From 19c95f261c6558d4c2cbbfacd2d8bb6501384601 Mon Sep 17 00:00:00 2001 From: Julien Thierry Date: Tue, 15 Oct 2019 18:25:44 +0100 Subject: [PATCH] arm64: entry.S: Do not preempt from IRQ before all cpufeatures are enabled Preempting from IRQ-return means that the task has its PSTATE saved on the stack, which will get restored when the task is resumed and does the actual IRQ return. However, enabling some CPU features requires modifying the PSTATE. This means that, if a task was scheduled out during an IRQ-return before all CPU features are enabled, the task might restore a PSTATE that does not include the feature enablement changes once scheduled back in. * Task 1: PAN == 0 ---| |--------------- | |<- return from IRQ, PSTATE.PAN = 0 | <- IRQ | +--------+ <- preempt() +-- ^ | reschedule Task 1, PSTATE.PAN == 1 * Init: --------------------+------------------------ ^ | enable_cpu_features set PSTATE.PAN on all CPUs Worse than this, since PSTATE is untouched when task switching is done, a task missing the new bits in PSTATE might affect another task, if both do direct calls to schedule() (outside of IRQ/exception contexts). Fix this by preventing preemption on IRQ-return until features are enabled on all CPUs. This way the only PSTATE values that are saved on the stack are from synchronous exceptions. These are expected to be fatal this early, the exception is BRK for WARN_ON(), but as this uses do_debug_exception() which keeps IRQs masked, it shouldn't call schedule(). Signed-off-by: Julien Thierry [james: Replaced a really cool hack, with an even simpler static key in C. expanded commit message with Julien's cover-letter ascii art] Signed-off-by: James Morse Signed-off-by: Will Deacon --- arch/arm64/kernel/entry.S | 2 +- arch/arm64/kernel/process.c | 18 ++++++++++++++++++ include/linux/sched.h | 1 + 3 files changed, 20 insertions(+), 1 deletion(-) diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index e304fe0..e1859e0 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -680,7 +680,7 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKING orr x24, x24, x0 alternative_else_nop_endif cbnz x24, 1f // preempt count != 0 || NMI return path - bl preempt_schedule_irq // irq en/disable is done inside + bl arm64_preempt_schedule_irq // irq en/disable is done inside 1: #endif diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 1fb2819..71f788c 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -17,6 +17,7 @@ #include #include #include +#include #include #include #include @@ -44,6 +45,7 @@ #include #include #include +#include #include #include #include @@ -631,3 +633,19 @@ static int __init tagged_addr_init(void) core_initcall(tagged_addr_init); #endif /* CONFIG_ARM64_TAGGED_ADDR_ABI */ + +asmlinkage void __sched arm64_preempt_schedule_irq(void) +{ + lockdep_assert_irqs_disabled(); + + /* + * Preempting a task from an IRQ means we leave copies of PSTATE + * on the stack. cpufeature's enable calls may modify PSTATE, but + * resuming one of these preempted tasks would undo those changes. + * + * Only allow a task to be preempted once cpufeatures have been + * enabled. + */ + if (static_branch_likely(&arm64_const_caps_ready)) + preempt_schedule_irq(); +} diff --git a/include/linux/sched.h b/include/linux/sched.h index 2c2e56b..67a1d86 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -223,6 +223,7 @@ extern long schedule_timeout_uninterruptible(long timeout); extern long schedule_timeout_idle(long timeout); asmlinkage void schedule(void); extern void schedule_preempt_disabled(void); +asmlinkage void preempt_schedule_irq(void); extern int __must_check io_schedule_prepare(void); extern void io_schedule_finish(int token); -- 2.7.4