[v3,0/4] arm64: Enable BTI for the executable as well as the interpreter

  Deployments of BTI on arm64 have run into issues interacting with
systemd's MemoryDenyWriteExecute feature.  Currently for dynamically
linked executables the kernel will only handle architecture specific
properties like BTI for the interpreter, the expectation is that the
interpreter will then handle any properties on the main executable.
For BTI this means remapping the executable segments PROT_EXEC |
PROT_BTI.

This interacts poorly with MemoryDenyWriteExecute since that is
implemented using a seccomp filter which prevents setting PROT_EXEC on
already mapped memory and lacks the context to be able to detect that
memory is already mapped with PROT_EXEC.  This series resolves this by
handling the BTI property for both the interpreter and the main
executable.

This does mean that we may get more code with BTI enabled if running on
a system without BTI support in the dynamic linker, this is expected to
be a safe configuration and testing seems to confirm that. It also
reduces the flexibility userspace has to disable BTI but it is expected
that for cases where there are problems which require BTI to be disabled
it is more likely that it will need to be disabled on a system level.

v3:
 - Fix passing of properties for parsing by the main executable.
 - Drop has_interp from arch_parse_elf_property().
 - Coding style tweaks.
v2:
 - Add a patch dropping has_interp from arch_adjust_elf_prot()
 - Fix bisection issue with static executables on arm64 in the first
   patch.

Mark Brown (4):
  elf: Allow architectures to parse properties on the main executable
  arm64: Enable BTI for main executable as well as the interpreter
  elf: Remove has_interp property from arch_adjust_elf_prot()
  elf: Remove has_interp property from arch_parse_elf_property()

 arch/arm64/include/asm/elf.h | 13 ++++++++++---
 arch/arm64/kernel/process.c  | 23 +++++++++++------------
 fs/binfmt_elf.c              | 33 ++++++++++++++++++++++++---------
 include/linux/elf.h          |  8 +++++---
 4 files changed, 50 insertions(+), 27 deletions(-)


base-commit: c4681547bcce777daf576925a966ffa824edd09d
  

Hi,

On 6/14/21 5:32 PM, Mark Brown wrote:
> Deployments of BTI on arm64 have run into issues interacting with
> systemd's MemoryDenyWriteExecute feature.  Currently for dynamically
> linked executables the kernel will only handle architecture specific
> properties like BTI for the interpreter, the expectation is that the
> interpreter will then handle any properties on the main executable.
> For BTI this means remapping the executable segments PROT_EXEC |
> PROT_BTI.
> 
> This interacts poorly with MemoryDenyWriteExecute since that is
> implemented using a seccomp filter which prevents setting PROT_EXEC on
> already mapped memory and lacks the context to be able to detect that
> memory is already mapped with PROT_EXEC.  This series resolves this by
> handling the BTI property for both the interpreter and the main
> executable.
> 
> This does mean that we may get more code with BTI enabled if running on
> a system without BTI support in the dynamic linker, this is expected to
> be a safe configuration and testing seems to confirm that. It also
> reduces the flexibility userspace has to disable BTI but it is expected
> that for cases where there are problems which require BTI to be disabled
> it is more likely that it will need to be disabled on a system level.

It looks like its working as expected now (the previously detailed test 
is now failing) in a MDWE enviroment, and the smaps/etc looks as 
expected too.

Thanks for fixing this!

tested-by: Jeremy Linton <jeremy.linton@arm.com>


> 
> v3:
>   - Fix passing of properties for parsing by the main executable.
>   - Drop has_interp from arch_parse_elf_property().
>   - Coding style tweaks.
> v2:
>   - Add a patch dropping has_interp from arch_adjust_elf_prot()
>   - Fix bisection issue with static executables on arm64 in the first
>     patch.
> 
> Mark Brown (4):
>    elf: Allow architectures to parse properties on the main executable
>    arm64: Enable BTI for main executable as well as the interpreter
>    elf: Remove has_interp property from arch_adjust_elf_prot()
>    elf: Remove has_interp property from arch_parse_elf_property()
> 
>   arch/arm64/include/asm/elf.h | 13 ++++++++++---
>   arch/arm64/kernel/process.c  | 23 +++++++++++------------
>   fs/binfmt_elf.c              | 33 ++++++++++++++++++++++++---------
>   include/linux/elf.h          |  8 +++++---
>   4 files changed, 50 insertions(+), 27 deletions(-)
> 
> 
> base-commit: c4681547bcce777daf576925a966ffa824edd09d
>