The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 2a9baf5ad4884108b3c6d56a50e8105ccf8a4ee7
Gitweb: https://git.kernel.org/tip/2a9baf5ad4884108b3c6d56a50e8105ccf8a4ee7
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Tue, 27 Oct 2020 10:15:05 +01:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 27 Oct 2020 23:15:23 +01:00
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@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Kyle Huey <me@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201027093607.956147736@xxxxxxxxxxxxx
---
arch/x86/kernel/traps.c | 28 +++++++++++++++++++++-------
1 file changed, 21 insertions(+), 7 deletions(-)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 3c70fb3..b5208aa 100644
--- 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.
@@ -936,6 +943,13 @@ static __always_inline void exc_debug_user(struct pt_regs *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.
* User wants a sigtrap for that.
|