| Message ID | 20230125182201.800076-3-mark.rutland@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64: pointer auth cleanup | expand |
On Wed, 25 Jan 2023 at 19:22, Mark Rutland <mark.rutland@arm.com> wrote: > > Currently, when CONFIG_ARM64_PTR_AUTH_KERNEL=y (and > CONFIG_UNWIND_PATCH_PAC_INTO_SCS=n), we enable pointer authentication > for all functions, including leaf functions. This isn't necessary, and > is unfortunate for a few reasons: > > * Any PACIASP instruction is implicitly a `BTI C` landing pad, and > forcing the addition of a PACIASP in every function introduces a > larger set of BTI gadgets than is necessary. > > * The PACIASP and AUTIASP instructions make leaf functions larger than > necessary, bloating the kernel Image. For a defconfig v6.2-rc3 kernel, > this appears to add ~64KiB relative to not signing leaf functions, > which is unfortunate but not entirely onerous. > > * The PACIASP and AUTIASP instructions potentially make leaf functions > more expensive in terms of performance and/or power. For many trivial > leaf functions, this is clearly unnecessary, e.g. > > | <arch_local_save_flags>: > | d503233f paciasp > | d53b4220 mrs x0, daif > | d50323bf autiasp > | d65f03c0 ret > > | <calibration_delay_done>: > | d503233f paciasp > | d50323bf autiasp > | d65f03c0 ret > | d503201f nop > > * When CONFIG_UNWIND_PATCH_PAC_INTO_SCS=y we disable pointer > authentication for leaf functions, so clearly this is not functionally > necessary, indicates we have an inconsistent threat model, and > convolutes the Makefile logic. > > We've used pointer authentication in leave functions since the leaf > introduction of in-kernel pointer authentication in commit: > > 74afda4016a7437e ("arm64: compile the kernel with ptrauth return address signing") > > ... but at the time we had no rationale for signing leaf functions. > > Subsequently, we considered avoiding signing leaf functions: > > https://lore.kernel.org/linux-arm-kernel/1586856741-26839-1-git-send-email-amit.kachhap@arm.com/ > https://lore.kernel.org/linux-arm-kernel/1588149371-20310-1-git-send-email-amit.kachhap@arm.com/ > > ... however at the time we didn't have an abundance of reasons to avoid > signing leaf functions as above (e.g. the BTI case), we had no hardware > to make performance measurements, and it was reasoned that this gave > some level of protection against a limited set of code-reuse gadgets > which would fall through to a RET. We documented this in commit: > > 717b938e22f8dbf0 ("arm64: Document why we enable PAC support for leaf functions") > > Notably, this was before we supported any forward-edge CFI scheme (e.g. > Arm BTI, or Clang CFI/kCFI), which would prevent jumping into the middle > of a function. > > In addition, even with signing forced for leaf functions, AUTIASP may be > placed before a number of instructions which might constitute such a > gadget, e.g. > > | <user_regs_reset_single_step>: > | f9400022 ldr x2, [x1] > | d503233f paciasp > | d50323bf autiasp > | f9408401 ldr x1, [x0, #264] > | 720b005f tst w2, #0x200000 > | b26b0022 orr x2, x1, #0x200000 > | 926af821 and x1, x1, #0xffffffffffdfffff > | 9a820021 csel x1, x1, x2, eq // eq = none > | f9008401 str x1, [x0, #264] > | d65f03c0 ret > > | <fpsimd_cpu_dead>: > | 2a0003e3 mov w3, w0 > | 9000ff42 adrp x2, ffff800009ffd000 <xen_dynamic_chip+0x48> > | 9120e042 add x2, x2, #0x838 > | 52800000 mov w0, #0x0 // #0 > | d503233f paciasp > | f000d041 adrp x1, ffff800009a20000 <this_cpu_vector> > | d50323bf autiasp > | 9102c021 add x1, x1, #0xb0 > | f8635842 ldr x2, [x2, w3, uxtw #3] > | f821685f str xzr, [x2, x1] > | d65f03c0 ret > | d503201f nop > > So generally, trying to use AUTIASP to detect such gadgetization is not > robust, and this is dealt with far better by forward-edge CFI (which is > designed to prevent such cases). We should bite the buller and stop bullet > pretending that AUTIASP is a mitigation for such forward-edge > gadgetisation. > Nit: this has an 's' whereas the previous occurrence had a 'z' > For the above reasons, this patch has the kernel consistently sign > non-leaf functions and avoid signing leaf functions. > > Considering a defconfig v6.2-rc3 kernel built with LLVM 15.0.6: > > * The vmlinux is ~43KiB smaller: > > | [mark@lakrids:~/src/linux]% ls -al vmlinux-* > | -rwxr-xr-x 1 mark mark 338547808 Jan 25 17:17 vmlinux-after > | -rwxr-xr-x 1 mark mark 338591472 Jan 25 17:22 vmlinux-before > > * The resulting Image is 64KiB smaller: > > | [mark@lakrids:~/src/linux]% ls -al Image-* > | -rwxr-xr-x 1 mark mark 32702976 Jan 25 17:17 Image-after > | -rwxr-xr-x 1 mark mark 32768512 Jan 25 17:22 Image-before > > * There are ~400 fewer BTI gadgets: > > | [mark@lakrids:~/src/linux]% usekorg 12.1.0 aarch64-linux-objdump -d vmlinux-before 2> /dev/null | grep -ow 'paciasp\|bti\sc\?' | sort | uniq -c > | 1219 bti c > | 61982 paciasp > > | [mark@lakrids:~/src/linux]% usekorg 12.1.0 aarch64-linux-objdump -d vmlinux-after 2> /dev/null | grep -ow 'paciasp\|bti\sc\?' | sort | uniq -c > | 10099 bti c > | 52699 paciasp > > Which is +8880 BTIs, and -9283 PACIASPs, for -403 unnecessary BTI > gadgets. While this is relatively small relative to the total, > distinguishing the two cases will make it easier to analyse and reduce > this set further in future. > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Amit Daniel Kachhap <amit.kachhap@arm.com> > Cc: Ard Biesheuvel <ardb@kernel.org> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Mark Brown <broonie@kernel.org> > Cc: Will Deacon <will@kernel.org> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> > --- > arch/arm64/Makefile | 28 ++++++++-------------------- > 1 file changed, 8 insertions(+), 20 deletions(-) > > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile > index e176eb76345b5..ab1f12b4f339a 100644 > --- a/arch/arm64/Makefile > +++ b/arch/arm64/Makefile > @@ -63,30 +63,18 @@ stack_protector_prepare: prepare0 > include/generated/asm-offsets.h)) > endif > > -# Ensure that if the compiler supports branch protection we default it > -# off, this will be overridden if we are using branch protection. > -branch-prot-flags-y += $(call cc-option,-mbranch-protection=none) > - > -ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y) > -branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all > -# We enable additional protection for leaf functions as there is some > -# narrow potential for ROP protection benefits and no substantial > -# performance impact has been observed. > -PACRET-y := pac-ret+leaf > - > -# Using a shadow call stack in leaf functions is too costly, so avoid PAC there > -# as well when we may be patching PAC into SCS > -PACRET-$(CONFIG_UNWIND_PATCH_PAC_INTO_SCS) := pac-ret > - > ifeq ($(CONFIG_ARM64_BTI_KERNEL),y) > -branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET_BTI) := -mbranch-protection=$(PACRET-y)+bti > + KBUILD_CFLAGS += -mbranch-protection=pac-ret+bti > +else ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y) > + ifeq ($(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET),y) > + KBUILD_CFLAGS += -mbranch-protection=pac-ret > + else > + KBUILD_CFLAGS += -msign-return-address=non-leaf > + endif > else > -branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=$(PACRET-y) > -endif > + KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none) > endif > > -KBUILD_CFLAGS += $(branch-prot-flags-y) > - > # Tell the assembler to support instructions from the latest target > # architecture. > # > -- > 2.30.2 >
On Thu, Jan 26, 2023 at 09:40:33AM +0100, Ard Biesheuvel wrote: > On Wed, 25 Jan 2023 at 19:22, Mark Rutland <mark.rutland@arm.com> wrote: > > We've used pointer authentication in leave functions since the > > leaf Thanks; fixed locally. [...] > > So generally, trying to use AUTIASP to detect such gadgetization is not > > robust, and this is dealt with far better by forward-edge CFI (which is > > designed to prevent such cases). We should bite the buller and stop > > bullet Thanks; fixed locally. > > pretending that AUTIASP is a mitigation for such forward-edge > > gadgetisation. > > > > Nit: this has an 's' whereas the previous occurrence had a 'z' Thanks; I've made those both use 'z'. [...] > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > > Cc: Amit Daniel Kachhap <amit.kachhap@arm.com> > > Cc: Ard Biesheuvel <ardb@kernel.org> > > Cc: Catalin Marinas <catalin.marinas@arm.com> > > Cc: Mark Brown <broonie@kernel.org> > > Cc: Will Deacon <will@kernel.org> > > Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Thanks! Mark.
