This is a note to let you know that I've just added the patch titled
x86/bhi: Avoid warning in #DB handler due to BHI mitigation
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-bhi-avoid-warning-in-db-handler-due-to-bhi-mitig.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 0fb43e036b2c97caeec4da28848c7a61949142da
Author: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Date: Fri May 24 09:04:59 2024 +0200
x86/bhi: Avoid warning in #DB handler due to BHI mitigation
[ Upstream commit ac8b270b61d48fcc61f052097777e3b5e11591e0 ]
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 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>
[ bp: Massage commit message. ]
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>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240524070459.3674025-1-alexandre.chartre@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 4c1dfc51c56e4..ebfccadf918cb 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -90,10 +90,6 @@ 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
@@ -117,6 +113,16 @@ 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
/* XEN PV guests always use IRET path */
