Re: [PATCH v18 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

From: Kees Cook
Date: Mon Jun 24 2019 - 11:04:15 EST

On Mon, Jun 24, 2019 at 04:32:47PM +0200, Andrey Konovalov wrote:
> From: Catalin Marinas <catalin.marinas@xxxxxxx>
>
> It is not desirable to relax the ABI to allow tagged user addresses into
> the kernel indiscriminately. This patch introduces a prctl() interface
> for enabling or disabling the tagged ABI with a global sysctl control
> for preventing applications from enabling the relaxed ABI (meant for
> testing user-space prctl() return error checking without reconfiguring
> the kernel). The ABI properties are inherited by threads of the same
> application and fork()'ed children but cleared on execve(). A Kconfig
> option allows the overall disabling of the relaxed ABI.
>
> The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
> MTE-specific settings like imprecise vs precise exceptions.
>
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> ---
> arch/arm64/Kconfig | 9 ++++
> arch/arm64/include/asm/processor.h | 8 ++++
> arch/arm64/include/asm/thread_info.h | 1 +
> arch/arm64/include/asm/uaccess.h | 4 +-
> arch/arm64/kernel/process.c | 72 ++++++++++++++++++++++++++++
> include/uapi/linux/prctl.h | 5 ++
> kernel/sys.c | 12 +++++
> 7 files changed, 110 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 697ea0510729..55fbaf20af2d 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1107,6 +1107,15 @@ config ARM64_SW_TTBR0_PAN
> zeroed area and reserved ASID. The user access routines
> restore the valid TTBR0_EL1 temporarily.
>
> +config ARM64_TAGGED_ADDR_ABI
> + bool "Enable the tagged user addresses syscall ABI"
> + default y
> + help
> + When this option is enabled, user applications can opt in to a
> + relaxed ABI via prctl() allowing tagged addresses to be passed
> + to system calls as pointer arguments. For details, see
> + Documentation/arm64/tagged-address-abi.txt.
> +
> menuconfig COMPAT
> bool "Kernel support for 32-bit EL0"
> depends on ARM64_4K_PAGES || EXPERT
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index fd5b1a4efc70..ee86070a28d4 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -296,6 +296,14 @@ extern void __init minsigstksz_setup(void);
> /* PR_PAC_RESET_KEYS prctl */
> #define PAC_RESET_KEYS(tsk, arg) ptrauth_prctl_reset_keys(tsk, arg)
>
> +#ifdef CONFIG_ARM64_TAGGED_ADDR_ABI
> +/* PR_{SET,GET}_TAGGED_ADDR_CTRL prctl */
> +long set_tagged_addr_ctrl(unsigned long arg);
> +long get_tagged_addr_ctrl(void);
> +#define SET_TAGGED_ADDR_CTRL(arg) set_tagged_addr_ctrl(arg)
> +#define GET_TAGGED_ADDR_CTRL() get_tagged_addr_ctrl()
> +#endif
> +
> /*
> * For CONFIG_GCC_PLUGIN_STACKLEAK
> *
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 2372e97db29c..4f81c4f15404 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -88,6 +88,7 @@ void arch_release_task_struct(struct task_struct *tsk);
> #define TIF_SVE 23 /* Scalable Vector Extension in use */
> #define TIF_SVE_VL_INHERIT 24 /* Inherit sve_vl_onexec across exec */
> #define TIF_SSBD 25 /* Wants SSB mitigation */
> +#define TIF_TAGGED_ADDR 26 /* Allow tagged user addresses */
>
> #define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
> #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index a138e3b4f717..097d6bfac0b7 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -62,7 +62,9 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
> {
> unsigned long ret, limit = current_thread_info()->addr_limit;
>
> - addr = untagged_addr(addr);
> + if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
> + test_thread_flag(TIF_TAGGED_ADDR))
> + addr = untagged_addr(addr);
>
> __chk_user_ptr(addr);
> asm volatile(
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 9856395ccdb7..60e70158a4a1 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -19,6 +19,7 @@
> #include <linux/kernel.h>
> #include <linux/mm.h>
> #include <linux/stddef.h>
> +#include <linux/sysctl.h>
> #include <linux/unistd.h>
> #include <linux/user.h>
> #include <linux/delay.h>
> @@ -307,11 +308,18 @@ static void tls_thread_flush(void)
> }
> }
>
> +static void flush_tagged_addr_state(void)
> +{
> + if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI))
> + clear_thread_flag(TIF_TAGGED_ADDR);
> +}
> +
> void flush_thread(void)
> {
> fpsimd_flush_thread();
> tls_thread_flush();
> flush_ptrace_hw_breakpoint(current);
> + flush_tagged_addr_state();
> }
>
> void release_thread(struct task_struct *dead_task)
> @@ -541,3 +549,67 @@ void arch_setup_new_exec(void)
>
> ptrauth_thread_init_user(current);
> }
> +
> +#ifdef CONFIG_ARM64_TAGGED_ADDR_ABI
> +/*
> + * Control the relaxed ABI allowing tagged user addresses into the kernel.