On Wed, Jan 25, 2023 at 06:22:01PM +0000, Mark Rutland wrote: > Currently, when CONFIG_ARM64_PTR_AUTH_KERNEL=y (and > CONFIG_UNWIND_PATCH_PAC_INTO_SCS=n), we enable pointer authentication > for all functions, including leaf functions. This isn't necessary, and > is unfortunate for a few reasons: Reviewed-by: Mark Brown <broonie@kernel.org>
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile index e176eb76345b5..ab1f12b4f339a 100644 --- a/arch/arm64/Makefile +++ b/arch/arm64/Makefile @@ -63,30 +63,18 @@ stack_protector_prepare: prepare0 include/generated/asm-offsets.h)) endif -# Ensure that if the compiler supports branch protection we default it -# off, this will be overridden if we are using branch protection. -branch-prot-flags-y += $(call cc-option,-mbranch-protection=none) - -ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y) -branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all -# We enable additional protection for leaf functions as there is some -# narrow potential for ROP protection benefits and no substantial -# performance impact has been observed. -PACRET-y := pac-ret+leaf - -# Using a shadow call stack in leaf functions is too costly, so avoid PAC there -# as well when we may be patching PAC into SCS -PACRET-$(CONFIG_UNWIND_PATCH_PAC_INTO_SCS) := pac-ret - ifeq ($(CONFIG_ARM64_BTI_KERNEL),y) -branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET_BTI) := -mbranch-protection=$(PACRET-y)+bti + KBUILD_CFLAGS += -mbranch-protection=pac-ret+bti +else ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y) + ifeq ($(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET),y) + KBUILD_CFLAGS += -mbranch-protection=pac-ret + else + KBUILD_CFLAGS += -msign-return-address=non-leaf + endif else -branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=$(PACRET-y) -endif + KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none) endif -KBUILD_CFLAGS += $(branch-prot-flags-y) - # Tell the assembler to support instructions from the latest target # architecture. #
Currently, when CONFIG_ARM64_PTR_AUTH_KERNEL=y (and CONFIG_UNWIND_PATCH_PAC_INTO_SCS=n), we enable pointer authentication for all functions, including leaf functions. This isn't necessary, and is unfortunate for a few reasons: * Any PACIASP instruction is implicitly a `BTI C` landing pad, and forcing the addition of a PACIASP in every function introduces a larger set of BTI gadgets than is necessary. * The PACIASP and AUTIASP instructions make leaf functions larger than necessary, bloating the kernel Image. For a defconfig v6.2-rc3 kernel, this appears to add ~64KiB relative to not signing leaf functions, which is unfortunate but not entirely onerous. * The PACIASP and AUTIASP instructions potentially make leaf functions more expensive in terms of performance and/or power. For many trivial leaf functions, this is clearly unnecessary, e.g. | <arch_local_save_flags>: | d503233f paciasp | d53b4220 mrs x0, daif | d50323bf autiasp | d65f03c0 ret | <calibration_delay_done>: | d503233f paciasp | d50323bf autiasp | d65f03c0 ret | d503201f nop * When CONFIG_UNWIND_PATCH_PAC_INTO_SCS=y we disable pointer authentication for leaf functions, so clearly this is not functionally necessary, indicates we have an inconsistent threat model, and convolutes the Makefile logic. We've used pointer authentication in leave functions since the introduction of in-kernel pointer authentication in commit: 74afda4016a7437e ("arm64: compile the kernel with ptrauth return address signing") ... but at the time we had no rationale for signing