| Message ID | 20171204025801.12161-1-takahiro.akashi@linaro.org |
|---|---|
| Headers | show |
| Series | arm64: kexec: add kexec_file_load() support | expand |
Hi Akashi, I'm still getting my head round how all this works, so please forgive what may be stupid questions! On 04/12/17 02:57, AKASHI Takahiro wrote: > This is the seventh round of implementing kexec_file_load() support > on arm64.[1] > Most of the code is based on kexec-tools (along with some kernel code > from x86, which also came from kexec-tools). > > > This patch series enables us to > * load the kernel, Image, via kexec_file_load() system call, and > * optionally verify its signature at load time for trusted boot. Is kdump using kexec_file_load() possible? (questions on patch 3) I can't work out why additional elf-generating code would be necessary if kdump works today without it... > To load the kernel via kexec_file_load() system call, a small change > is also required on kexec-tools. See [2]. This enables '-s' option. > (Please use v7.2.1+ crash utility for v4.14+ kernel) (what does the -s option do?) > As we discussed a long time ago, users may not be allowed to specify > device-tree file of the 2nd kernel explicitly with kexec-tools, hence > re-using the blob of the first kernel. > > Regarding a kernel image verification, a signature must be presented > along with the binary itself. A signature is basically a hash value > calculated against the whole binary data and encrypted by a key which > will be authenticated by the system's trusted certificate. > Any attempt to read and load a to-be-kexec-ed kernel image through > a system call will be checked and blocked if the binary's hash value > doesn't match its associated signature. > Concerns(or future works): (lets keep this stuff in the future) > * Even if the kernel is configured with CONFIG_RANDOMIZE_BASE, the 2nd > kernel won't be placed at a randomized address. We will have to > add some boot code similar to efi-stub to implement the randomization. I think there are two parts to this. The efistub may copy the kernel to a new ~random location in physical memory. It also adds a seed used to randomise the virtual-addresses the kernel executes from. For kexec_file_load() the first-kernel could apply some randomness to the physical offset when it re-assembles the kexec-kernel. i.e. code in arm64_relocate_new_kernel(). I don't think we should do this without some hint that the new kernel supports this... For the virtual-addresses it would need to add a new kaslr-seed to the DT/chosen, which should be harmless. > for approach (1), > * While big-endian kernel can support kernel signing, I'm not sure that > Image can be recognized as in PE format because x86 standard only > defines little-endian-based format. What does the recognizing? (I don't think we should invent a new format..) > * vmlinux support (Patch 3 is why I'm here) I don't think we need to support this. I can't boot a vmlinux file via UEFI. As I understand it kexec_file_load() is all about the signature verification for UEFI:SecureBoot. The chances of me having a vmlinux signed for SecureBoot use is pretty low, chances are its a self-signed image I just built, in which case I can use the arm64 Image file that was built at the same time. Supporting two file formats is going to be a headache. Distributions ship separate debug info packages for debugging, I don't think we need to make them bootable... Thanks, James
Hi James, On Wed, Feb 07, 2018 at 06:37:21PM +0000, James Morse wrote: > Hi Akashi, > > I'm still getting my head round how all this works, so please forgive what may > be stupid questions! It is my pleasure. Hopefully I will be able to address all of your concerns before submitting a new version sometime next week. > > On 04/12/17 02:57, AKASHI Takahiro wrote: > > This is the seventh round of implementing kexec_file_load() support > > on arm64.