[RFC][PATCH v2 21/21] x86/pti: Use a different stack canary with the user and kernel page-table

Alexandre Chartre
2020-11-16T14:48:38Z

Using stack protector requires the stack canary to be mapped into
the current page-table. Now that the page-table switch between the
user and kernel page-table is deferred to C code, stack protector can
be used while the user page-table is active and so the stack canary
is mapped into the user page-table.

To prevent leaking the stack canary used with the kernel page-table,
use a different canary with the user and kernel page-table. The stack
canary is changed when switching the page-table.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
arch/x86/include/asm/entry-common.h | 56 ++++++++++++++++++++++++++-
arch/x86/include/asm/stackprotector.h | 35 +++++++++++------
arch/x86/kernel/sev-es.c | 18 +++++++++
include/linux/sched.h | 8 ++++
kernel/fork.c | 3 ++
5 files changed, 107 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index e01735a181b8..5b4d0e3237a3 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -96,6 +96,52 @@ static __always_inline void arch_exit_to_user_mode(void)
#define PTI_USER_PGTABLE_AND_PCID_MASK \
(PTI_USER_PCID_MASK | PTI_USER_PGTABLE_MASK)

+/*
+ * Functions to set the stack canary to the kernel or user value:
+ *
+ * The kernel stack canary should be used when running with the kernel
+ * page-table, and the user stack canary should be used when running
+ * with the user page-table. Also the kernel stack canary should not
+ * leak to the user page-table.
+ *
+ * So the stack canary should be set to the kernel value when entering
+ * the kernel from userspace *after* switching to the kernel page-table.
+ * And the stack canary should be set to the user value when returning
+ * to userspace *before* switching to the user page-table.
+ *
+ * In both cases, there is a window (between the page-table switch and
+ * the stack canary setting) where we will be running with the kernel
+ * page-table and the user stack canary. This window should be as small
+ * as possible and, ideally, it should:
+ * - not call functions which require the stack protector to be used;
+ * - have interrupt disabled to prevent interrupt handlers from being
+ * processed with the user stack canary (but there is nothing we can
+ * do for NMIs).
+ */
+static __always_inline void set_stack_canary_kernel(void)
+{
+ this_cpu_write(fixed_percpu_data.stack_canary,
+ current->stack_canary);
+}
+
+static __always_inline void set_stack_canary_user(void)
+{
+ this_cpu_write(fixed_percpu_data.stack_canary,
+ current->stack_canary_user);
+}
+
+static __always_inline void switch_to_kernel_stack_canary(unsigned long cr3)
+{
+ if (cr3 & PTI_USER_PGTABLE_MASK)
+ set_stack_canary_kernel();
+}
+
+static __always_inline void restore_stack_canary(unsigned long cr3)
+{
+ if (cr3 & PTI_USER_PGTABLE_MASK)
+ set_stack_canary_user();
+}
+
static __always_inline void write_kernel_cr3(unsigned long cr3)
{
if (static_cpu_has(X86_FEATURE_PCID))
@@ -155,8 +201,10 @@ static __always_inline unsigned long save_and_switch_to_kernel_cr3(void)
return 0;

cr3 = __native_read_cr3();
- if (cr3 & PTI_USER_PGTABLE_MASK)
+ if (cr3 & PTI_USER_PGTABLE_MASK) {
switch_to_kernel_cr3(cr3);
+ set_stack_canary_kernel();
+ }

return cr3;
}
@@ -167,6 +215,7 @@ static __always_inline void restore_cr3(unsigned long cr3)
return;

if (cr3 & PTI_USER_PGTABLE_MASK) {
+ set_stack_canary_user();
switch_to_user_cr3(cr3);
} else {
/*
@@ -182,6 +231,7 @@ static __always_inline void user_pagetable_enter(void)
if (!static_cpu_has(X86_FEATURE_PTI))
return;

+ set_stack_canary_user();
switch_to_user_cr3(__native_read_cr3());
}

@@ -191,6 +241,7 @@ static __always_inline void user_pagetable_exit(void)
return;

switch_to_kernel_cr3(__native_read_cr3());
+ set_stack_canary_kernel();
}

static __always_inline void user_pagetable_return(struct pt_regs *regs)
@@ -218,6 +269,9 @@ static __always_inline void user_pagetable_exit(void) {};
static __always_inline void user_pagetable_return(struct pt_regs *regs) {};
static __always_inline void user_pagetable_escape(struct pt_regs *regs) {};

