[PATCH v2] x86/bhi: BHI mitigation can trigger warning in #DB handler

[Alexandre Chartre]

When BHI mitigation is enabled, if sysenter is invoked with the TF flag
set then entry_SYSENTER_compat uses CLEAR_BRANCH_HISTORY and calls the
clear_bhb_loop() before the TF flag is cleared. This causes the #DB
handler (exc_debug_kernel) to issue a warning because single-step is
used outside the entry_SYSENTER_compat function.

To address this issue, entry_SYSENTER_compat() should use
CLEAR_BRANCH_HISTORY after making sure flag the TF flag is cleared.

The problem can be reproduced with the following sequence:

 $ cat sysenter_step.c
 int main()
 { asm("pushf; pop %ax; bts $8,%ax; push %ax; popf; sysenter"); }

 $ gcc -o sysenter_step sysenter_step.c

 $ ./sysenter_step
 Segmentation fault (core dumped)

The program is expected to crash, and the #DB handler will issue a warning.

Kernel log:

  WARNING: CPU: 27 PID: 7000 at arch/x86/kernel/traps.c:1009 exc_debug_kernel+0xd2/0x160
  ...
  RIP: 0010:exc_debug_kernel+0xd2/0x160
  ...
  Call Trace:
  <#DB>
   ? show_regs+0x68/0x80
   ? __warn+0x8c/0x140
   ? exc_debug_kernel+0xd2/0x160
   ? report_bug+0x175/0x1a0
   ? handle_bug+0x44/0x90
   ? exc_invalid_op+0x1c/0x70
   ? asm_exc_invalid_op+0x1f/0x30
   ? exc_debug_kernel+0xd2/0x160
   exc_debug+0x43/0x50
   asm_exc_debug+0x1e/0x40
  RIP: 0010:clear_bhb_loop+0x0/0xb0
  ...
  </#DB>
  <TASK>
   ? entry_SYSENTER_compat_after_hwframe+0x6e/0x8d
  </TASK>

Fixes: 7390db8aea0d ("x86/bhi: Add support for clearing branch history at syscall entry")
Reported-by: Suman Maity <suman.m.maity@xxxxxxxxxx>
Signed-off-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
---
 arch/x86/entry/entry_64_compat.S | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 11c9b8efdc4c..ed0a5f2dc129 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -89,10 +89,6 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 
 	cld
 
-	IBRS_ENTER
-	UNTRAIN_RET
-	CLEAR_BRANCH_HISTORY
-
 	/*
 	 * SYSENTER doesn't filter flags, so we need to clear NT and AC
 	 * ourselves.  To save a few cycles, we can check whether
@@ -116,6 +112,16 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
 	jnz	.Lsysenter_fix_flags
 .Lsysenter_flags_fixed:
 
+	/*
+	 * CPU bugs mitigations mechanisms can call other functions. They
+	 * should be invoked after making sure TF is cleared because
+	 * single-step is ignored only for instructions inside the
+	 * entry_SYSENTER_compat function.
+	 */
+	IBRS_ENTER
+	UNTRAIN_RET
+	CLEAR_BRANCH_HISTORY
+
 	movq	%rsp, %rdi
 	call	do_SYSENTER_32
 	jmp	sysret32_from_system_call
-- 
2.39.3