[PATCH 2/2] arm64: pauth: don't sign leaf functions

[Mark Rutland <mark.rutland@arm.com>]

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(-)

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


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