On Sat, 22 Sep 2018, Thomas Gleixner wrote:
> On Sat, 22 Sep 2018, Peter Zijlstra wrote:
> > This has some unfortunate duplication.
> >
> > Lets go with it for now, but I'll see if I can do something about that
> > later.
>
> Yes, I know. I tried to make the duplication smaller, but all attempts
> ended up being a convoluted mess. I'll try again after applying more
> coffee.
Lunch and coffee indeed made brain work better. The simple solution was way
too obvious.
Thanks,
tglx
8<--------------------
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -7,6 +7,7 @@
#include <linux/export.h>
#include <linux/cpu.h>
#include <linux/debugfs.h>
+#include <linux/ptrace.h>
#include <asm/tlbflush.h>
#include <asm/mmu_context.h>
@@ -180,6 +181,19 @@ static void sync_current_stack_to_mm(str
}
}
+static bool ibpb_needed(struct task_struct *tsk, u64 last_ctx_id)
+{
+ /*
+ * Check if the current (previous) task has access to the memory
+ * of the @tsk (next) task. If access is denied, make sure to
+ * issue a IBPB to stop user->user Spectre-v2 attacks.
+ *
+ * Note: __ptrace_may_access() returns 0 or -ERRNO.
+ */
+ return (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
+ ptrace_may_access_sched(tsk, PTRACE_MODE_SPEC_IBPB));
+}
+
void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk)
{
@@ -262,18 +276,13 @@ void switch_mm_irqs_off(struct mm_struct
* one process from doing Spectre-v2 attacks on another.
*
* As an optimization, flush indirect branches only when
- * switching into processes that disable dumping. This
- * protects high value processes like gpg, without having
- * too high performance overhead. IBPB is *expensive*!
- *
- * This will not flush branches when switching into kernel
- * threads. It will also not flush if we switch to idle
- * thread and back to the same process. It will flush if we
- * switch to a different non-dumpable process.
+ * switching into a processes that can't be ptrace by the
+ * current one (as in such case, attacker has much more
+ * convenient way how to tamper with the next process than
+ * branch buffer poisoning).
*/
- if (tsk && tsk->mm &&
- tsk->mm->context.ctx_id != last_ctx_id &&
- get_dumpable(tsk->mm) != SUID_DUMP_USER)
+ if (static_cpu_has(X86_FEATURE_USE_IBPB) &&
+ ibpb_needed(tsk, last_ctx_id))
indirect_branch_prediction_barrier();
if (IS_ENABLED(CONFIG_VMAP_STACK)) {
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -62,14 +62,17 @@ extern void exit_ptrace(struct task_stru
#define PTRACE_MODE_READ 0x01
#define PTRACE_MODE_ATTACH 0x02
#define PTRACE_MODE_NOAUDIT 0x04
-#define PTRACE_MODE_FSCREDS 0x08
-#define PTRACE_MODE_REALCREDS 0x10
+#define PTRACE_MODE_FSCREDS 0x08
+#define PTRACE_MODE_REALCREDS 0x10
+#define PTRACE_MODE_SCHED 0x20
+#define PTRACE_MODE_IBPB 0x40
/* shorthands for READ/ATTACH and FSCREDS/REALCREDS combinations */
#define PTRACE_MODE_READ_FSCREDS (PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)
#define PTRACE_MODE_READ_REALCREDS (PTRACE_MODE_READ | PTRACE_MODE_REALCREDS)
#define PTRACE_MODE_ATTACH_FSCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS)
#define PTRACE_MODE_ATTACH_REALCREDS (PTRACE_MODE_ATTACH |
PTRACE_MODE_REALCREDS)
+#define PTRACE_MODE_SPEC_IBPB (PTRACE_MODE_ATTACH_REALCREDS | PTRACE_MOD_IBPB)
/**
* ptrace_may_access - check whether the caller is permitted to access
@@ -87,6 +90,20 @@ extern void exit_ptrace(struct task_stru
*/
extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
+/**
+ * ptrace_may_access - check whether the caller is permitted to access
+ * a target task.
+ * @task: target task
+ * @mode: selects type of access and caller credentials
+ *
+ * Returns true on success, false on denial.
+ *
+ * Similar to ptrace_may_access(). Only to be called from context switch
+ * code. Does not call into audit and the regular LSM hooks due to locking
+ * constraints.
+ */
+extern bool ptrace_may_access_sched(struct task_struct *task, unsigned int
mode);
+
static inline int ptrace_reparented(struct task_struct *child)
{
return !same_thread_group(child->real_parent, child->parent);
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -261,6 +261,9 @@ static int ptrace_check_attach(struct ta
static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
{
+ if (mode & PTRACE_MODE_SCHED)
+ return false;
+
if (mode & PTRACE_MODE_NOAUDIT)
return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
else
@@ -328,9 +331,16 @@ static int __ptrace_may_access(struct ta
!ptrace_has_cap(mm->user_ns, mode)))
return -EPERM;
+ if (mode & PTRACE_MODE_SCHED)
+ return 0;
return security_ptrace_access_check(task, mode);
}
+bool ptrace_may_access_sched(struct task_struct *task, unsigned int mode)
+{
+ return __ptrace_may_access(task, mode | PTRACE_MODE_SCHED);
+}
+
bool ptrace_may_access(struct task_struct *task, unsigned int mode)
{
int err;
