| Message ID | 20180623022058.10935-14-takahiro.akashi@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi Akashi, On 23/06/18 03:20, AKASHI Takahiro wrote: > With this patch, kernel verification can be done without IMA security > subsystem enabled. Turn on CONFIG_KEXEC_VERIFY_SIG instead. > > 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. > > You can create a signed kernel image with: > $ sbsign --key ${KEY} --cert ${CERT} Image > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > index f68318f61c85..5133c22a01ab 100644 > --- a/arch/arm64/Kconfig > +++ b/arch/arm64/Kconfig > @@ -845,6 +845,30 @@ config KEXEC_FILE > for kernel and initramfs as opposed to list of segments as > accepted by previous system call. > > +config KEXEC_VERIFY_SIG > + bool "Verify kernel signature during kexec_file_load() syscall" > + depends on KEXEC_FILE > + help > + Select this option to verify a signature with loaded kernel > + image. If configured, any attempt of loading a image without > + valid signature will fail. > + > + In addition to that option, you need to enable signature > + verification for the corresponding kernel image type being > + loaded in order for this to work. > + > +config KEXEC_IMAGE_VERIFY_SIG > + bool "Enable Image signature verification support" > + default y > + depends on KEXEC_VERIFY_SIG > + depends on EFI && SIGNED_PE_FILE_VERIFICATION > + help > + Enable Image signature verification support. > + > +comment "Image signature verification is missing yet" > + depends on KEXEC_VERIFY_SIG > + depends on !EFI || !SIGNED_PE_FILE_VERIFICATION This comment thing is a good idea, but its also a bit confusing... it took me quite a while to work out what was missing. Could we phrase it something like: "Support for PE file signature verification disabled!" This tells us its about PE files, and its probably a missing config option somewhere, not some code that hasn't been written yet. (which was my first assumption!). KEXEC_VERIFY_SIG presumably turns on just the IMA verification, which verifies the Image, but not in the same way as KEXEC_IMAGE_VERIFY_SIG.... (if I've understood it properly) Is there any reason to have these as separate enables? Couldn't we 'select SIGNED_PE_FILE_VERIFICATION if EFI' in KEXEC_VERIFY_SIG? This would mean there is one option to verify signatures, instead of two... (does it really depend on EFI?) Thanks, James
On Tue, Jul 03, 2018 at 06:47:38PM +0100, James Morse wrote: > Hi Akashi, > > On 23/06/18 03:20, AKASHI Takahiro wrote: > > With this patch, kernel verification can be done without IMA security > > subsystem enabled. Turn on CONFIG_KEXEC_VERIFY_SIG instead. > > > > 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. > > > > You can create a signed kernel image with: > > $ sbsign --key ${KEY} --cert ${CERT} Image > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig > > index f68318f61c85..5133c22a01ab 100644 > > --- a/arch/arm64/Kconfig > > +++ b/arch/arm64/Kconfig > > @@ -845,6 +845,30 @@ config KEXEC_FILE > > for kernel and initramfs as opposed to list of segments as > > accepted by previous system call. > > > > +config KEXEC_VERIFY_SIG > > + bool "Verify kernel signature during kexec_file_load() syscall" > > + depends on KEXEC_FILE > > + help > > + Select this option to verify a signature with loaded kernel > > + image. If configured, any attempt of loading a image without > > + valid signature will fail. > > + > > + In addition to that option, you need to enable signature > > + verification for the corresponding kernel image type being > > + loaded in order for this to work. > > + > > +config KEXEC_IMAGE_VERIFY_SIG > > + bool "Enable Image signature verification support" > > + default y > > + depends on KEXEC_VERIFY_SIG > > + depends on EFI && SIGNED_PE_FILE_VERIFICATION > > + help > > + Enable Image signature verification support. > > + > > +comment "Image signature verification is missing yet" > > + depends on KEXEC_VERIFY_SIG > > + depends on !EFI || !SIGNED_PE_FILE_VERIFICATION > > > This comment thing is a good idea, but its also a bit confusing... it took me > quite a while to work out what was missing. Could we phrase it something like: > "Support for PE file signature verification disabled!" OK. > This tells us its about PE files, and its probably a missing config option > somewhere, not some code that hasn't been written yet. (which was my first > assumption!). > > KEXEC_VERIFY_SIG presumably turns on just the IMA verification, which verifies > the Image, but not in the same way as