[1] > > Most of the code is based on kexec-tools (along with some kernel code > > from x86, which also came from kexec-tools). > > > > > > This patch series enables us to > > * load the kernel, Image, via kexec_file_load() system call, and > > * optionally verify its signature at load time for trusted boot. > > Is kdump using kexec_file_load() possible? (questions on patch 3) > I can't work out why additional elf-generating code would be necessary if kdump > works today without it... The code that probably you are mentioning is the one for creating a content of an ELF header (elfcoreheader) of /proc/vmcore. The memory region is allocated by the kernel, but the content itself will be filled up by kexec-tools for kexec_load syscall, while by the kernel for kexec_file_load syscall. (In former case, a "user" buffer is passed via kexec_load syscall and copied into the crash dump kernel memory.) Please see kexec/crashdump.c and crashdump-elf.c on kexec-tools. > > > > To load the kernel via kexec_file_load() system call, a small change > > is also required on kexec-tools. See [2]. This enables '-s' option. > > (Please use v7.2.1+ crash utility for v4.14+ kernel) > > (what does the -s option do?) It is a switch for using kexec_file_load syscall instead of kexec_load syscall. > > > As we discussed a long time ago, users may not be allowed to specify > > device-tree file of the 2nd kernel explicitly with kexec-tools, hence > > re-using the blob of the first kernel. > > > > Regarding a kernel image verification, a signature must be presented > > along with the binary itself. A signature is basically a hash value > > calculated against the whole binary data and encrypted by a key which > > will be authenticated by the system's trusted certificate. > > Any attempt to read and load a to-be-kexec-ed kernel image through > > a system call will be checked and blocked if the binary's hash value > > doesn't match its associated signature. > > > Concerns(or future works): > > (lets keep this stuff in the future) Sure. > > * Even if the kernel is configured with CONFIG_RANDOMIZE_BASE, the 2nd > > kernel won't be placed at a randomized address. We will have to > > add some boot code similar to efi-stub to implement the randomization. > > I think there are two parts to this. The efistub may copy the kernel to a new > ~random location in physical memory. It also adds a seed used to randomise the > virtual-addresses the kernel executes from. > For kexec_file_load() the first-kernel could apply some randomness to the > physical offset when it re-assembles the kexec-kernel. i.e. code in > arm64_relocate_new_kernel(). I don't think we should do this without some hint > that the new kernel supports this... Indeed. arm64_relocate_new_kernel belongs to the first kernel, while efistub to crash dump kernel. > For the virtual-addresses it would need to add a new kaslr-seed to the > DT/chosen, which should be harmless. Yeah, adding a seed is quite easy. > > > for approach (1), > > * While big-endian kernel can support kernel signing, I'm not sure that > > Image can be recognized as in PE format because x86 standard only > > defines little-endian-based format. > > What does the recognizing? (I don't think we should invent a new format..) The obvious problem is that there exists no defition of PE format for BE architecture. (I believe that MS only defines the format for x86). For exmaple, what magic number should we use? Are the fields in a header in BE or LE? I don't invent a new format, neither. Anyhow, if we allow for IMA-base kexec_file_load, it perfectly resolve the issue without any changes (maybe). > > > > * vmlinux support > > (Patch 3 is why I'm here) > > I don't think we need to support this. I can't boot a vmlinux file via UEFI. As > I understand it kexec_file_load() is all about the signature verification for > UEFI:SecureBoot. The chances of me having a vmlinux signed for SecureBoot use is > pretty low, chances are its a self-signed image I just built, in which case I > can use the arm64 Image file that was built at the same time. > > Supporting two file formats is going to be a headache. Distributions ship > separate debug info packages for debugging, I don't think we need to make them > bootable... As I said somewhere before, the only reason that we may want to have vmlinux support is that kexec-tools (and in turn kexec_load syscall) supports both file types. (I don't know why Jeoff added this support first.) Thanks, -Takahiro AKASHI > > Thanks, > > James
