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
