[PATCH] arm64: add prctl(PR_PAC_SET_ENABLED_KEYS)
Dave Martin
Dave.Martin at arm.com
Wed Aug 19 06:18:12 EDT 2020
On Fri, Jul 31, 2020 at 06:11:52PM -0700, Peter Collingbourne wrote:
> This prctl allows the user program to control which PAC keys are enabled
> in a particular task. The main reason why this is useful is to enable a
> userspace ABI that uses PAC to sign and authenticate function pointers
> and other pointers exposed outside of the function, while still allowing
> binaries conforming to the ABI to interoperate with legacy binaries that
> do not sign or authenticate pointers.
>
> The idea is that a dynamic loader or early startup code would issue
> this prctl very early after establishing that a process may load legacy
> binaries, but before executing any PAC instructions.
Apologies for the slow response on this, I'd had it on my list for a
while...
> ---
> .../arm64/pointer-authentication.rst | 27 +++++++++++++++
> arch/arm64/include/asm/asm_pointer_auth.h | 19 +++++++++++
> arch/arm64/include/asm/pointer_auth.h | 10 ++++--
> arch/arm64/include/asm/processor.h | 5 +++
> arch/arm64/kernel/asm-offsets.c | 1 +
> arch/arm64/kernel/pointer_auth.c | 34 +++++++++++++++++++
> include/uapi/linux/prctl.h | 3 ++
> kernel/sys.c | 8 +++++
> 8 files changed, 105 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/arm64/pointer-authentication.rst b/Documentation/arm64/pointer-authentication.rst
> index 30b2ab06526b..1f7e064deeb3 100644
> --- a/Documentation/arm64/pointer-authentication.rst
> +++ b/Documentation/arm64/pointer-authentication.rst
> @@ -107,3 +107,30 @@ filter out the Pointer Authentication system key registers from
> KVM_GET/SET_REG_* ioctls and mask those features from cpufeature ID
> register. Any attempt to use the Pointer Authentication instructions will
> result in an UNDEFINED exception being injected into the guest.
> +
> +
> +Enabling and disabling keys
> +---------------------------
> +
> +The prctl PR_PAC_SET_ENABLED_KEYS allows the user program to control which
> +PAC keys are enabled in a particular task. It takes two arguments, the
> +first being a bitmask of PR_PAC_APIAKEY, PR_PAC_APIBKEY, PR_PAC_APDAKEY
> +and PR_PAC_APDBKEY specifying which keys shall be affected by this prctl,
> +and the second being a bitmask of the same bits specifying whether the key
> +should be enabled or disabled. For example::
> +
> + prctl(PR_PAC_SET_ENABLED_KEYS,
> + PR_PAC_APIAKEY | PR_PAC_APIBKEY | PR_PAC_APDAKEY | PR_PAC_APDBKEY,
> + PR_PAC_APIBKEY, 0, 0);
> +
> +disables all keys except the IB key.
> +
> +The main reason why this is useful is to enable a userspace ABI that uses PAC
> +instructions to sign and authenticate function pointers and other pointers
> +exposed outside of the function, while still allowing binaries conforming to
> +the ABI to interoperate with legacy binaries that do not sign or authenticate
> +pointers.
What actually breaks without this?
Since the keys are all enabled by default, the only purpose of this
prctl seems to be to disable keys. I'm not sure what this is giving us.
> +
> +The idea is that a dynamic loader or early startup code would issue this
> +prctl very early after establishing that a process may load legacy binaries,
> +but before executing any PAC instructions.
> diff --git a/arch/arm64/include/asm/asm_pointer_auth.h b/arch/arm64/include/asm/asm_pointer_auth.h
> index 52dead2a8640..d121fa5fed5f 100644
> --- a/arch/arm64/include/asm/asm_pointer_auth.h
> +++ b/arch/arm64/include/asm/asm_pointer_auth.h
> @@ -31,6 +31,14 @@ alternative_else_nop_endif
> ldp \tmp2, \tmp3, [\tmp1, #PTRAUTH_USER_KEY_APDB]
> msr_s SYS_APDBKEYLO_EL1, \tmp2
> msr_s SYS_APDBKEYHI_EL1, \tmp3
> +
> + ldr \tmp2, [\tsk, #THREAD_SCTLR_ENXX_MASK]
> + cbz \tmp2, .Laddr_auth_skip_\@
I wonder whether it would make sense to simple store the thread's base
SCTLR value (containing the ENxx bits), rather than storing the ENxx
bits separately. There may be reasons outside this snippet why this
isn't such a good idea though -- I haven't gone digging so far.
