From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1792608815; b=zguP3QIn1v45yKzfFUlZPH4SY5z5aqGUwET2nflwY1exEYYG07wJiy7VUtsthfMLD4Q+ WlzSI74hR+BKhT973fhxLvt82ve6wsKpPbx4hnkdsK0pYUadInDvPWgkfoG3P8oRSHtTW 5rqOX1P/3r7KduH+hZiZzjzKWeU5l9orMXSP/7vk7hG0rEBO/LIKeVZQ4OvYtVz4FVoNY 48LYe6JVyxWGD5/xIkKRyQBUpfcKkJ34Iib9ruFVbTFjRSHnGdIAPB/cDzC3A7nXy2YCe 1w6B1J0sHWatisDWtp26cok18BB7omxyx8hXn3MqZ27FC7QfNsUp8HQnMudg+E/CSGA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1792608815; h=Received:DKIM-Signature:Subject:To:Cc:From:Date:Message-ID: MIME-Version:Content-Type:Content-Transfer-Encoding; bh=MwM1jDiGFXGz7WkyABxnWue4nvX51qTunpgsMfBndy8=; b=CHwHx8nNCW5Es4GPO0TMZeATCv1eCxfcATJKnEtqudYIow0PB6qCkfzJF97cjFAJe3u8 bG7nT3Q+8RWtNTj8hOxVXKSqavIm3Hlk8lJwQiFCZdTa54BYYAgHUFLEFAjNDWPgRXC3N GEAGf6f88n7sZ8E46NcTyYj9KZVGP8XtqE9lHvbI1j9NGSM5tSjQl9jP2tCgY4poJMV9v kFPG6tSvKV3SgDJLoLU0BsTsy7ped84qVJItNaUO3YAnJevBryz62HGXOz9VJ1GBUFQGi aJXhXl1uklWu4J/woeRQpsBpBq85JoLgW+x2ggW8MJIkUCBmVAdUm8JkwCpx3ASW1/g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=N0iqbdQQ; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0C422606A319 for ; Wed, 21 Oct 2026 18:53:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=N0iqbdQQ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0980D1F000C1; Wed, 21 Oct 2026 18:53:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1792608814; bh=MwM1jDiGFXGz7WkyABxnWue4nvX51qTunpgsMfBndy8=; h=Subject:To:Cc:From:Date; b=N0iqbdQQIfNr9UvtN0f578PNlLKshW2oabEnjbIV0JRmLCai8h3sGqkqYcj31EBPW /85JmpsCZa6xiZquFxy7kjoPoiJNmsl0nAwgUtIAS2h2TWzg+hlrPy/DRxd2iW599z +4R45pq9FfwUQyMKYGMoVSx9q+Zq8pXND/kUcel4= Subject: FAILED: patch "[PATCH] x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI" failed to apply to 6.1-stable tree To: ejc3@meta.com,pbonzini@redhat.com Cc: From: Date: Wed, 21 Oct 2026 20:50:15 +0200 Message-ID: <2026102115-flatterer-resolute-532f@gregkh> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=ANSI_X3.4-1968 Content-Transfer-Encoding: 8bit The patch below does not apply to the 6.1-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to . To reproduce the conflict and resubmit, you may use the following commands: git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y git checkout FETCH_HEAD git cherry-pick -x 81d99abe2bd81379abfb595e403bbbee6fbfc624 # git commit -s git send-email --to '' --in-reply-to '2026102115-flatterer-resolute-532f@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..' Possible dependencies: thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 81d99abe2bd81379abfb595e403bbbee6fbfc624 Mon Sep 17 00:00:00 2001 From: EJ Campbell Date: Tue, 29 Sep 2026 11:12:32 -0700 Subject: [PATCH] x86/kvm: Don't consume async #PF flags on a kernel #PF in NMI context An async #PF "page not present" event arrives through the #PF vector, and its reason sits in the per-CPU apf_reason.flags until __kvm_handle_async_pf() reads and clears it. If an NMI arrives after the event is injected and before the #PF handler reads the flags, and the NMI handler itself takes a #PF, that fault reads and clears the flags belonging to the #PF it interrupted. It then finds interrupts disabled in the NMI context it interrupted, and panics: Kernel panic - not syncing: Host injected async #PF in interrupt disabled region Call Trace: panic+0x52/0x60 __kvm_handle_async_pf+0xb0/0xb0 exc_page_fault+0x71/0xb0 asm_exc_page_fault+0x27/0x30 RIP: 0010:__get_user_nocheck_8+0x6/0x20 perf_callchain_user+0x147/0x1a0 get_perf_callchain+0x12d/0x1c0 ... perf_event_nmi_handler+0x26/0x50 exc_nmi+0xe8/0x110 RIP: 0010:asm_exc_page_fault+0x0/0x30 The last RIP line shows the NMI landing on the first instruction of the #PF entry. perf with user call chains takes such a fault: the