Re: [PATCH v10 00/13] arm64: Branch Target Identification support

Szabolcs Nagy
2020-03-20T17:40:20Z

The 03/16/2020 16:50, Mark Brown wrote:
> This patch series implements support for ARMv8.5-A Branch Target
> Identification (BTI), which is a control flow integrity protection
> feature introduced as part of the ARMv8.5-A extensions.

i was playing with this and it seems the kernel does not add
PROT_BTI to non-static executables (i.e. there is an interpreter).

i thought any elf that the kernel maps would get PROT_BTI from the
kernel. (i want to remove the mprotect in glibc when not necessary)

i tested by linking a hello world exe with -Wl,-z,force-bti (and
verified that the property note is there) and expected it to crash
(with SIGILL) when the dynamic linker jumps to _start in the exe,
but it executed without errors (if i do the mprotect in glibc then
i get SIGILL as expected).

is this deliberate? does the kernel map static exe and dynamic
linked exe differently?

i cant tell looking at the patches where this logic comes from.

>
> Changes:
>
> v10:
> - Fix build for !COMPAT configurations.
> v9:
> - Move Kconfig addition to final patch in series.
> - Add patch from Daniel Kiss adding BTI information to smaps, this has
> a trivial conflict with a .rst conversion in -next.
> v8:
> - Remove a redundant IS_ENABLED(CONFIG_ARM64_BTI) check.
> v7:
> - Rebase onto v5.6-rc3.
> - Move comment about keeping NT_GNU_PROPERTY_TYPE_0 internal into first
> patch.
> - Add an explicit check for system_supports_bti() when parsing BTI ELF
> property for improved robustness.
> v6:
> - Rebase onto v5.6-rc1.
> - Fix typos s/BYTPE/BTYPE/ in commit log for "arm64: BTI: Decode BYTPE
> bits when printing PSTATE".
> v5:
> - Changed a bunch of -EIO to -ENOEXEC in the ELF parsing code.
> - Move PSR_BTYPE defines to UAPI.
> - Use compat_user_mode() rather than open coding.
> - Fix a typo s/BYTPE/BTYPE/ in syscall.c
> v4:
> - Dropped patch fixing existing documentation as it has already been merged.
> - Convert WARN_ON() to WARN_ON_ONCE() in "ELF: Add ELF program property
> parsing support".
> - Added display of guarded pages to ptdump.
> - Updated for conversion of exception handling from assembler to C.
>
> Notes:
>
> * GCC 9 can compile backwards-compatible BTI-enabled code with
> -mbranch-protection=bti or -mbranch-protection=standard.
>
> * Binutils 2.33 and later support the new ELF note.
>
> Creation of a BTI-enabled binary requires _everything_ linked in to
> be BTI-enabled. For now ld --force-bti can be used to override this,
> but some things may break until the required C library support is in
> place.
>
> There is no straightforward way to mark a .s file as BTI-enabled:
> scraping the output from gcc -S works as a quick hack for now.
>
> readelf -n can be used to examing the program properties in an ELF
> file.
>
> * Runtime mmap() and mprotect() can be used to enable BTI on a
> page-by-page basis using the new PROT_BTI, but the code in the
> affected pages still needs to be written or compiled to contain the
> appropriate BTI landing pads.
>
> Daniel Kiss (1):
> mm: smaps: Report arm64 guarded pages in smaps
>
> Dave Martin (11):
> ELF: UAPI and Kconfig additions for ELF program properties
> ELF: Add ELF program property parsing support
> arm64: Basic Branch Target Identification support
> elf: Allow arch to tweak initial mmap prot flags
> arm64: elf: Enable BTI at exec based on ELF program properties
> arm64: BTI: Decode BYTPE bits when printing PSTATE
> arm64: unify native/compat instruction skipping
> arm64: traps: Shuffle code to eliminate forward declarations
> arm64: BTI: Reset BTYPE when skipping emulated instructions
> KVM: arm64: BTI: Reset BTYPE when skipping emulated instructions
> arm64: BTI: Add Kconfig entry for userspace BTI
>
> Mark Brown (1):
> arm64: mm: Display guarded pages in ptdump
>
> Documentation/arm64/cpu-feature-registers.rst | 2 +
> Documentation/arm64/elf_hwcaps.rst | 5 +
> Documentation/filesystems/proc.txt | 1 +
> arch/arm64/Kconfig | 25 +++
> arch/arm64/include/asm/cpucaps.h | 3 +-
> arch/arm64/include/asm/cpufeature.h | 6 +
> arch/arm64/include/asm/elf.h | 50 ++++++
> arch/arm64/include/asm/esr.h | 2 +-
> arch/arm64/include/asm/exception.h | 1 +
> arch/arm64/include/asm/hwcap.h | 1 +
> arch/arm64/include/asm/kvm_emulate.h | 6 +-
> arch/arm64/include/asm/mman.h | 37 +++++
> arch/arm64/include/asm/pgtable-hwdef.h | 1 +
> arch/arm64/include/asm/pgtable.h | 2 +-
> arch/arm64/include/asm/ptrace.h | 1 +
> arch/arm64/include/asm/sysreg.h | 4 +
> arch/arm64/include/uapi/asm/hwcap.h | 1 +
> arch/arm64/include/uapi/asm/mman.h | 9 ++
> arch/arm64/include/uapi/asm/ptrace.h | 9 ++
> arch/arm64/kernel/cpufeature.c | 33 ++++
> arch/arm64/kernel/cpuinfo.c | 1 +
> arch/arm64/kernel/entry-common.c | 11 ++
> arch/arm64/kernel/process.c | 36 ++++-
> arch/arm64/kernel/ptrace.c | 2 +-
> arch/arm64/kernel/signal.c | 16 ++
> arch/arm64/kernel/syscall.c | 18 +++
> arch/arm64/kernel/traps.c | 131 ++++++++--------
> arch/arm64/mm/dump.c | 5 +
> fs/Kconfig.binfmt | 6 +
> fs/binfmt_elf.c | 145 +++++++++++++++++-
> fs/compat_binfmt_elf.c | 4 +
> fs/proc/task_mmu.c | 3 +
> include/linux/elf.h | 43 ++++++
> include/linux/mm.h | 3 +
> include/uapi/linux/elf.h | 11 ++
> 35 files changed, 560 insertions(+), 74 deletions(-)
> create mode 100644 arch/arm64/include/asm/mman.h
> create mode 100644 arch/arm64/include/uapi/asm/mman.h
>
>
> base-commit: f8788d86ab28f61f7b46eb6be375f8a726783636
> --
> 2.20.1
>