This is the seventh round of implementing kexec_file_load() support on arm64.[1] Most of the code is based on kexec-tools (along with some kernel code from x86, which also came from kexec-tools). This patch series enables us to * load the kernel, Image, via kexec_file_load() system call, and * optionally verify its signature at load time for trusted boot. To load the kernel via kexec_file_load() system call, a small change is also required on kexec-tools. See [2]. This enables '-s' option. (Please use v7.2.1+ crash utility for v4.14+ kernel) As we discussed a long time ago, users may not be allowed to specify device-tree file of the 2nd kernel explicitly with kexec-tools, hence re-using the blob of the first kernel. Regarding a kernel image verification, a signature must be presented along with the binary itself. A signature is basically a hash value calculated against the whole binary data and encrypted by a key which will be authenticated by the system's trusted certificate. Any attempt to read and load a to-be-kexec-ed kernel image through a system call will be checked and blocked if the binary's hash value doesn't match its associated signature. There are two methods available now: 1. implementing arch-specific verification hook of kexec_file_load() 2. utilizing IMA(Integrity Measurement Architecture)[3] appraisal framework Before my v7, I believed that my patch only supports (1) but am now confident that (2) comes free if IMA is enabled and properly configured. (1) Arch-specific verification hook If CONFIG_KEXEC_VERIFY_SIG is enabled, kexec_file_load() invokes an arch- defined (and hence file-format-specific) hook function to check for the validity of kernel binary. On x86, a signature is embedded into a PE file (Microsoft's format) header of binary. Since arm64's "Image" can also be seen as a PE file as far as CONFIG_EFI is enabled, we adopt this format for kernel signing. As in the case of UEFI applications, we can create a signed kernel image: $ sbsign --key ${KEY} --cert ${CERT} Image You may want to use certs/signing_key.pem, which is intended to be used for module sigining (CONFIG_MODULE_SIG), as ${KEY} and ${CERT} for test purpose. (2) IMA appraisal-based IMA was first introduced in linux in order to meet TCG (Trusted Computing Group) requirement that all the sensitive files be *measured* before reading/executing them to detect any untrusted changes/modification. Then appraisal feature, which allows us to ensure the integrity of files and even prevent them from reading/executing, was added later. Meanwhile, kexec_file_load() has been merged since v3.17 and evolved to enable IMA-appraisal type verification by the commit b804defe4297 ("kexec: replace call to copy_file_from_fd() with kernel version"). In this scheme, a signature will be stored in a extended file attribute, "security.ima" while a decryption key is hold in a dedicated keyring, ".ima" or "_ima". All the necessary process of verification is confined in a secure API, kernel_read_file_from_fd(), called by kexec_file_load(). Please note that powerpc is one of the two architectures now supporting KEXEC_FILE, and that it wishes to exntend IMA, where a signature may be appended to "vmlinux" file[4], like module signing, instead of using an extended file attribute. While IMA meant to be used with TPM (Trusted Platform Module) on secure platform, IMA is still usable without TPM. Here is an example procedure about how we can give it a try to run the feature using a self-signed root ca for demo/test purposes: 1) Generate needed keys and certificates, following "Generate trusted keys" section in README of ima-evm-utils[5]. 2) Build the kernel with the following kernel configurations, specifying "ima-local-ca.pem" for CONFIG_SYSTEM_TRUSTED_KEYS: CONFIG_EXT4_FS_SECURITY CONFIG_INTEGRITY_SIGNATURE CONFIG_INTEGRITY_ASYMMETRIC_KEYS CONFIG_INTEGRITY_TRUSTED_KEYRING CONFIG_IMA CONFIG_IMA_WRITE_POLICY CONFIG_IMA_READ_POLICY CONFIG_IMA_APPRAISE CONFIG_IMA_APPRAISE_BOOTPARAM CONFIG_SYSTEM_TRUSTED_KEYS Please note that CONFIG_KEXEC_VERIFY_SIG is not, actually should not be, enabled. 