From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DEB363AA519 for ; Fri, 20 Mar 2026 11:54:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774007663; cv=none; b=R4+ftSm86cqG+vDGBy7Ouk75w5shhwR87hVakMhj2DedOsShQjkgPY37/FbFDZlHLxhYmxCdZJUNP5uc5jlcJnd+/jpjQdvYiLr/dM6klTk2CnhMlFdkkX80W/T0zCgfpPc2x5qtALaoDEhzO3jkYiYzCtIozrHA6tCyts8cJ5E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774007663; c=relaxed/simple; bh=z43NRHGb+77g/nx7QHI/SfLzvalWf0s45xHjUBa0D4E=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=BJszeCNlJD9b+cYJTjxHfVkpA0341XE5PEHhFkSgPTVxDjmaeYqAvi5Yi5iM4WCn20h+vdv5OxfcL10oR2A8/ucR9bx2U9Sje1Q+yNEYjk2Q0jTPiYm1rXqMC6EpBaMGuOhQMT5dUgQxkujxwnHA8eHOodSpWsCvU0fxBYPtydY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kApNwq4h; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kApNwq4h" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 370EDC2BCAF; Fri, 20 Mar 2026 11:54:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774007663; bh=z43NRHGb+77g/nx7QHI/SfLzvalWf0s45xHjUBa0D4E=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=kApNwq4hUwafRHX6oRvk7hRxDxFo6ZVymhpu4zv8toomi4YQYwRUG+vYTPv7dsUVU SjzL34UqU2iWrf1IJyofc+LZfaBsN9LXrfXtk/Nhl3MEYaa3srv8nmyvvk6QA+4zNB G0E93Y8xDcpKNf8E20qv3hVmWuPG0FDOOWW9XquNYKjj2/8YIeK3NzSW+YiMCEUfo3 nZHPkFL4zXghHgx9NfU4zsxIf4s0Dj0cqZgjw2MHvi1qZ4BBN2VlZVOMKrohSXCNQh nWuuR34ShQ0eJVvYSUnDYUablDrLit4jsCuoWt1O0r0IhuH+GUpWoMT0L1wXHGP7e0 ApP7D4hDD+kcg== From: Borislav Petkov To: X86 ML Cc: Nikunj A Dadhania , LKML , Borislav Petkov , Sohil Mehta , stable@kernel.org Subject: [PATCH 1/4] x86/cpu: Enable FSGSBASE early in cpu_init_exception_handling() Date: Fri, 20 Mar 2026 12:54:14 +0100 Message-ID: <20260320115417.4156-2-bp@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260320115417.4156-1-bp@kernel.org> References: <20260320115417.4156-1-bp@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Nikunj A Dadhania Move FSGSBASE enablement from identify_cpu() to cpu_init_exception_handling() to ensure it is enabled before any exceptions can occur on both boot and secondary CPUs. == Background == Exception entry code (paranoid_entry()) uses ALTERNATIVE patching based on X86_FEATURE_FSGSBASE to decide whether to use RDGSBASE/WRGSBASE instructions or the slower RDMSR/SWAPGS sequence for saving/restoring GSBASE. On boot CPU, ALTERNATIVE patching happens after enabling FSGSBASE in CR4. When the feature is available, the code is permanently patched to use RDGSBASE/WRGSBASE, which require CR4.FSGSBASE=1 to execute without triggering == Boot Sequence == Boot CPU (with CR pinning enabled): trap_init() cpu_init() <- Uses unpatched code (RDMSR/SWAPGS) x2apic_setup() ... arch_cpu_finalize_init() identify_boot_cpu() identify_cpu() cr4_set_bits(X86_CR4_FSGSBASE) # Enables the feature # This becomes part of cr4_pinned_bits ... alternative_instructions() <- Patches code to use RDGSBASE/WRGSBASE Secondary CPUs (with CR pinning enabled): start_secondary() cr4_init() <- Code already patched, CR4.FSGSBASE=1 set implicitly via cr4_pinned_bits cpu_init() <- exceptions work because FSGSBASE is already enabled Secondary CPU (with CR pinning disabled): start_secondary() cr4_init() <- Code already patched, CR4.FSGSBASE=0 cpu_init() x2apic_setup() rdmsrq(MSR_IA32_APICBASE) <- Triggers #VC in SNP guests exc_vmm_communication() paranoid_entry() <- Uses RDGSBASE with CR4.FSGSBASE=0 (patched code) ... ap_starting() identify_secondary_cpu() identify_cpu() cr4_set_bits(X86_CR4_FSGSBASE) <- Enables the feature, which is too late == CR Pinning == Currently, for secondary CPUs, CR4.FSGSBASE is set implicitly through CR-pinning: the boot CPU sets it during identify_cpu(), it becomes part of cr4_pinned_bits, and cr4_init() applies those pinned bits to secondary CPUs. This works but creates an undocumented dependency between cr4_init() and the pinning mechanism. == Problem == Secondary CPUs boot after alternatives have been applied globally. They execute already-patched paranoid_entry() code that uses RDGSBASE/WRGSBASE instructions, which require CR4.FSGSBASE=1. Upcoming changes to CR pinning behavior will break the implicit dependency, causing secondary CPUs to generate #UD. This issue manifests itself on AMD SEV-SNP guests, where the rdmsrq() in x2apic_setup() triggers a #VC exception early during cpu_init(). The #VC handler (exc_vmm_communication()) executes the patched paranoid_entry() path. Without CR4.FSGSBASE enabled, RDGSBASE instructions trigger #UD. == Fix == Enable FSGSBASE explicitly in cpu_init_exception_handling() before loading exception handlers. This makes the dependency explicit and ensures both boot and secondary CPUs have FSGSBASE enabled before paranoid_entry() executes. Fixes: c82965f9e530 ("x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit") Reported-by: Borislav Petkov Suggested-by: Sohil Mehta Signed-off-by: Nikunj A Dadhania Signed-off-by: Borislav Petkov (AMD) Reviewed-by: Sohil Mehta Cc: Link: https://patch.msgid.link/20260318075654.1792916-2-nikunj@amd.com --- arch/x86/kernel/cpu/common.c | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-) diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index a8ff4376c286..7840b224d6a7 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -2050,12 +2050,6 @@ static void identify_cpu(struct cpuinfo_x86 *c) setup_umip(c); setup_lass(c); - /* Enable FSGSBASE instructions if available. */ - if (cpu_has(c, X86_FEATURE_FSGSBASE)) { - cr4_set_bits(X86_CR4_FSGSBASE); - elf_hwcap2 |= HWCAP2_FSGSBASE; - } - /* * The vendor-specific functions might have changed features. * Now we do "generic changes." @@ -2416,6 +2410,18 @@ void cpu_init_exception_handling(bool boot_cpu) /* GHCB needs to be setup to handle #VC. */ setup_ghcb(); + /* + * On CPUs with FSGSBASE support, paranoid_entry() uses + * ALTERNATIVE-patched RDGSBASE/WRGSBASE instructions. Secondary CPUs + * boot after alternatives are patched globally, so early exceptions + * execute patched code that depends on FSGSBASE. Enable the feature + * before any exceptions occur. + */ + if (cpu_feature_enabled(X86_FEATURE_FSGSBASE)) { + cr4_set_bits(X86_CR4_FSGSBASE); + elf_hwcap2 |= HWCAP2_FSGSBASE; + } + if (cpu_feature_enabled(X86_FEATURE_FRED)) { /* The boot CPU has enabled FRED during early boot */ if (!boot_cpu) -- 2.51.0