KEXEC_IMAGE_VERIFY_SIG.... (if I've > understood it properly) I'm afraid that I'm not clear at the cover letter. Those two mechanisms, IMA verification and kexec-specific verification, are totally different. The former is relatively new as well as generic, and doesn't even require KEXEC_VERIFY_SIG at all as all the stuff is done under IMA framework (via security hooks) with extended file attributes. On the other hand, KEXEC_VERIFY_SIG is just an option that turns on verification check in a kexec-specific (and more importantly arch-specific and file-format-specific) manner through 'kexec_file_ops->verify interface.' > Is there any reason to have these as separate enables? If you are talking about KEXEC_VERIFY_SIG and KEXEC_IMAGE_VERIFY_SIG, it is a leftover when "vmlinux" image was also supported in my old versions of kexec_file patch set. But please note that x86 also retains two separate configuration options, KEXEC_VERIFY_SIG and KEXEC_BZIMAGE_VERIFY_SIG. I simply followed that. > Couldn't we 'select SIGNED_PE_FILE_VERIFICATION if EFI' in KEXEC_VERIFY_SIG? I didn't "select" SIGNED_PE_FILE_VERIFICATION here following "kbuild/kconfig-language.txt" which suggests, "use select only for non-visible symbols (no prompts anywhere)." > This would mean there is one option to verify signatures, instead of two... > (does it really depend on EFI?) Strictly speaking, SIGNED_PE_FILE_VERIFICATION depends on the fact that a binary file is in PE format, which means that EFI is enabled on arm64. It is possible to support KEXEC_VERIFY_SIG for non-PE binaries, but in that case, we will have to invent a new (arm64-specific) way of verification. (For instance, we might want to add a kexec-specific ELF segment to vmlinux.) Thanks, -Takahiro AKASHI > > Thanks, > > James
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index f68318f61c85..5133c22a01ab 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -845,6 +845,30 @@ config KEXEC_FILE for kernel and initramfs as opposed to list of segments as accepted by previous system call. +config KEXEC_VERIFY_SIG + bool "Verify kernel signature during kexec_file_load() syscall" + depends on KEXEC_FILE + help + Select this option to verify a signature with loaded kernel + image. If configured, any attempt of loading a image without + valid signature will fail. + + In addition to that option, you need to enable signature + verification for the corresponding kernel image type being + loaded in order for this to work. + +config KEXEC_IMAGE_VERIFY_SIG + bool "Enable Image signature verification support" + default y + depends on KEXEC_VERIFY_SIG + depends on EFI && SIGNED_PE_FILE_VERIFICATION + help + Enable Image signature verification support. + +comment "Image signature verification is missing yet" + depends on KEXEC_VERIFY_SIG + depends on !EFI || !SIGNED_PE_FILE_VERIFICATION + config CRASH_DUMP bool "Build kdump crash kernel" help diff --git a/arch/arm64/kernel/kexec_image.c b/arch/arm64/kernel/kexec_image.c index df1e341d3a28..bb0a95add197 100644 --- a/arch/arm64/kernel/kexec_image.c +++ b/arch/arm64/kernel/kexec_image.c @@ -13,6 +13,7 @@ #include <linux/kernel.h> #include <linux/kexec.h> #include <linux/string.h> +#include <linux/verification.h> #include <asm/boot.h> #include <asm/byteorder.h> #include <asm/cpufeature.h> @@ -28,6 +29,9 @@ static int image_probe(const char *kernel_buf, unsigned long kernel_len) !memcmp(&h->magic, ARM64_MAGIC, sizeof(ARM64_MAGIC))) return -EINVAL; + pr_debug("PE format: %s\n", + memcmp(&h->mz_magic, "MZ", 2) ? "no" : "yes"); + return 0; } @@ -107,7 +111,18 @@ static void *image_load(struct kimage *image, return ERR_PTR(ret); } +#ifdef CONFIG_KEXEC_IMAGE_VERIFY_SIG +static int image_verify_sig(const char *kernel, unsigned long kernel_len) +{ + return verify_pefile_signature(kernel, kernel_len, NULL, + VERIFYING_KEXEC_PE_SIGNATURE); +} +#endif + const struct kexec_file_ops kexec_image_ops = { .probe = image_probe, .load = image_load, +#ifdef CONFIG_KEXEC_IMAGE_VERIFY_SIG + .verify_sig = image_verify_sig, +#endif };
With this patch, kernel verification can be done without IMA security subsystem enabled. Turn on CONFIG_KEXEC_VERIFY_SIG instead. 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. You can create a signed kernel image with: $ sbsign --key ${KEY} --cert ${CERT} Image Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will.deacon@arm.com> --- arch/arm64/Kconfig | 24 ++++++++++++++++++++++++ arch/arm64/kernel/kexec_image.c | 15 +++++++++++++++ 2 files changed, 39 insertions(+)