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

Kees Cook
2020-02-26T21:43:20Z

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