On Tue, Dec 29, 2020 at 10:59:14PM -0800, Peter Collingbourne wrote: > This change introduces a prctl that 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. > > This change adds a small amount of overhead to kernel entry and exit > due to additional required instruction sequences. > > On a DragonBoard 845c (Cortex-A75) with the powersave governor, the > overhead of similar instruction sequences was measured as 4.9ns when > simulating the common case where IA is left enabled, or 43.7ns when > simulating the uncommon case where IA is disabled. These numbers can > be seen as the worst case scenario, since in more realistic scenarios > a better performing governor would be used and a newer chip would be > used that would support PAC unlike Cortex-A75 and would be expected > to be faster than Cortex-A75. > > On an Apple M1 under a hypervisor, the overhead of the entry/exit > instruction sequences introduced by this patch was measured as 0.3ns > in the case where IA is left enabled, and 33.0ns in the case where > IA is disabled. > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx> > Link: https://linux-review.googlesource.com/id/Ibc41a5e6a76b275efbaa126b31119dc197b927a5 [...] > diff --git a/arch/arm64/kernel/pointer_auth.c b/arch/arm64/kernel/pointer_auth.c > index adb955fd9bdd..f03e5bfe4490 100644 > --- a/arch/arm64/kernel/pointer_auth.c > +++ b/arch/arm64/kernel/pointer_auth.c > @@ -46,3 +46,65 @@ 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; > + > + WARN_ON(arg & ~PR_PAC_ENABLED_KEYS_MASK); > + 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_set_enabled_keys(struct task_struct *tsk, unsigned long keys, > + unsigned long enabled) > +{ > + u64 sctlr = tsk->thread.sctlr_user; > + > + if (!system_supports_address_auth()) > + return -EINVAL; > + > + if (is_compat_thread(task_thread_info(tsk))) > + return -EINVAL; > + > + if ((keys & ~PR_PAC_ENABLED_KEYS_MASK) || (enabled & ~keys)) > + return -EINVAL; > + > + sctlr &= ~arg_to_enxx_mask(keys); > + sctlr |= arg_to_enxx_mask(enabled); > + if (tsk == current) > + set_task_sctlr_el1(sctlr); > + else > + tsk->thread.sctlr_user = sctlr; Who synchronizes all these modifications to 'sctlr_user'? Seems like it gets hit by two independent prctl()s as well as ptrace. Will
