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[91.219.240.17]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b2aecc89sm88150895e9.9.2025.05.02.05.09.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 02 May 2025 05:10:00 -0700 (PDT) From: Vitaly Kuznetsov To: Ard Biesheuvel Cc: x86@kernel.org, linux-efi@vger.kernel.org, Thomas Gleixner , Ingo Molnar , Dave Hansen , "H. Peter Anvin" , Peter Jones , Daniel Berrange , Emanuele Giuseppe Esposito , Gerd Hoffmann , Greg KH , Luca Boccassi , Peter Zijlstra , Matthew Garrett , James Bottomley , Eric Snowberg , Paolo Bonzini , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] x86/efi: Implement support for embedding SBAT data for x86 In-Reply-To: References: <20250424080950.289864-1-vkuznets@redhat.com> <20250424080950.289864-3-vkuznets@redhat.com> <87ldrka6w6.fsf@redhat.com> Date: Fri, 02 May 2025 14:09:59 +0200 Message-ID: <87a57v9pt4.fsf@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Ard Biesheuvel writes: > On Mon, 28 Apr 2025 at 12:59, Vitaly Kuznetsov wrote: >> >> Ard Biesheuvel writes: >> >> > On Thu, 24 Apr 2025 at 10:10, Vitaly Kuznetsov wrote: ... >> >> $(obj)/vmlinux: $(vmlinux-objs-y) $(vmlinux-libs-y) FORCE >> >> $(call if_changed,ld) >> >> >> >> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S >> >> index 3b2bc61c9408..d0a27905de90 100644 >> >> --- a/arch/x86/boot/compressed/vmlinux.lds.S >> >> +++ b/arch/x86/boot/compressed/vmlinux.lds.S >> >> @@ -49,9 +49,22 @@ SECTIONS >> >> *(.data.*) >> >> >> >> /* Add 4 bytes of extra space for the obsolete CRC-32 checksum */ >> >> +#ifndef CONFIG_EFI_SBAT >> >> . = ALIGN(. + 4, 0x200); >> >> +#else >> >> + /* Avoid gap between '.data' and '.sbat' */ >> >> + . = ALIGN(. + 4, 0x1000); >> >> +#endif >> >> _edata = . ; >> >> } >> >> +#ifdef CONFIG_EFI_SBAT >> >> + .sbat : ALIGN(0x1000) { >> >> + _sbat = . ; >> >> + *(.sbat) >> >> + _esbat = ALIGN(0x200); >> >> + . = _esbat; >> >> + } >> >> +#endif >> >> . = ALIGN(L1_CACHE_BYTES); >> >> .bss : { >> >> _bss = . ; >> > >> > This looks a bit odd - see below >> > >> >> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S >> >> index b5c79f43359b..ab851490ef74 100644 >> >> --- a/arch/x86/boot/header.S >> >> +++ b/arch/x86/boot/header.S >> >> @@ -207,6 +207,19 @@ pecompat_fstart: >> >> IMAGE_SCN_MEM_READ | \ >> >> IMAGE_SCN_MEM_WRITE # Characteristics >> >> >> >> +#ifdef CONFIG_EFI_SBAT >> >> + .ascii ".sbat\0\0\0" >> >> + .long ZO__esbat - ZO__sbat # VirtualSize >> >> + .long setup_size + ZO__sbat # VirtualAddress >> >> + .long ZO__esbat - ZO__sbat # SizeOfRawData >> >> + .long setup_size + ZO__sbat # PointerToRawData >> >> + >> >> + .long 0, 0, 0 >> >> + .long IMAGE_SCN_CNT_INITIALIZED_DATA | \ >> >> + IMAGE_SCN_MEM_READ | \ >> >> + IMAGE_SCN_MEM_DISCARDABLE # Characteristics >> >> +#endif >> >> + >> > >> > This puts the .sbat section at the very end of the file. However, the >> > virtual size of .data is 'ZO__end - ZO__data' not 'ZO__edata - >> > ZO__data', and so the .sbat section will overlap with .bss in the >> > memory view of the image. >> >> Missed that, will fix, thanks! A stupid question though: does this >> matter in practice for SBAT? I don't think anyone needs SBAT data after >> kernel starts booting so we can consider it 'discarded'. BSS data can >> then do whatever it wants. >> > > It violates the PE/COFF spec, and some PE loaders and signing tools > are very pedantic about the layout. Turns out it the problem is slightly harder to address then I initially thought. On x86, arch/x86/boot/bzImage is composed of setup.bin and vmlinux.bin and the later is produced from vmlinux with objcopy -O binary -R .note -R .comment -S arch/x86/boot/compressed/vmlinux arch/x86/boot/vmlinux.bin "objcopy -O binary" basically dumps memory representation of vmlinux. In case we put '.sbat' before '.bss' we get: Sections: Idx Name Size VMA LMA File off Algn ... 4 .data 00002000 0000000000aef000 0000000000aef000 00af0000 2**12 CONTENTS, ALLOC, LOAD, DATA 5 .sbat 00001000 0000000000af1000 0000000000af1000 00af2000 2**12 CONTENTS, ALLOC, LOAD, READONLY, DATA 6 .bss 00023078 0000000000af2000 0000000000af2000 00af3000 2**12 ALLOC 7 .pgtable 00021000 0000000000b16000 0000000000b16000 00af3000 2**12 ALLOC ... so we can't have a single '.data' section in PE covering both '.data' and '.bss'/'.pgtable' as '.sbat' is in the middle and we want it to be a separate section. If we try putting '.sbat' after '.bss' we are going to get a hole size of '.bss' + '.pgtable' in arch/x86/boot/vmlinux.bin because 'objcopy -O binary' is not going to squeeze anything (and it has no idea what '.sbat' is and if it can be moved). The problem is similar for zboot. I have two ideas: 1) Get back to the idea of putting '.sbat' between '.text' and '.data' (was in my RFC). 2) Introduce a separate '.bss' section to the PE binary, basically: diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S index b5c79f43359b..dae8202705c4 100644 --- a/arch/x86/boot/header.S +++ b/arch/x86/boot/header.S @@ -197,7 +197,7 @@ pecompat_fstart: IMAGE_SCN_MEM_EXECUTE # Characteristics .ascii ".data\0\0\0" - .long ZO__end - ZO__data # VirtualSize + .long ZO__edata - ZO__data # VirtualSize .long setup_size + ZO__data # VirtualAddress .long ZO__edata - ZO__data # SizeOfRawData .long setup_size + ZO__data # PointerToRawData @@ -207,6 +207,17 @@ pecompat_fstart: IMAGE_SCN_MEM_READ | \ IMAGE_SCN_MEM_WRITE # Characteristics + .ascii ".bss\0\0\0\0" + .long ZO__end - ZO__edata # VirtualSize + .long setup_size + ZO__edata # VirtualAddress + .long 0 # SizeOfRawData + .long 0 # PointerToRawData + + .long 0, 0, 0 + .long IMAGE_SCN_CNT_UNINITIALIZED_DATA| \ + IMAGE_SCN_MEM_READ | \ + IMAGE_SCN_MEM_WRITE # Characteristics + .set section_count, (. - section_table) / 40 #endif /* CONFIG_EFI_STUB */ This way '.data' doesn't need to be the last meaninful section in 'vmlinux' and we can then pub '.sbat' right after it. I'm going to give it a try but also I'm wondering why do we have '.data' covering '.bss' and '.pgtable' in the first place. -- Vitaly