[RFC][PATCH v2 14/21] x86/pti: Execute IDT handlers on the kernel stack

Alexandre Chartre
2020-11-16T14:48:00Z

After an interrupt/exception in userland, the kernel is entered
and it switches the stack to the PTI stack which is mapped both in
the kernel and in the user page-table. When executing the interrupt
function, switch to the kernel stack (which is mapped only in the
kernel page-table) so that no kernel data leak to the userland
through the stack.

For now, only changes IDT handlers which have no argument other
than the pt_regs registers.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
arch/x86/include/asm/idtentry.h | 43 +++++++++++++++++++++++++++++++--
arch/x86/kernel/cpu/mce/core.c | 2 +-
arch/x86/kernel/traps.c | 4 +--
3 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 4b4aca2b1420..3595a31947b3 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -10,10 +10,49 @@
#include <linux/hardirq.h>

#include <asm/irq_stack.h>
+#include <asm/pti.h>

bool idtentry_enter_nmi(struct pt_regs *regs);
void idtentry_exit_nmi(struct pt_regs *regs, bool irq_state);

+/*
+ * The CALL_ON_STACK_* macro call the specified function either directly
+ * if no stack is provided, or on the specified stack.
+ */
+#define CALL_ON_STACK_1(stack, func, arg1) \
+ ((stack) ? \
+ asm_call_on_stack_1(stack, \
+ (void (*)(void))(func), (void *)(arg1)) : \
+ func(arg1))
+
+/*
+ * Functions to return the top of the kernel stack if we are using the
+ * user page-table (and thus not running with the kernel stack). If we
+ * are using the kernel page-table (and so already using the kernel
+ * stack) when it returns NULL.
+ */
+static __always_inline void *pti_kernel_stack(struct pt_regs *regs)
+{
+ unsigned long stack;
+
+ if (pti_enabled() && user_mode(regs)) {
+ stack = (unsigned long)task_top_of_kernel_stack(current);
+ return (void *)(stack - 8);
+ } else {
+ return NULL;
+ }
+}
+
+/*
+ * Wrappers to run an IDT handler on the kernel stack if we are not
+ * already using this stack.
+ */
+static __always_inline
+void run_idt(void (*func)(struct pt_regs *), struct pt_regs *regs)
+{
+ CALL_ON_STACK_1(pti_kernel_stack(regs), func, regs);
+}
+
/**
* DECLARE_IDTENTRY - Declare functions for simple IDT entry points
* No error code pushed by hardware
@@ -55,7 +94,7 @@ __visible noinstr void func(struct pt_regs *regs) \
irqentry_state_t state = irqentry_enter(regs); \
\
instrumentation_begin(); \
- __##func (regs); \
+ run_idt(__##func, regs); \
instrumentation_end(); \
irqentry_exit(regs, state); \
} \
@@ -271,7 +310,7 @@ __visible noinstr void func(struct pt_regs *regs) \
instrumentation_begin(); \
__irq_enter_raw(); \
kvm_set_cpu_l1tf_flush_l1d(); \
- __##func (regs); \
+ run_idt(__##func, regs); \
__irq_exit_raw(); \
instrumentation_end(); \
irqentry_exit(regs, state); \
diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 4102b866e7c0..9407c3cd9355 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -2035,7 +2035,7 @@ DEFINE_IDTENTRY_MCE_USER(exc_machine_check)
unsigned long dr7;

dr7 = local_db_save();
- exc_machine_check_user(regs);
+ run_idt(exc_machine_check_user, regs);
local_db_restore(dr7);
}
#else
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 09b22a611d99..5161385b3670 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -257,7 +257,7 @@ DEFINE_IDTENTRY_RAW(exc_invalid_op)

state = irqentry_enter(regs);
instrumentation_begin();
- handle_invalid_op(regs);
+ run_idt(handle_invalid_op, regs);
instrumentation_end();
irqentry_exit(regs, state);
}
@@ -647,7 +647,7 @@ DEFINE_IDTENTRY_RAW(exc_int3)
if (user_mode(regs)) {
irqentry_enter_from_user_mode(regs);
instrumentation_begin();
- do_int3_user(regs);
+ run_idt(do_int3_user, regs);
instrumentation_end();
irqentry_exit_to_user_mode(regs);
} else {
--
2.18.4