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

From: Szabolcs Nagy <[email-masked]>
Date: Fri, 20 Mar 2020 17:39:46 +0000

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
> 

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