> +
> + mrs_s \tmp3, SYS_SCTLR_EL1
> + bic \tmp3, \tmp3, \tmp2
> + msr_s SYS_SCTLR_EL1, \tmp3
> +
> .Laddr_auth_skip_\@:
> alternative_if ARM64_HAS_GENERIC_AUTH
> ldp \tmp2, \tmp3, [\tmp1, #PTRAUTH_USER_KEY_APGA]
> @@ -45,6 +53,17 @@ alternative_else_nop_endif
> ldp \tmp2, \tmp3, [\tmp1, #PTRAUTH_KERNEL_KEY_APIA]
> msr_s SYS_APIAKEYLO_EL1, \tmp2
> msr_s SYS_APIAKEYHI_EL1, \tmp3
> +
> + ldr \tmp2, [\tsk, #THREAD_SCTLR_ENXX_MASK]
> + cbz \tmp2, .Lset_sctlr_skip_\@
> +
> + mrs_s \tmp1, SYS_SCTLR_EL1
> + mov \tmp2, #(SCTLR_ELx_ENIA | SCTLR_ELx_ENIB | SCTLR_ELx_ENDA)
(Nit: harmless but unnecessary (). # is not an operator as such, just
random syntax. Whatever follows is greedily parsed as an immediate
expression.)
> + movk \tmp2, #SCTLR_ELx_ENDB
Why do we check THREAD_SCTLR_ENXX_MASK, and then proceed to set all the
ENxx bits unconditionally? I may be missing something here.
> + orr \tmp1, \tmp1, \tmp2
> + msr_s SYS_SCTLR_EL1, \tmp1
> +
> +.Lset_sctlr_skip_\@:
> .endm
>
> .macro ptrauth_keys_install_kernel_nosync tsk, tmp1, tmp2, tmp3
> diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
> index c6b4f0603024..d4c375454a36 100644
> --- a/arch/arm64/include/asm/pointer_auth.h
> +++ b/arch/arm64/include/asm/pointer_auth.h
> @@ -70,14 +70,19 @@ static __always_inline void ptrauth_keys_switch_kernel(struct ptrauth_keys_kerne
> }
>
> extern int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg);
> +extern int ptrauth_prctl_set_enabled_keys(struct task_struct *tsk,
> + unsigned long keys,
> + unsigned long enabled);
>
> static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
> {
> return ptrauth_clear_pac(ptr);
> }
>
> -#define ptrauth_thread_init_user(tsk) \
> - ptrauth_keys_init_user(&(tsk)->thread.keys_user)
> +#define ptrauth_thread_init_user(tsk) do { \
> + ptrauth_keys_init_user(&(tsk)->thread.keys_user); \
> + (tsk)->thread.sctlr_enxx_mask = 0; \
> + } while (0)
> #define ptrauth_thread_init_kernel(tsk) \
> ptrauth_keys_init_kernel(&(tsk)->thread.keys_kernel)
> #define ptrauth_thread_switch_kernel(tsk) \
> @@ -85,6 +90,7 @@ static inline unsigned long ptrauth_strip_insn_pac(unsigned long ptr)
>
> #else /* CONFIG_ARM64_PTR_AUTH */
> #define ptrauth_prctl_reset_keys(tsk, arg) (-EINVAL)
> +#define ptrauth_prctl_set_enabled_keys(tsk, keys, enabled) (-EINVAL)
> #define ptrauth_strip_insn_pac(lr) (lr)
> #define ptrauth_thread_init_user(tsk)
> #define ptrauth_thread_init_kernel(tsk)
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index 240fe5e5b720..6974d227b01f 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -150,6 +150,7 @@ struct thread_struct {
> #ifdef CONFIG_ARM64_PTR_AUTH
> struct ptrauth_keys_user keys_user;
> struct ptrauth_keys_kernel keys_kernel;
> + u64 sctlr_enxx_mask;
> #endif
> };
>
> @@ -313,6 +314,10 @@ extern void __init minsigstksz_setup(void);
> /* PR_PAC_RESET_KEYS prctl */
> #define PAC_RESET_KEYS(tsk, arg) ptrauth_prctl_reset_keys(tsk, arg)
>
> +/* PR_PAC_SET_ENABLED_KEYS prctl */
> +#define PAC_SET_ENABLED_KEYS(tsk, keys, enabled) \
> + ptrauth_prctl_set_enabled_keys(tsk, keys, enabled)
> +
> #ifdef CONFIG_ARM64_TAGGED_ADDR_ABI
> /* PR_{SET,GET}_TAGGED_ADDR_CTRL prctl */
> long set_tagged_addr_ctrl(unsigned long arg);
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 0577e2142284..dac80e16fe35 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -47,6 +47,7 @@ int main(void)
> #ifdef CONFIG_ARM64_PTR_AUTH
