x86/debug: Fix BTF handling

The SDM states that #DB clears DEBUGCTLMSR_BTF, this means that when the
bit is set for userspace (TIF_BLOCKSTEP) and a kernel #DB happens first,
the BTF bit meant for userspace execution is lost.

Have the kernel #DB handler restore the BTF bit when it was requested
for userspace.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kyle Huey <me@kylehuey.com>
Link: https://lore.kernel.org/r/20201027093607.956147736@infradead.org

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 3c70fb3..b5208aa 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -799,13 +799,6 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
         */
        current->thread.virtual_dr6 = 0;
 
-       /*
-        * The SDM says "The processor clears the BTF flag when it
-        * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
-        * TIF_BLOCKSTEP in sync with the hardware BTF flag.
-        */
-       clear_thread_flag(TIF_BLOCKSTEP);
-
        return dr6;
 }
 
@@ -873,6 +866,20 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
         */
        WARN_ON_ONCE(user_mode(regs));
 
+       if (test_thread_flag(TIF_BLOCKSTEP)) {
+               /*
+                * The SDM says "The processor clears the BTF flag when it
+                * generates a debug exception." but PTRACE_BLOCKSTEP requested
+                * it for userspace, but we just took a kernel #DB, so re-set
+                * BTF.
+                */
+               unsigned long debugctl;
+
+               rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
+               debugctl |= DEBUGCTLMSR_BTF;
+               wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
+       }
+
        /*
         * Catch SYSENTER with TF set and clear DR_STEP. If this hit a
         * watchpoint at the same time then that will still be handled.
@@ -935,6 +942,13 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
        irqentry_enter_from_user_mode(regs);
        instrumentation_begin();
 
+       /*
+        * The SDM says "The processor clears the BTF flag when it
+        * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
+        * TIF_BLOCKSTEP in sync with the hardware BTF flag.
+        */
+       clear_thread_flag(TIF_BLOCKSTEP);
+
        /*
         * If dr6 has no reason to give us about the origin of this trap,
         * then it's very likely the result of an icebp/int01 trap.