+static __always_inline void switch_to_kernel_stack_canary(unsigned long cr3) {}
+static __always_inline void restore_stack_canary(unsigned long cr3) {}
+
#endif /* CONFIG_PAGE_TABLE_ISOLATION */
#endif /* MODULE */

diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
index 7fb482f0f25b..be6c051bafe3 100644
--- a/arch/x86/include/asm/stackprotector.h
+++ b/arch/x86/include/asm/stackprotector.h
@@ -52,6 +52,25 @@
#define GDT_STACK_CANARY_INIT \
[GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),

+static __always_inline u64 boot_get_random_canary(void)
+{
+ u64 canary;
+ u64 tsc;
+
+ /*
+ * We both use the random pool and the current TSC as a source
+ * of randomness. The TSC only matters for very early init,
+ * there it already has some randomness on most systems. Later
+ * on during the bootup the random pool has true entropy too.
+ */
+ get_random_bytes(&canary, sizeof(canary));
+ tsc = rdtsc();
+ canary += tsc + (tsc << 32UL);
+ canary &= CANARY_MASK;
+
+ return canary;
+}
+
/*
* Initialize the stackprotector canary value.
*
@@ -66,23 +85,15 @@
static __always_inline void boot_init_stack_canary(void)
{
u64 canary;
- u64 tsc;

#ifdef CONFIG_X86_64
BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
#endif
- /*
- * We both use the random pool and the current TSC as a source
- * of randomness. The TSC only matters for very early init,
- * there it already has some randomness on most systems. Later
- * on during the bootup the random pool has true entropy too.
- */
- get_random_bytes(&canary, sizeof(canary));
- tsc = rdtsc();
- canary += tsc + (tsc << 32UL);
- canary &= CANARY_MASK;
-
+ canary = boot_get_random_canary();
current->stack_canary = canary;
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+ current->stack_canary_user = boot_get_random_canary();
+#endif
#ifdef CONFIG_X86_64
this_cpu_write(fixed_percpu_data.stack_canary, canary);
#else
diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 59fc7c472525..614fbef497bd 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -1387,6 +1387,14 @@ DEFINE_IDTENTRY_VC_SETUP_STACK(exc_vmm_communication)
frame->regs = *regs;
frame->saved_cr3 = saved_cr3;

+ /*
+ * save_and_switch_to_kernel_cr3() has switched the stack canary
+ * to the kernel stack canary. However, depending on the saved
+ * CR3 value, this function may have been entered with the user
+ * stack canary. So restore the stack canary before returning.
+ */
+ restore_stack_canary(saved_cr3);
+
return sp;
}

@@ -1394,6 +1402,16 @@ DEFINE_IDTENTRY_VC(exc_vmm_communication)
{
struct exc_vc_frame *frame = (struct exc_vc_frame *)regs;

+ /*
+ * The VC setup stack function has switched to the kernel CR3
+ * but not to the kernel stack canary. Switch to the kernel
+ * stack canary now that we are using the kernel page-table.
+ *
+ * The original stack canary will be restored by the
+ * restore_cr3() function.
+ */
+ switch_to_kernel_stack_canary(frame->saved_cr3);
+
if (likely(!on_vc_fallback_stack(regs)))
safe_stack_exc_vmm_communication(regs, error_code);
else
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 063cd120b459..a0199c5d8ae1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -816,6 +816,14 @@ struct task_struct {
#ifdef CONFIG_STACKPROTECTOR
/* Canary value for the -fstack-protector GCC feature: */
unsigned long stack_canary;
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+ /*
+ * With PTI, stack_canary_user is used when we are in the
+ * kernel but using the user page-table. Once we have switched
+ * to the kernel page-table, stack_canary is used instead.
+ */
+ unsigned long stack_canary_user;
+#endif
#endif
/*
* Pointers to the (original) parent process, youngest child, younger sibling,
diff --git a/kernel/fork.c b/kernel/fork.c
index 31cd77dbdba3..70eaba4d8191 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -909,6 +909,9 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)

#ifdef CONFIG_STACKPROTECTOR
tsk->stack_canary = get_random_canary();
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+ tsk->stack_canary_user = get_random_canary();
+#endif
#endif
if (orig->cpus_ptr == &orig->cpus_mask)
tsk->cpus_ptr = &tsk->cpus_mask;
--
2.18.4