sampling NMI walks the user stack with __get_user(), which faults when a saved frame pointer is not a valid user address or the walk runs off the top of the stack. The same window exists when KVM, running as an L1 hypervisor with DELIVERY_AS_PF_VMEXIT, reads the flags itself after a #PF VM-exit. The NMI entry code saves and restores CR2 for this nesting, but not the async #PF reason. Andy Lutomirski described the race, with perf as the example, when async #PF delivery was reworked in 2020 [1]. Saving and clearing the flags across the NMI, next to CR2, was discussed there but not merged. Since that rework, Linux has stopped setting KVM_ASYNC_PF_SEND_ALWAYS, so the host delivers "page not present" only for user mode faults and a kernel-mode fault in NMI context (including kernel-mode #MC and #DB) cannot be an async page fault. In that case, return without touching the flags so that the interrupted handler finds them intact. Tested on v7.3-rc5 in a 4-vCPU, 2 GiB guest restored from a snapshot whose memory is served on demand through userfaultfd, so that reading it back takes async #PFs, while every thread samples hardware cycles with user call chains and points its frame pointer at an unmapped page. Without this patch the guest panicked as above in 3 of 3 runs. With it, the guest survived 3 of 3 runs of 60 seconds, and the host counted 148,441 to 153,334 kvm_async_pf_not_present events in the first 20 seconds of each. On 6.18.50 the results were the same: the guest panicked without the patch and survived with it. Link: https://lore.kernel.org/all/ed71d0967113a35f670a9625a058b8e6e0b2f104.1583547991.git.luto@kernel.org/ [1] Link: https://lore.kernel.org/all/20211126123145.2772-1-jiangshanlai@gmail.com/ [2] Cc: stable@vger.kernel.org # v5.9+ Signed-off-by: EJ Campbell Link: https://patch.msgid.link/CADkxfh-uQTca_6jB1JPDq1MsKffF_UKGQR339oQSJNcoV75GNQ@mail.gmail.com Signed-off-by: Paolo Bonzini diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index 6b0a5861ccb8..d8dd2cadcbf7 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -265,11 +265,39 @@ noinstr u32 kvm_read_and_reset_apf_flags(void) } EXPORT_SYMBOL_FOR_KVM(kvm_read_and_reset_apf_flags); +static __always_inline bool kvm_apf_reason_pending(void) +{ + return __this_cpu_read(async_pf_enabled) && + __this_cpu_read(apf_reason.flags); +} + noinstr bool __kvm_handle_async_pf(struct pt_regs *regs, u32 token) { - u32 flags = kvm_read_and_reset_apf_flags(); irqentry_state_t state; + u32 flags; + if (!kvm_apf_reason_pending()) + return false; + + /* + * The guest does not enable KVM_ASYNC_PF_SEND_ALWAYS, so the host + * delivers async #PF only to user mode (or as a #PF VM-exit, which KVM + * reads itself). A #PF from kernel mode can still arrive between that + * delivery and the read of the reason flags, inside an NMI that + * interrupted it, for instance perf reading user memory for a call + * chain. Such a fault is not async, so leave the async PF flags + * untouched for the outer handler. Outside NMI context, on the other + * hand, async PF flags on a kernel-mode fault mean the host is broken. + */ + if (!user_mode(regs)) { + if (likely(in_nmi())) + return false; + state = irqentry_enter(regs); + instrumentation_begin(); + panic("Host injected async #PF in kernel mode\n"); + } + + flags = kvm_read_and_reset_apf_flags(); if (!flags) return false; @@ -285,8 +313,6 @@ noinstr bool __kvm_handle_async_pf(struct pt_regs *regs, u32 token) panic("Host injected async #PF in interrupt disabled region\n"); if (flags & KVM_PV_REASON_PAGE_NOT_PRESENT) { - if (unlikely(!(user_mode(regs)))) - panic("Host injected async #PF in kernel mode\n"); /* Page is swapped out by the host. */ kvm_async_pf_task_wait_schedule(token); } else {