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Tue, 29 Sep 2026 00:16:09 -0700 (PDT) From: "Mukesh Kumar Chaurasiya (IBM)" To: maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, ritesh.list@gmail.com, sshegde@linux.ibm.com, mkchauras@linux.ibm.com, mkchauras@gmail.com, ruanjinjie@huawei.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Cc: Venkat Rao Bagalkote Subject: [PATCH] powerpc: fix NMI exit ordering: restore soft masks after irqentry_nmi_exit() Date: Tue, 29 Sep 2026 12:46:00 +0530 Message-ID: <20260929071600.2963954-1-mkchauras@gmail.com> X-Mailer: git-send-email 2.55.0 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When running with CONFIG_PPC_IRQ_SOFT_MASK_DEBUG=y and LOCKDEP=y, an Oops is triggered during NMI exit: [ 202.258332] Oops: Exception in kernel mode, sig: 4 [#1] NIP [000000000003e178] 0x3e178 LR [c00000000003e174] arch_local_irq_restore+0x2a4/0x328 TRAP: 0700 MSR: 0000000000081000 The faulting instruction at NIP is a 'trap' (0fe00000) inside the rfi_flush_fallback trampoline at a userspace-looking low address, reached after a bad rfid caused by an incorrect SRR0 value. Root cause ---------- DEFINE_INTERRUPT_HANDLER_NMI used to call arch_interrupt_nmi_exit_prepare() *before* irqentry_nmi_exit(), giving this sequence: arch_interrupt_nmi_enter_prepare() // saves irq_soft_mask, irq_happened irqentry_nmi_enter() // __nmi_enter() -> nmi_count++, in_nmi()=true ____func(regs) // NMI handler body arch_interrupt_nmi_exit_prepare() // RESTORES irq_soft_mask=0 (IRQS_ENABLED) // RESTORES irq_happened with pending bits irqentry_nmi_exit() // instrumentation block runs here... trace_hardirqs_on_prepare() // ...with in_nmi()==true still lockdep_hardirqs_on_prepare() // ...and irq_soft_mask already restored to 0 -> arch_local_irq_restore(0) // mask=0 -> enable path entered WARN_ON_ONCE(in_nmi()) // FIRES: __nmi_exit() hasn't run yet __nmi_exit() // nmi_count-- (too late) arch_interrupt_nmi_exit_prepare() restores irq_soft_mask to whatever the interrupted thread had (IRQS_ENABLED / 0) and restores irq_happened with any pending deferred-interrupt bits that were present when the NMI fired. This is correct and necessary before rfid back to the interrupted context, but it must not happen until irqentry_nmi_exit() has fully completed. With irq_soft_mask=0 and pending bits in irq_happened, the trace_hardirqs_on_prepare() call inside irqentry_nmi_exit()'s instrumentation block reaches lockdep_hardirqs_on_prepare() which calls __trace_hardirqs_on_caller(), which eventually calls arch_local_irq_restore(0) (the IRQS_ENABLED / enable path). arch_local_irq_restore() with mask=0 hits the CONFIG_PPC_IRQ_SOFT_MASK_DEBUG guards at irq_64.c:217: WARN_ON_ONCE(in_nmi()); // TRUE: __nmi_exit() not yet called The WARN_ON_ONCE fires a 'trap' instruction (tw 31,0,0 / 0fe00000) in kernel mode with MSR_PR=0, MSR_IR=0 (real-mode). The program check handler (TRAP 0700) takes the sig:4 (SIGILL) path via do_program_check() and calls die(), producing the Oops. The RESTART_TABLE mechanism in arch_local_irq_restore then causes the bad rfid that lands at the userspace-looking address visible in the NIP. The bug is invisible with LOCKDEP=n because trace_hardirqs_on_prepare() is a no-op when CONFIG_TRACE_IRQFLAGS is not set (TRACE_IRQFLAGS is selected by PROVE_LOCKING which selects LOCKDEP). Fix --- Move arch_interrupt_nmi_exit_prepare() to after the irqentry_nmi_exit() block, so that: irqentry_nmi_exit() // __nmi_exit() -> nmi_count--, in_nmi()=false arch_interrupt_nmi_exit_prepare() // now safe to restore soft mask state The instrumentation inside irqentry_nmi_exit() now executes while in_nmi() is still true (nmi_count not yet decremented), but irq_soft_mask is still IRQS_ALL_DISABLED (NMI-entry value), so arch_local_irq_restore() is never called with mask=0 from that path. After __nmi_exit() returns, arch_interrupt_nmi_exit_prepare() restores the pre-NMI soft mask and irq_happened, which is the correct state for the rfid back to the interrupted context. Reported-by: Venkat Rao Bagalkote Closes: https://lore.kernel.org/all/d4cccba9-ce5c-424b-864c-2248fb0d99e6@linux.ibm.com/ Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature") Signed-off-by: Mukesh Kumar Chaurasiya (IBM) --- arch/powerpc/include/asm/interrupt.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h index 1b45a49e9bed..56f855c6f07e 100644 --- a/arch/powerpc/include/asm/interrupt.h +++ b/arch/powerpc/include/asm/interrupt.h @@ -301,7 +301,6 @@ interrupt_handler long func(struct pt_regs *regs) \ state = irqentry_nmi_enter(regs); \ } \ ret = ____##func (regs); \ - arch_interrupt_nmi_exit_prepare(regs, &nmi_state); \ if (mfmsr() & MSR_DR) { \ /* nmi_exit if relocations are on */ \ irqentry_nmi_exit(regs, state); \ @@ -317,6 +316,7 @@ interrupt_handler long func(struct pt_regs *regs) \ } else { \ irqentry_nmi_exit(regs, state); \ } \ + arch_interrupt_nmi_exit_prepare(regs, &nmi_state); \ \ return ret; \ } \ -- 2.55.0