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SubjectRe: [PATCH v6 10/13] arm64: add prctl control for resetting ptrauth keys
On Fri, Dec 07, 2018 at 06:39:28PM +0000, Kristina Martsenko wrote:
> Add an arm64-specific prctl to allow a thread to reinitialize its
> pointer authentication keys to random values. This can be useful when
> exec() is not used for starting new processes, to ensure that different
> processes still have different keys.
>
> Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
> ---
> arch/arm64/include/asm/pointer_auth.h | 3 +++
> arch/arm64/include/asm/processor.h | 4 +++
> arch/arm64/kernel/Makefile | 1 +
> arch/arm64/kernel/pointer_auth.c | 47 +++++++++++++++++++++++++++++++++++
> include/uapi/linux/prctl.h | 8 ++++++
> kernel/sys.c | 8 ++++++
> 6 files changed, 71 insertions(+)
> create mode 100644 arch/arm64/kernel/pointer_auth.c
>
> diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
> index 89190d93c850..7797bc346c6b 100644
> --- a/arch/arm64/include/asm/pointer_auth.h
> +++ b/arch/arm64/include/asm/pointer_auth.h
> @@ -59,6 +59,8 @@ static inline void ptrauth_keys_switch(struct ptrauth_keys *keys)
> __ptrauth_key_install(APGA, keys->apga);
> }
>
> +extern int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg);
> +
> /*
> * The EL0 pointer bits used by a pointer authentication code.
> * This is dependent on TBI0 being enabled, or bits 63:56 would also apply.
> @@ -82,6 +84,7 @@ do { \
> ptrauth_keys_switch(&(tsk)->thread_info.keys_user)
>
> #else /* CONFIG_ARM64_PTR_AUTH */
> +#define ptrauth_prctl_reset_keys(tsk, arg) (-EINVAL)
> #define ptrauth_strip_insn_pac(lr) (lr)
> #define ptrauth_thread_init_user(tsk)
> #define ptrauth_thread_switch(tsk)
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 6b0d4dff5012..40ccfb7605b6 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -46,6 +46,7 @@
> #include <asm/hw_breakpoint.h>
> #include <asm/lse.h>
> #include <asm/pgtable-hwdef.h>
> +#include <asm/pointer_auth.h>
> #include <asm/ptrace.h>
> #include <asm/types.h>
>
> @@ -270,6 +271,9 @@ extern void __init minsigstksz_setup(void);
> #define SVE_SET_VL(arg) sve_set_current_vl(arg)
> #define SVE_GET_VL() sve_get_current_vl()
>
> +/* PR_PAC_RESET_KEYS prctl */
> +#define PAC_RESET_KEYS(tsk, arg) ptrauth_prctl_reset_keys(tsk, arg)
> +
> /*
> * For CONFIG_GCC_PLUGIN_STACKLEAK
> *
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index 4c8b13bede80..096740ab81d2 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -57,6 +57,7 @@ arm64-obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
> arm64-obj-$(CONFIG_CRASH_CORE) += crash_core.o
> arm64-obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o
> arm64-obj-$(CONFIG_ARM64_SSBD) += ssbd.o
> +arm64-obj-$(CONFIG_ARM64_PTR_AUTH) += pointer_auth.o
>
> obj-y += $(arm64-obj-y) vdso/ probes/
> obj-m += $(arm64-obj-m)
> diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
> new file mode 100644
> index 000000000000..b9f6f5f3409a
> --- /dev/null
> +++ b/arch/arm64/kernel/pointer_auth.c
> @@ -0,0 +1,47 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/errno.h>
> +#include <linux/prctl.h>
> +#include <linux/random.h>
> +#include <linux/sched.h>
> +#include <asm/cpufeature.h>
> +#include <asm/pointer_auth.h>
> +
> +int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
> +{
> + struct ptrauth_keys *keys = &tsk->thread_info.keys_user;
> + unsigned long addr_key_mask = PR_PAC_APIAKEY | PR_PAC_APIBKEY |
> + PR_PAC_APDAKEY | PR_PAC_APDBKEY;
> + unsigned long key_mask = addr_key_mask | PR_PAC_APGAKEY;
> +
> + if (!system_supports_address_auth() && !system_supports_generic_auth())
> + return -EINVAL;
> +
> + if (!arg) {
> + ptrauth_keys_init(keys);
> + ptrauth_keys_switch(keys);
> + return 0;
> + }
> +
> + if (arg & ~key_mask)
> + return -EINVAL;
> +
> + if (((arg & addr_key_mask) && !system_supports_address_auth()) ||
> + ((arg & PR_PAC_APGAKEY) && !system_supports_generic_auth()))
> + return -EINVAL;
> +
> + if (arg & PR_PAC_APIAKEY)
> + get_random_bytes(&keys->apia, sizeof(keys->apia));
> + if (arg & PR_PAC_APIBKEY)
> + get_random_bytes(&keys->apib, sizeof(keys->apib));
> + if (arg & PR_PAC_APDAKEY)
> + get_random_bytes(&keys->apda, sizeof(keys->apda));
> + if (arg & PR_PAC_APDBKEY)
> + get_random_bytes(&keys->apdb, sizeof(keys->apdb));
> + if (arg & PR_PAC_APGAKEY)
> + get_random_bytes(&keys->apga, sizeof(keys->apga));
> +
> + ptrauth_keys_switch(keys);
> +
> + return 0;
> +}
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index b17201edfa09..b4875a93363a 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -220,4 +220,12 @@ struct prctl_mm_map {
> # define PR_SPEC_DISABLE (1UL << 2)
> # define PR_SPEC_FORCE_DISABLE (1UL << 3)
>
> +/* Reset arm64 pointer authentication keys */
> +#define PR_PAC_RESET_KEYS 54
> +# define PR_PAC_APIAKEY (1UL << 0)
> +# define PR_PAC_APIBKEY (1UL << 1)
> +# define PR_PAC_APDAKEY (1UL << 2)
> +# define PR_PAC_APDBKEY (1UL << 3)
> +# define PR_PAC_APGAKEY (1UL << 4)
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 123bd73046ec..64b5a230f38d 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -121,6 +121,9 @@
> #ifndef SVE_GET_VL
> # define SVE_GET_VL() (-EINVAL)
> #endif
> +#ifndef PAC_RESET_KEYS
> +# define PAC_RESET_KEYS(a, b) (-EINVAL)
> +#endif
>
> /*
> * this is where the system-wide overflow UID and GID are defined, for
> @@ -2476,6 +2479,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> return -EINVAL;
> error = arch_prctl_spec_ctrl_set(me, arg2, arg3);
> break;
> + case PR_PAC_RESET_KEYS:
> + if (arg3 || arg4 || arg5)
> + return -EINVAL;
> + error = PAC_RESET_KEYS(me, arg2);
> + break;

If this only ever operated on current, can we drop the task argument?

(Last time I looked, the task argument is useless for all existing
prctls -- I have some outstanding refactoring to get rid of it entirely.)

Since arg2 contains unused flag bits and we already return -EINVAL if
any of those are set, we can define a new flag in arg2 in the future if
we want to extend this interface.

So I think we can drop the checks on arg3..arg5 and avoid the need
for the caller to supply , 0, 0, 0) in every call to this prctl.

(But if someone else objects to this approach I'm happy to concede on
this point).

Cheers
---Dave
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