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([2a02:6b6f:e750:1b00:1cfc:9209:4810:3ae5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-47494a91c47sm49161355e9.1.2025.10.22.16.49.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Oct 2025 16:49:26 -0700 (PDT) Message-ID: <1a7a28d2-5a49-4522-b6e6-a42a14397aa0@gmail.com> Date: Thu, 23 Oct 2025 00:49:26 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] x86/boot: Fix page table access in 5-level to 4-level paging transition Content-Language: en-GB To: Dave Hansen , dwmw@amazon.co.uk, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, ardb@kernel.org, hpa@zytor.com Cc: x86@kernel.org, apopple@nvidia.com, thuth@redhat.com, nik.borisov@suse.com, kas@kernel.org, linux-kernel@vger.kernel.org, linux-efi@vger.kernel.org, kernel-team@meta.com, Michael van der Westhuizen , Tobias Fleig References: <20251022220755.1026144-1-usamaarif642@gmail.com> <20251022220755.1026144-2-usamaarif642@gmail.com> <8283c1b6-1487-49e2-b220-7dbd043a2913@intel.com> From: Usama Arif In-Reply-To: <8283c1b6-1487-49e2-b220-7dbd043a2913@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 23/10/2025 00:16, Dave Hansen wrote: > On 10/22/25 15:06, Usama Arif wrote: >> + pgdp = (pgd_t *)read_cr3_pa(); >> + new_cr3 = (u64 *)(pgd_val(pgdp[0]) & PTE_PFN_MASK); >> + memcpy(trampoline_32bit, new_cr3, PAGE_SIZE); > > Heh, somebody like casting, I see! haha yeah its a lot here. > > But seriously, read_cr3_pa() should be returning a physical address. No? > Today it does: > > static inline unsigned long read_cr3_pa(void) > { > return __read_cr3() & CR3_ADDR_MASK; > } > > So shouldn't CR3_ADDR_MASK be masking out any naughty non-address bits? > Shouldn't we fix read_cr3_pa() and not do this in its caller? So we need to mask 2 things here: - cr3, which is done by read_cr3_pa using CR3_ADDR_MASK/(__sme_clr(PHYSICAL_PAGE_MASK)) as you pointed out. - pgdp[0] (the deferenced value), i.e. the p4d table pointer (This was previously *(unsigned long *)__native_read_cr3()). This needs to be masked by PTE_PFN_MASK and and not PAGE_MASK which was done previously in order to take care of _PAGE_BIT_NOPTISHADOW.