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Thu, 09 Apr 2020 09:35:33 -0700 (PDT) Received: from rani.riverdale.lan ([2001:470:1f07:5f3::b55f]) by smtp.gmail.com with ESMTPSA id 10sm21471275qtt.54.2020.04.09.09.35.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Apr 2020 09:35:32 -0700 (PDT) From: Arvind Sankar X-Google-Original-From: Arvind Sankar Date: Thu, 9 Apr 2020 12:35:30 -0400 To: Ard Biesheuvel Cc: Arvind Sankar , Dave Young , pjones@redhat.com, daniel.kiper@oracle.com, Leif Lindholm , Borislav Petkov , Sergey Shatunov , hpa@zytor.com, Linux Kernel Mailing List , mingo@redhat.com, Thomas Gleixner , X86 ML , linux-efi , initramfs@vger.kernel.org, Donovan Tremura , Harald Hoyer Subject: Re: [PATCH 1/2] efi/x86: Move efi stub globals from .bss to .data Message-ID: <20200409163530.GA785575@rani.riverdale.lan> References: <20200406180614.429454-1-nivedita@alum.mit.edu> <20200408074334.GA21886@dhcp-128-65.nay.redhat.com> <20200409143910.GA727557@rani.riverdale.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 09, 2020 at 04:47:55PM +0200, Ard Biesheuvel wrote: > On Thu, 9 Apr 2020 at 16:39, Arvind Sankar wrote: > > > > On Wed, Apr 08, 2020 at 09:49:15AM +0200, Ard Biesheuvel wrote: > > > (add Peter, Leif and Daniel) > > > > > > On Wed, 8 Apr 2020 at 09:43, Dave Young wrote: > > > > > > > > On 04/06/20 at 02:06pm, Arvind Sankar wrote: > > > > > Commit > > > > > > > > > > 3ee372ccce4d ("x86/boot/compressed/64: Remove .bss/.pgtable from > > > > > bzImage") > > > > > > > > > > removed the .bss section from the bzImage. > > > > > > > > > > However, while a PE loader is required to zero-initialize the .bss > > > > > section before calling the PE entry point, the EFI handover protocol > > > > > does not currently document any requirement that .bss be initialized by > > > > > the bootloader prior to calling the handover entry. > > > > > > > > > > When systemd-boot is used to boot a unified kernel image [1], the image > > > > > is constructed by embedding the bzImage as a .linux section in a PE > > > > > executable that contains a small stub loader from systemd together with > > > > > additional sections and potentially an initrd. As the .bss section > > > > > within the bzImage is no longer explicitly present as part of the file, > > > > > it is not initialized before calling the EFI handover entry. > > > > > Furthermore, as the size of the embedded .linux section is only the size > > > > > of the bzImage file itself, the .bss section's memory may not even have > > > > > been allocated. > > > > > > > > I did not follow up the old report, maybe I missed something. But not > > > > sure why only systemd-boot is mentioned here. I also have similar issue > > > > with early efi failure. With these two patches applied, it works well > > > > then. > > > > > > > > BTW, I use Fedora 31 + Grub2 > > > > > > > > > > OK, so I take it this means that GRUB's PE/COFF loader does not > > > zero-initialize BSS either? Does it honor the image size in memory if > > > it exceeds the file size? > > > > Dave, that comment was because the previous report was for systemd-boot > > stub. > > > > Ard, should I revise the commit message to make it clear it's not > > restricted to systemd-boot but anything using handover entry may be > > affected? Maybe just a "for example, when systemd-boot..." and then a > > line to say grub2 with the EFI stub patches is also impacted? > > > > Well, the fact the /some/ piece of software is used in production that > relies on the ill-defined EFI handover protocol is sufficient > justification, so I don't think it is hugely important to update it. > > > https://src.fedoraproject.org/rpms/grub2/blob/f31/f/0001-Add-support-for-Linux-EFI-stub-loading.patch#_743 > > > > + kernel_mem = grub_efi_allocate_pages_max(lh.pref_address, > > + BYTES_TO_PAGES(lh.init_size)); > > > > Looking at this, grub does allocate init_size for the image, but it > > doesn't zero it out. > > > > This call also looks wrong to me though. It allocates at max address of > > pref_address, which, if it succeeds, will guarantee that the kernel gets > > loaded entirely below pref_address == LOAD_PHYSICAL_ADDR. In native > > mode, if it weren't for the EFI stub copying the kernel again, this > > would cause the startup code to relocate the kernel into unallocated > > memory. On a mixed-mode boot, this would cause the early page tables > > setup prior to transitioning to 64-bit mode to be in unallocated memory > > and potentially get clobbered by the EFI stub. > > > > The first try to allocate pref_address should be calling > > grub_efi_allocate_fixed instead. > > Thanks Arvind. I'm sure the Fedora/RedHat folks on cc should be able > to get these logged somewhere. Ok. For dracut, the process for building the unified kernel image needs a check to make sure the kernel can fit in the space provided for it -- there is 16MiB of space and the distro bzImage's are up to 10-11MiB in size, so there's some slack left at present. Additionally, in mixed-mode, the unified kernel images are quite likely to end up with early pgtables from startup_32 clobbering the initrd, independently of the recent kernel changes. Hopefully no-one actually uses these in mixed-mode.