leaf functions. Subsequently, we considered avoiding signing leaf functions: https://lore.kernel.org/linux-arm-kernel/1586856741-26839-1-git-send-email-amit.kachhap@arm.com/ https://lore.kernel.org/linux-arm-kernel/1588149371-20310-1-git-send-email-amit.kachhap@arm.com/ ... however at the time we didn't have an abundance of reasons to avoid signing leaf functions as above (e.g. the BTI case), we had no hardware to make performance measurements, and it was reasoned that this gave some level of protection against a limited set of code-reuse gadgets which would fall through to a RET. We documented this in commit: 717b938e22f8dbf0 ("arm64: Document why we enable PAC support for leaf functions") Notably, this was before we supported any forward-edge CFI scheme (e.g. Arm BTI, or Clang CFI/kCFI), which would prevent jumping into the middle of a function. In addition, even with signing forced for leaf functions, AUTIASP may be placed before a number of instructions which might constitute such a gadget, e.g. | <user_regs_reset_single_step>: | f9400022 ldr x2, [x1] | d503233f paciasp | d50323bf autiasp | f9408401 ldr x1, [x0, #264] | 720b005f tst w2, #0x200000 | b26b0022 orr x2, x1, #0x200000 | 926af821 and x1, x1, #0xffffffffffdfffff | 9a820021 csel x1, x1, x2, eq // eq = none | f9008401 str x1, [x0, #264] | d65f03c0 ret | <fpsimd_cpu_dead>: | 2a0003e3 mov w3, w0 | 9000ff42 adrp x2, ffff800009ffd000 <xen_dynamic_chip+0x48> | 9120e042 add x2, x2, #0x838 | 52800000 mov w0, #0x0 // #0 | d503233f paciasp | f000d041 adrp x1, ffff800009a20000 <this_cpu_vector> | d50323bf autiasp | 9102c021 add x1, x1, #0xb0 | f8635842 ldr x2, [x2, w3, uxtw #3] | f821685f str xzr, [x2, x1] | d65f03c0 ret | d503201f nop So generally, trying to use AUTIASP to detect such gadgetization is not robust, and this is dealt with far better by forward-edge CFI (which is designed to prevent such cases). We should bite the buller and stop pretending that AUTIASP is a mitigation for such forward-edge gadgetisation. For the above reasons, this patch has the kernel consistently sign non-leaf functions and avoid signing leaf functions. Considering a defconfig v6.2-rc3 kernel built with LLVM 15.0.6: * The vmlinux is ~43KiB smaller: | [mark@lakrids:~/src/linux]% ls -al vmlinux-* | -rwxr-xr-x 1 mark mark 338547808 Jan 25 17:17 vmlinux-after | -rwxr-xr-x 1 mark mark 338591472 Jan 25 17:22 vmlinux-before * The resulting Image is 64KiB smaller: | [mark@lakrids:~/src/linux]% ls -al Image-* | -rwxr-xr-x 1 mark mark 32702976 Jan 25 17:17 Image-after | -rwxr-xr-x 1 mark mark 32768512 Jan 25 17:22 Image-before * There are ~400 fewer BTI gadgets: | [mark@lakrids:~/src/linux]% usekorg 12.1.0 aarch64-linux-objdump -d vmlinux-before 2> /dev/null | grep -ow 'paciasp\|bti\sc\?' | sort | uniq -c | 1219 bti c | 61982 paciasp | [mark@lakrids:~/src/linux]% usekorg 12.1.0 aarch64-linux-objdump -d vmlinux-after 2> /dev/null | grep -ow 'paciasp\|bti\sc\?' | sort | uniq -c | 10099 bti c | 52699 paciasp Which is +8880 BTIs, and -9283 PACIASPs, for -403 unnecessary BTI gadgets. While this is relatively small relative to the total, distinguishing the two cases will make it easier to analyse and reduce this set further in future. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Amit Daniel Kachhap <amit.kachhap@arm.com> Cc: Ard Biesheuvel <ardb@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Mark Brown <broonie@kernel.org> Cc: Will Deacon <will@kernel.org> --- arch/arm64/Makefile | 28 ++++++++-------------------- 1 file changed, 8 insertions(+), 20 deletions(-)