, I dropped your Tested-by and Reviewed-by tags
since the relevant patches changed a decent amount.

v2:
- Use a NULL handler and crash_ipi_issued instead of a magic nop
handler. [tglx]
- Add comments to call out that modifying the existing handler
once the NMI is sent may cause explosions.
- Add a patch to cleanup cpu_emergency_vmxoff().

v1: https://lore.kernel.org/all/20220511234332.3654455-1-seanjc@google.com

Sean Christopherson (3):
x86/crash: Disable virt in core NMI crash handler to avoid double
shootdown
x86/reboot: Disable virtualization in an emergency if SVM is supported
x86/virt: Fold __cpu_emergency_vmxoff() into its sole caller

arch/x86/include/asm/reboot.h | 1 +
arch/x86/include/asm/virtext.h | 14 +-----
arch/x86/kernel/crash.c | 16 +-----
arch/x86/kernel/reboot.c | 89 +++++++++++++++++++++++++---------
4 files changed, 69 insertions(+), 51 deletions(-)


base-commit: a7fed5c0431dbfa707037848830f980e0f93cfb3
--
2.36.0.550.gb090851708-goog

\
 
 \ /
  Last update: 2022-05-18 02:17    [W:0.116 / U:0.056 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site