> DEFINE(THREAD_KEYS_USER, offsetof(struct task_struct, thread.keys_user));
> DEFINE(THREAD_KEYS_KERNEL, offsetof(struct task_struct, thread.keys_kernel));
> + DEFINE(THREAD_SCTLR_ENXX_MASK,offsetof(struct task_struct, thread.sctlr_enxx_mask));
> #endif
> BLANK();
> DEFINE(S_X0, offsetof(struct pt_regs, regs[0]));
> diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c
> index 1e77736a4f66..8c385b7f324a 100644
> --- a/arch/arm64/kernel/pointer_auth.c
> +++ b/arch/arm64/kernel/pointer_auth.c
> @@ -42,3 +42,37 @@ int ptrauth_prctl_reset_keys(struct task_struct *tsk, unsigned long arg)
>
> return 0;
> }
> +
> +static u64 arg_to_enxx_mask(unsigned long arg)
> +{
> + u64 sctlr_enxx_mask = 0;
> + if (arg & PR_PAC_APIAKEY)
> + sctlr_enxx_mask |= SCTLR_ELx_ENIA;
> + if (arg & PR_PAC_APIBKEY)
> + sctlr_enxx_mask |= SCTLR_ELx_ENIB;
> + if (arg & PR_PAC_APDAKEY)
> + sctlr_enxx_mask |= SCTLR_ELx_ENDA;
> + if (arg & PR_PAC_APDBKEY)
> + sctlr_enxx_mask |= SCTLR_ELx_ENDB;
> + return sctlr_enxx_mask;
> +}
> +
> +int ptrauth_prctl_set_enabled_keys(struct task_struct *tsk, unsigned long keys,
> + unsigned long enabled)
> +{
> + u64 sctlr_enxx_mask = tsk->thread.sctlr_enxx_mask;
> + unsigned long addr_key_mask = PR_PAC_APIAKEY | PR_PAC_APIBKEY |
> + PR_PAC_APDAKEY | PR_PAC_APDBKEY;
> +
> + if (!system_supports_address_auth())
> + return -EINVAL;
> +
> + if ((keys & ~addr_key_mask) || (enabled & ~keys))
> + return -EINVAL;
Should we take the types of authentication supported?
I don't recall whether we expose ptrauth to userspace if only
instruction authentication or only data authentication is supported. If
so, should we reject attempts to configure unsupported keys here?
We should probably try to do a consistent thing both here and in
PR_PAC_RESET_KEYS if so.
> +
> + sctlr_enxx_mask |= arg_to_enxx_mask(keys);
> + sctlr_enxx_mask &= ~arg_to_enxx_mask(enabled);
> +
> + tsk->thread.sctlr_enxx_mask = sctlr_enxx_mask;
> + return 0;
Do we need a way to query the enabled keys?
We could either have a _GET_ prctl (the common approach), or have this
prctl return the mask of enabled keys (avoids the extra prctl, but
weirder).
As above, we might
Things like CRIU may need a GET in order to save/restore this setting
properly.
> +}
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 07b4f8131e36..18e1ae4a37a2 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -238,4 +238,7 @@ struct prctl_mm_map {
> #define PR_SET_IO_FLUSHER 57
> #define PR_GET_IO_FLUSHER 58
>
> +/* Set enabled arm64 pointer authentication keys */
> +#define PR_PAC_SET_ENABLED_KEYS 59
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 00a96746e28a..623df216183b 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -119,6 +119,9 @@
> #ifndef PAC_RESET_KEYS
> # define PAC_RESET_KEYS(a, b) (-EINVAL)
> #endif
> +#ifndef PAC_SET_ENABLED_KEYS
> +# define PAC_SET_ENABLED_KEYS(a, b, c) (-EINVAL)
> +#endif
> #ifndef SET_TAGGED_ADDR_CTRL
> # define SET_TAGGED_ADDR_CTRL(a) (-EINVAL)
> #endif
> @@ -2494,6 +2497,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> return -EINVAL;
> error = PAC_RESET_KEYS(me, arg2);
> break;
> + case PR_PAC_SET_ENABLED_KEYS:
> + if (arg4 || arg5)
> + return -EINVAL;
> + error = PAC_SET_ENABLED_KEYS(me, arg2, arg3);
> + break;
> case PR_SET_TAGGED_ADDR_CTRL:
> if (arg3 || arg4 || arg5)
> return -EINVAL;
[...]
Cheers
---Dave
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