[PATCH v7 05/22] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs

Will Deacon
2021-05-25T15:19:27Z

Scheduling a 32-bit application on a 64-bit-only CPU is a bad idea.

Ensure that 32-bit applications always take the slow-path when returning
to userspace on a system with mismatched support at EL0, so that we can
avoid trying to run on a 64-bit-only CPU and force a SIGKILL instead.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/process.c | 19 ++++++++++++++++++-
arch/arm64/kernel/signal.c | 26 ++++++++++++++++++++++++++
2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index b4bb67f17a2c..f4a91bf1ce0c 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -527,6 +527,15 @@ static void erratum_1418040_thread_switch(struct task_struct *prev,
write_sysreg(val, cntkctl_el1);
}

+static void compat_thread_switch(struct task_struct *next)
+{
+ if (!is_compat_thread(task_thread_info(next)))
+ return;
+
+ if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+ set_tsk_thread_flag(next, TIF_NOTIFY_RESUME);
+}
+
static void update_sctlr_el1(u64 sctlr)
{
/*
@@ -568,6 +577,7 @@ __notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
ssbs_thread_switch(next);
erratum_1418040_thread_switch(prev, next);
ptrauth_thread_switch_user(next);
+ compat_thread_switch(next);

/*
* Complete any pending TLB or cache maintenance on this CPU in case
@@ -633,8 +643,15 @@ unsigned long arch_align_stack(unsigned long sp)
*/
void arch_setup_new_exec(void)
{
- current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
+ unsigned long mmflags = 0;
+
+ if (is_compat_task()) {
+ mmflags = MMCF_AARCH32;
+ if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+ set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+ }

+ current->mm->context.flags = mmflags;
ptrauth_thread_init_user();
mte_thread_init_user();

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 6237486ff6bb..f8192f4ae0b8 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -911,6 +911,19 @@ static void do_signal(struct pt_regs *regs)
restore_saved_sigmask();
}

+static bool cpu_affinity_invalid(struct pt_regs *regs)
+{
+ if (!compat_user_mode(regs))
+ return false;
+
+ /*
+ * We're preemptible, but a reschedule will cause us to check the
+ * affinity again.
+ */
+ return !cpumask_test_cpu(raw_smp_processor_id(),
+ system_32bit_el0_cpumask());
+}
+
asmlinkage void do_notify_resume(struct pt_regs *regs,
unsigned long thread_flags)
{
@@ -938,6 +951,19 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
if (thread_flags & _TIF_NOTIFY_RESUME) {
tracehook_notify_resume(regs);
rseq_handle_notify_resume(NULL, regs);
+
+ /*
+ * If we reschedule after checking the affinity
+ * then we must ensure that TIF_NOTIFY_RESUME
+ * is set so that we check the affinity again.
+ * Since tracehook_notify_resume() clears the
+ * flag, ensure that the compiler doesn't move
+ * it after the affinity check.
+ */
+ barrier();
+
+ if (cpu_affinity_invalid(regs))
+ force_sig(SIGKILL);
}

if (thread_flags & _TIF_FOREIGN_FPSTATE)
--
2.31.1.818.g46aad6cb9e-goog