6.6-stable review patch. If anyone has any objections, please let me know.
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From: Paul E. McKenney paulmck@kernel.org
[ Upstream commit f44075ecafb726830e63d33fbca29413149eeeb8 ]
The ->idt_seq and ->recv_jiffies variables added by:
1a3ea611fc10 ("x86/nmi: Accumulate NMI-progress evidence in exc_nmi()")
... place the exit-time check of the bottom bit of ->idt_seq after the this_cpu_dec_return() that re-enables NMI nesting. This can result in the following sequence of events on a given CPU in kernels built with CONFIG_NMI_CHECK_CPU=y:
o An NMI arrives, and ->idt_seq is incremented to an odd number. In addition, nmi_state is set to NMI_EXECUTING==1.
o The NMI is processed.
o The this_cpu_dec_return(nmi_state) zeroes nmi_state and returns NMI_EXECUTING==1, thus opting out of the "goto nmi_restart".
o Another NMI arrives and ->idt_seq is incremented to an even number, triggering the warning. But all is just fine, at least assuming we don't get so many closely spaced NMIs that the stack overflows or some such.
Experience on the fleet indicates that the MTBF of this false positive is about 70 years. Or, for those who are not quite that patient, the MTBF appears to be about one per week per 4,000 systems.
Fix this false-positive warning by moving the "nmi_restart" label before the initial ->idt_seq increment/check and moving the this_cpu_dec_return() to follow the final ->idt_seq increment/check. This way, all nested NMIs that get past the NMI_NOT_RUNNING check get a clean ->idt_seq slate. And if they don't get past that check, they will set nmi_state to NMI_LATCHED, which will cause the this_cpu_dec_return(nmi_state) to restart.
Fixes: 1a3ea611fc10 ("x86/nmi: Accumulate NMI-progress evidence in exc_nmi()") Reported-by: Chris Mason clm@fb.com Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Ingo Molnar mingo@kernel.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Andy Lutomirski luto@kernel.org Cc: "H. Peter Anvin" hpa@zytor.com Link: https://lore.kernel.org/r/0cbff831-6e3d-431c-9830-ee65ee7787ff@paulmck-lapto... Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kernel/nmi.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c index a0c551846b35f..4766b6bed4439 100644 --- a/arch/x86/kernel/nmi.c +++ b/arch/x86/kernel/nmi.c @@ -507,12 +507,13 @@ DEFINE_IDTENTRY_RAW(exc_nmi) } this_cpu_write(nmi_state, NMI_EXECUTING); this_cpu_write(nmi_cr2, read_cr2()); + +nmi_restart: if (IS_ENABLED(CONFIG_NMI_CHECK_CPU)) { WRITE_ONCE(nsp->idt_seq, nsp->idt_seq + 1); WARN_ON_ONCE(!(nsp->idt_seq & 0x1)); WRITE_ONCE(nsp->recv_jiffies, jiffies); } -nmi_restart:
/* * Needs to happen before DR7 is accessed, because the hypervisor can @@ -548,16 +549,16 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
if (unlikely(this_cpu_read(nmi_cr2) != read_cr2())) write_cr2(this_cpu_read(nmi_cr2)); - if (this_cpu_dec_return(nmi_state)) - goto nmi_restart; - - if (user_mode(regs)) - mds_user_clear_cpu_buffers(); if (IS_ENABLED(CONFIG_NMI_CHECK_CPU)) { WRITE_ONCE(nsp->idt_seq, nsp->idt_seq + 1); WARN_ON_ONCE(nsp->idt_seq & 0x1); WRITE_ONCE(nsp->recv_jiffies, jiffies); } + if (this_cpu_dec_return(nmi_state)) + goto nmi_restart; + + if (user_mode(regs)) + mds_user_clear_cpu_buffers(); }
#if IS_ENABLED(CONFIG_KVM_INTEL)