3) Sign(label) a kernel image binary to be kexec-ed on target filesystem: $ evmctl ima_sign --key /path/to/private_key.pem /your/Image 4) Add a command line parameter and boot the kernel: ima_appraise=enforce On live system, 5) Set a security policy: $ mount -t securityfs none /sys/kernel/security $ echo "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig" \ > /sys/kernel/security/ima/policy 6) Add a key for ima: $ keyctl padd asymmetric my_ima_key %:.ima < /path/to/x509_ima.der (or evmctl import /path/to/x509_ima.der <ima_keyring_id>) 7) Then try kexec as normal. Concerns(or future works): * Even if the kernel is configured with CONFIG_RANDOMIZE_BASE, the 2nd kernel won't be placed at a randomized address. We will have to add some boot code similar to efi-stub to implement the randomization. for approach (1), * While big-endian kernel can support kernel signing, I'm not sure that Image can be recognized as in PE format because x86 standard only defines little-endian-based format. * vmlinux support [1] http://git.linaro.org/people/takahiro.akashi/linux-aarch64.git branch:arm64/kexec_file [2] http://git.linaro.org/people/takahiro.akashi/kexec-tools.git branch:arm64/kexec_file [3] https://sourceforge.net/p/linux-ima/wiki/Home/ [4] http://lkml.iu.edu//hypermail/linux/kernel/1707.0/03669.html [5] https://sourceforge.net/p/linux-ima/ima-evm-utils/ci/master/tree/ Changes in v7 (Dec 4, 2017) * rebased to v4.15-rc2 * re-organize the patch set to separate KEXEC_FILE_VERIFY_SIG-related code from the others * revamp factored-out code in kernel/kexec_file.c due to the changes in original x86 code * redefine walk_sys_ram_res_rev() prototype due to change of callback type in the counterpart, walk_sys_ram_res() * make KEXEC_FILE_IMAGE_FMT defaut on if KEXEC_FILE selected Changes in v6 (Oct 24, 2017) * fix a for-loop bug in _kexec_kernel_image_probe() per Julien Changes in v5 (Oct 10, 2017) * fix kbuild errors around patch #3 per Julien's comments, * fix a bug in walk_system_ram_res_rev() with some cleanup * modify fdt_setprop_range() to use vmalloc() * modify fill_property() to use memset() Changes in v4 (Oct 2, 2017) * reinstate x86's arch_kexec_kernel_image_load() * rename weak arch_kexec_kernel_xxx() to _kexec_kernel_xxx() for better re-use * constify kexec_file_loaders[] Changes in v3 (Sep 15, 2017) * fix kbuild test error * factor out arch_kexec_kernel_*() & arch_kimage_file_post_load_cleanup() * remove CONFIG_CRASH_CORE guard from kexec_file.c * add vmapped kernel region to vmcore for gdb backtracing (see prepare_elf64_headers()) * merge asm/kexec_file.h into asm/kexec.h * and some cleanups Changes in v2 (Sep 8, 2017) * move core-header-related functions from crash_core.c to kexec_file.c * drop hash-check code from purgatory * modify purgatory asm to remove arch_kexec_apply_relocations_add() * drop older kernel support * drop vmlinux support (at least, for this series) Patch #1 to #9 are essential for KEXEC_FILE support (plus IMA-based verification): Patch #1 to #4 are all preparatory patches on generic side. Patch #5 to #8 are common for enabling kexec_file_load. Patch #9 is for 'Image'-specific loading. Patch #10 to #11 are for KEXEC_VERIFY_SIG (arch-specific verification) support AKASHI Takahiro (11): resource: add walk_system_ram_res_rev() kexec_file: factor out arch_kexec_kernel_*() from x86, powerpc kexec_file: factor out crashdump elf header function from x86 asm-generic: add kexec_file_load system call to unistd.h arm64: kexec_file: create purgatory arm64: kexec_file: load initrd, device-tree and purgatory segments arm64: kexec_file: set up for crash dump adding elf core header arm64: kexec_file: enable KEXEC_FILE config arm64: kexec_file: add Image format support include: pe.h: remove message[] from mz header definition arm64: kexec_file: enable KEXEC_VERIFY_SIG for Image arch/arm64/Kconfig | 30 +++ arch/arm64/Makefile | 1 + arch/arm64/include/asm/kexec.h | 93 +++++++ arch/arm64/kernel/Makefile | 4 +- arch/arm64/kernel/kexec_image.c | 105 ++++++++ arch/arm64/kernel/machine_kexec_file.c | 368 ++++++++++++++++++++++++++++ arch/arm64/purgatory/Makefile | 24 ++ arch/arm64/purgatory/entry.S | 55 +++++ arch/powerpc/include/asm/kexec.h | 2 +- arch/powerpc/kernel/kexec_elf_64.c | 2 +- arch/powerpc/kernel/machine_kexec_file_64.c | 39 +-- arch/x86/include/asm/kexec-bzimage64.h | 2 +- arch/x86/kernel/crash.c | 324 ------------------------ arch/x86/kernel/kexec-bzimage64.c | 2 +- arch/x86/kernel/machine_kexec_64.c | 45 +--- include/linux/ioport.h | 3 + include/linux/kexec.h | 32 ++- include/linux/pe.h | 2 +- include/uapi/asm-generic/unistd.h | 4 +- kernel/kexec_file.c | 365 ++++++++++++++++++++++++++- kernel/kexec_internal.h | 20 ++ kernel/resource.c | 57 +++++ 22 files changed, 1158 insertions(+), 421 deletions(-) create mode 100644 arch/arm64/kernel/kexec_image.c create mode 100644 arch/arm64/kernel/machine_kexec_file.c create mode 100644 arch/arm64/purgatory/Makefile create mode 100644 arch/arm64/purgatory/entry.S -- 2.14.1