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Tue, 29 Sep 2026 08:50:16 +0000 (GMT) Received: from smtpav03.wdc07v.mail.ibm.com (smtpav03.wdc07v.mail.ibm.com [10.39.53.230]) by smtprelay05.wdc07v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 68T8oFlm16581172 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 29 Sep 2026 08:50:15 GMT Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3444F5805C; Tue, 29 Sep 2026 08:50:15 +0000 (GMT) Received: from smtpav03.wdc07v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8F5215805D; Tue, 29 Sep 2026 08:50:11 +0000 (GMT) Received: from [9.61.249.121] (unknown [9.61.249.121]) by smtpav03.wdc07v.mail.ibm.com (Postfix) with ESMTP; Tue, 29 Sep 2026 08:50:11 +0000 (GMT) Message-ID: Date: Tue, 29 Sep 2026 14:20:09 +0530 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] powerpc: fix NMI exit ordering: restore soft masks after irqentry_nmi_exit() To: "Mukesh Kumar Chaurasiya (IBM)" , 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, ruanjinjie@huawei.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org References: <20260929071600.2963954-1-mkchauras@gmail.com> Content-Language: en-GB From: Venkat Rao Bagalkote In-Reply-To: <20260929071600.2963954-1-mkchauras@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Authority-Analysis: v=2.4 cv=FYWiV5+6 c=1 sm=1 tr=0 ts=6abb7bc9 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=VdqzKS8jKosA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=pGLkceISAAAA:8 a=IGkpBAwnVNaApzdPf5QA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Info: AW1haW4tMjYwOTI5MDAzNCBTYWx0ZWRfX9i5jOK7ScgcK iOO8PADE5zw1vG++d6TbrDwflEGWR+g1PYcUuF3xgVbUB+aPOaPgWRdJwRZXwCUOKatNeRoij4w 8BCt2p5CGRXiqG2NyFeNXYqabxKejQQ= X-Proofpoint-ORIG-GUID: 7QK2m4bCfmmzh1pE8hdvpiz-XEgcxjxy X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwOTI5MDAzNCBTYWx0ZWRfX0YkSDxQcQ2Y4 KxVVrpoiH6EYcVm+6A4MXy3Jrov5EbsXxZBMGeWrt25Gng5bKIRi31Qj0VXyqz9SoaHv5IAvRQ4 3lEouuBUB3MHTHPQWcYNTUcpkjV6JAeC7fiKh5OxmbL64S+91xegD/mR0/2C71CYbt0kILkppKh 3e57coEnf/KboqTHMYXqDfTv9ASP3YMzltz2fSlXrk0e9xryibnRTyZayZEFe55yidYf38Jd5hV OFbdy5FzMyvoRNsMdc9i6qYIz0fb30bEKniNqAPELAS5nkhWGSFUwtkrRUi/PjT0u/Ar+QSocv6 qc0pE9UxflOGoRPDuDo5xo8ZmVnJ4DB/7DHshAKu5i61FGzMfjopmJi0n735Jpt3mlf3vYKzhuN FdnpbBuFmRKQfdKr/vEPDEpxUDMe8U1BCHN2hA5sSSF/aeb0CnQEPAmRfp1zOGo6IepTuOBYWBQ XDic6BEM3ZOEwE7eZPA== X-Proofpoint-GUID: J7qtAU6BpZmlcLghR1WJdYDgOhbPF8du X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-09-29_03,2026-09-21_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 clxscore=1015 impostorscore=0 spamscore=0 phishscore=0 priorityscore=1501 malwarescore=0 bulkscore=0 lowpriorityscore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2609040000 definitions=main-2609290034 On 29/09/26 12:46 pm, Mukesh Kumar Chaurasiya (IBM) wrote: > 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) > --- This fixes reported problem. Tested-by: Venkat Rao Bagalkote Regards, Venkat. > 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; \ > } \