Re: [PATCH v7 05/11] arm64: elf: Enable BTI at exec based on ELF program properties

[Kees Cook <keescook@chromium.org>]

On Wed, Feb 26, 2020 at 03:57:08PM +0000, Mark Brown wrote:
> From: Dave Martin <Dave.Martin@arm.com>
> 
> For BTI protection to be as comprehensive as possible, it is
> desirable to have BTI enabled from process startup.  If this is not
> done, the process must use mprotect() to enable BTI for each of its
> executable mappings, but this is painful to do in the libc startup
> code.  It's simpler and more sound to have the kernel do it
> instead.
> 
> To this end, detect BTI support in the executable (or ELF
> interpreter, as appropriate), via the
> NT_GNU_PROGRAM_PROPERTY_TYPE_0 note, and tweak the initial prot
> flags for the process' executable pages to include PROT_BTI as
> appropriate.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/Kconfig           |  3 +++
>  arch/arm64/include/asm/elf.h | 51 ++++++++++++++++++++++++++++++++++++
>  arch/arm64/kernel/process.c  | 19 ++++++++++++++
>  include/uapi/linux/elf.h     |  6 +++++
>  4 files changed, 79 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index e37f4f07b990..d65d226a77ec 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -9,6 +9,7 @@ config ARM64
>  	select ACPI_MCFG if (ACPI && PCI)
>  	select ACPI_SPCR_TABLE if ACPI
>  	select ACPI_PPTT if ACPI
> +	select ARCH_BINFMT_ELF_STATE
>  	select ARCH_CLOCKSOURCE_DATA
>  	select ARCH_HAS_DEBUG_VIRTUAL
>  	select ARCH_HAS_DEVMEM_IS_ALLOWED
> @@ -33,6 +34,7 @@ config ARM64
>  	select ARCH_HAS_SYSCALL_WRAPPER
>  	select ARCH_HAS_TEARDOWN_DMA_OPS if IOMMU_SUPPORT
>  	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
> +	select ARCH_HAVE_ELF_PROT
>  	select ARCH_HAVE_NMI_SAFE_CMPXCHG
>  	select ARCH_INLINE_READ_LOCK if !PREEMPTION
>  	select ARCH_INLINE_READ_LOCK_BH if !PREEMPTION
> @@ -62,6 +64,7 @@ config ARM64
>  	select ARCH_INLINE_SPIN_UNLOCK_IRQRESTORE if !PREEMPTION
>  	select ARCH_KEEP_MEMBLOCK
>  	select ARCH_USE_CMPXCHG_LOCKREF
> +	select ARCH_USE_GNU_PROPERTY if BINFMT_ELF
>  	select ARCH_USE_QUEUED_RWLOCKS
>  	select ARCH_USE_QUEUED_SPINLOCKS
>  	select ARCH_SUPPORTS_MEMORY_FAILURE
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index b618017205a3..c72e381fa86d 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -114,7 +114,11 @@
>  
>  #ifndef __ASSEMBLY__
>  
> +#include <uapi/linux/elf.h>
>  #include <linux/bug.h>
> +#include <linux/errno.h>
> +#include <linux/fs.h>
> +#include <linux/types.h>
>  #include <asm/processor.h> /* for signal_minsigstksz, used by ARCH_DLINFO */
>  
>  typedef unsigned long elf_greg_t;
> @@ -224,6 +228,53 @@ extern int aarch32_setup_additional_pages(struct linux_binprm *bprm,
>  
>  #endif /* CONFIG_COMPAT */
>  
> +struct arch_elf_state {
> +	int flags;
> +};
> +
> +#define ARM64_ELF_BTI		(1 << 0)
> +
> +#define INIT_ARCH_ELF_STATE {			\
> +	.flags = 0,				\
> +}
> +
> +static inline int arch_parse_elf_property(u32 type, const void *data,
> +					  size_t datasz, bool compat,
> +					  struct arch_elf_state *arch)
> +{
> +	/* No known properties for AArch32 yet */
> +	if (IS_ENABLED(CONFIG_COMPAT) && compat)
> +		return 0;
> +
> +	if (type == GNU_PROPERTY_AARCH64_FEATURE_1_AND) {
> +		const u32 *p = data;
> +
> +		if (datasz != sizeof(*p))
> +			return -ENOEXEC;
> +
> +		if (IS_ENABLED(CONFIG_ARM64_BTI) &&
> +		    system_supports_bti() &&
> +		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
> +			arch->flags |= ARM64_ELF_BTI;
> +	}
> +
> +	return 0;
> +}
> +
> +static inline int arch_elf_pt_proc(void *ehdr, void *phdr,
> +				   struct file *f, bool is_interp,
> +				   struct arch_elf_state *state)
> +{
> +	return 0;
> +}
> +
> +static inline int arch_check_elf(void *ehdr, bool has_interp,
> +				 void *interp_ehdr,
> +				 struct arch_elf_state *state)
> +{
> +	return 0;
> +}
> +
>  #endif /* !__ASSEMBLY__ */
>  
>  #endif
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 00626057a384..b8e3faa8d406 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -11,6 +11,7 @@
>  
>  #include <linux/compat.h>
>  #include <linux/efi.h>
> +#include <linux/elf.h>
>  #include <linux/export.h>
>  #include <linux/sched.h>
>  #include <linux/sched/debug.h>
> @@ -18,6 +19,7 @@
>  #include <linux/sched/task_stack.h>
>  #include <linux/kernel.h>
>  #include <linux/lockdep.h>
> +#include <linux/mman.h>
>  #include <linux/mm.h>
>  #include <linux/stddef.h>
>  #include <linux/sysctl.h>
> @@ -654,3 +656,20 @@ asmlinkage void __sched arm64_preempt_schedule_irq(void)
>  	if (system_capabilities_finalized())
>  		preempt_schedule_irq();
>  }
> +
> +#ifdef CONFIG_BINFMT_ELF
> +int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> +			 bool has_interp, bool is_interp)
> +{
> +	if (is_interp != has_interp)
> +		return prot;
> +
> +	if (!(state->flags & ARM64_ELF_BTI))
> +		return prot;
> +
> +	if (prot & PROT_EXEC)
> +		prot |= PROT_BTI;
> +
> +	return prot;
> +}
> +#endif
> diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
> index 20900f4496b7..c6dd0215482e 100644
> --- a/include/uapi/linux/elf.h
> +++ b/include/uapi/linux/elf.h
> @@ -448,4 +448,10 @@ typedef struct elf64_note {
>    Elf64_Word n_type;	/* Content type */
>  } Elf64_Nhdr;
>  
> +/* .note.gnu.property types for EM_AARCH64: */
> +#define GNU_PROPERTY_AARCH64_FEATURE_1_AND	0xc0000000
> +
> +/* Bits for GNU_PROPERTY_AARCH64_FEATURE_1_BTI */
> +#define GNU_PROPERTY_AARCH64_FEATURE_1_BTI	(1U << 0)
> +
>  #endif /* _UAPI_LINUX_ELF_H */
> -- 
> 2.20.1
> 

-- 
Kees Cook