From: D Scott Phillips scott@os.amperecomputing.com

commit 38e0257e0e6f4fef2aa2966b089b56a8b1cfb75c upstream.

The erratum 1418040 workaround enables CNTVCT_EL1 access trapping in EL0 when executing compat threads. The workaround is applied when switching between tasks, but the need for the workaround could also change at an exec(), when a non-compat task execs a compat binary or vice versa. Apply the workaround in arch_setup_new_exec().

This leaves a small window of time between SET_PERSONALITY and arch_setup_new_exec where preemption could occur and confuse the old workaround logic that compares TIF_32BIT between prev and next. Instead, we can just read cntkctl to make sure it's in the state that the next task needs. I measured cntkctl read time to be about the same as a mov from a general-purpose register on N1. Update the workaround logic to examine the current value of cntkctl instead of the previous task's compat state.

Fixes: d49f7d7376d0 ("arm64: Move handling of erratum 1418040 into C code") Cc: stable@vger.kernel.org # 5.9.x Signed-off-by: D Scott Phillips scott@os.amperecomputing.com Reviewed-by: Marc Zyngier maz@kernel.org Link: https://lore.kernel.org/r/20211220234114.3926-1-scott@os.amperecomputing.com Signed-off-by: Catalin Marinas catalin.marinas@arm.com --- arch/arm64/kernel/process.c | 39 +++++++++++++++---------------------- 1 file changed, 16 insertions(+), 23 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 4999caff3281..22275d8518eb 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -511,34 +511,26 @@ static void entry_task_switch(struct task_struct *next)

/* * ARM erratum 1418040 handling, affecting the 32bit view of CNTVCT. - * Assuming the virtual counter is enabled at the beginning of times: - * - * - disable access when switching from a 64bit task to a 32bit task - * - enable access when switching from a 32bit task to a 64bit task + * Ensure access is disabled when switching to a 32bit task, ensure + * access is enabled when switching to a 64bit task. */ -static void erratum_1418040_thread_switch(struct task_struct *prev, - struct task_struct *next) +static void erratum_1418040_thread_switch(struct task_struct *next) { - bool prev32, next32; - u64 val; - - if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040)) - return; - - prev32 = is_compat_thread(task_thread_info(prev)); - next32 = is_compat_thread(task_thread_info(next)); - - if (prev32 == next32 || !this_cpu_has_cap(ARM64_WORKAROUND_1418040)) + if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) || + !this_cpu_has_cap(ARM64_WORKAROUND_1418040)) return;

- val = read_sysreg(cntkctl_el1); - - if (!next32) - val |= ARCH_TIMER_USR_VCT_ACCESS_EN; + if (is_compat_thread(task_thread_info(next))) + sysreg_clear_set(cntkctl_el1, ARCH_TIMER_USR_VCT_ACCESS_EN, 0); else - val &= ~ARCH_TIMER_USR_VCT_ACCESS_EN; + sysreg_clear_set(cntkctl_el1, 0, ARCH_TIMER_USR_VCT_ACCESS_EN); +}

- write_sysreg(val, cntkctl_el1); +static void erratum_1418040_new_exec(void) +{ + preempt_disable(); + erratum_1418040_thread_switch(current); + preempt_enable(); }

/* @@ -556,7 +548,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev, entry_task_switch(next); uao_thread_switch(next); ssbs_thread_switch(next); - erratum_1418040_thread_switch(prev, next); + erratum_1418040_thread_switch(next);

/* * Complete any pending TLB or cache maintenance on this CPU in case @@ -622,6 +614,7 @@ void arch_setup_new_exec(void) current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;

ptrauth_thread_init_user(current); + erratum_1418040_new_exec();

if (task_spec_ssb_noexec(current)) { arch_prctl_spec_ctrl_set(current, PR_SPEC_STORE_BYPASS,