[PATCHv3 01/17] arm64: ensure ERET from kthread is illegal

Mark Rutland
2020-10-26T13:34:22Z

For consistency, all tasks have a pt_regs reserved at the highest
portion of their task stack. Among other things, this ensures that a
task's SP is always pointing within its stack rather than pointing
immediately past the end.

While it is never legitimate to ERET from a kthread, we take pains to
initialize pt_regs for kthreads as if this were legitimate. As this is
never legitimate, the effects of an erroneous return are rarely tested.

Let's simplify things by initializing a kthread's pt_regs such that an
ERET is caught as an illegal exception return, and removing the explicit
initialization of other exception context. Note that as
spectre_v4_enable_task_mitigation() only manipulates the PSTATE within
the unused regs this is safe to remove.

As user tasks will have their exception context initialized via
start_thread() or start_compat_thread(), this should only impact cases
where something has gone very wrong and we'd like that to be clearly
indicated.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/process.c | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 4784011cecac9..855137daafbfb 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -422,16 +422,15 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
if (clone_flags & CLONE_SETTLS)
p->thread.uw.tp_value = tls;
} else {
+ /*
+ * A kthread has no context to ERET to, so ensure any buggy
+ * ERET is treated as an illegal exception return.
+ *
+ * When a user task is created from a kthread, childregs will
+ * be initialized by start_thread() or start_compat_thread().
+ */
memset(childregs, 0, sizeof(struct pt_regs));
- childregs->pstate = PSR_MODE_EL1h;
- if (IS_ENABLED(CONFIG_ARM64_UAO) &&
- cpus_have_const_cap(ARM64_HAS_UAO))
- childregs->pstate |= PSR_UAO_BIT;
-
- spectre_v4_enable_task_mitigation(p);
-
- if (system_uses_irq_prio_masking())
- childregs->pmr_save = GIC_PRIO_IRQON;
+ childregs->pstate = PSR_MODE_EL1h | PSR_IL_BIT;

p->thread.cpu_context.x19 = stack_start;
p->thread.cpu_context.x20 = stk_sz;