> + */
> +static unsigned int tagged_addr_prctl_allowed = 1;
> +
> +long set_tagged_addr_ctrl(unsigned long arg)
> +{
> + if (!tagged_addr_prctl_allowed)
> + return -EINVAL;
> + if (is_compat_task())
> + return -EINVAL;
> + if (arg & ~PR_TAGGED_ADDR_ENABLE)
> + return -EINVAL;
> +
> + update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);
> +
> + return 0;
> +}
> +
> +long get_tagged_addr_ctrl(void)
> +{
> + if (!tagged_addr_prctl_allowed)
> + return -EINVAL;
> + if (is_compat_task())
> + return -EINVAL;
> +
> + if (test_thread_flag(TIF_TAGGED_ADDR))
> + return PR_TAGGED_ADDR_ENABLE;
> +
> + return 0;
> +}
> +
> +/*
> + * Global sysctl to disable the tagged user addresses support. This control
> + * only prevents the tagged address ABI enabling via prctl() and does not
> + * disable it for tasks that already opted in to the relaxed ABI.
> + */
> +static int zero;
> +static int one = 1;
> +
> +static struct ctl_table tagged_addr_sysctl_table[] = {
> + {
> + .procname = "tagged_addr",
> + .mode = 0644,
> + .data = &tagged_addr_prctl_allowed,
> + .maxlen = sizeof(int),
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = &zero,
> + .extra2 = &one,
> + },
> + { }
> +};
> +
> +static int __init tagged_addr_init(void)
> +{
> + if (!register_sysctl("abi", tagged_addr_sysctl_table))
> + return -EINVAL;
> + return 0;
> +}
> +
> +core_initcall(tagged_addr_init);
> +#endif /* CONFIG_ARM64_TAGGED_ADDR_ABI */
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 094bb03b9cc2..2e927b3e9d6c 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -229,4 +229,9 @@ struct prctl_mm_map {
> # define PR_PAC_APDBKEY (1UL << 3)
> # define PR_PAC_APGAKEY (1UL << 4)
>
> +/* Tagged user address controls for arm64 */
> +#define PR_SET_TAGGED_ADDR_CTRL 55
> +#define PR_GET_TAGGED_ADDR_CTRL 56
> +# define PR_TAGGED_ADDR_ENABLE (1UL << 0)
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 2969304c29fe..c6c4d5358bd3 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -124,6 +124,12 @@
> #ifndef PAC_RESET_KEYS
> # define PAC_RESET_KEYS(a, b) (-EINVAL)
> #endif
> +#ifndef SET_TAGGED_ADDR_CTRL
> +# define SET_TAGGED_ADDR_CTRL(a) (-EINVAL)
> +#endif
> +#ifndef GET_TAGGED_ADDR_CTRL
> +# define GET_TAGGED_ADDR_CTRL() (-EINVAL)
> +#endif
>
> /*
> * this is where the system-wide overflow UID and GID are defined, for
> @@ -2492,6 +2498,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> return -EINVAL;
> error = PAC_RESET_KEYS(me, arg2);
> break;
> + case PR_SET_TAGGED_ADDR_CTRL:
> + error = SET_TAGGED_ADDR_CTRL(arg2);
> + break;
> + case PR_GET_TAGGED_ADDR_CTRL:
> + error = GET_TAGGED_ADDR_CTRL();
> + break;
> default:
> error = -EINVAL;
> break;
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook