This is a note to let you know that I've just added the patch titled
x86/bhi: Add support for clearing branch history at syscall entry
to the 6.8-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-add-support-for-clearing-branch-history-at-syscall-entry.patch
and it can be found in the queue-6.8 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From b72071e8912a7c517dca366bf1711038701e6cb0 Mon Sep 17 00:00:00 2001
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Date: Mon, 11 Mar 2024 08:56:58 -0700
Subject: x86/bhi: Add support for clearing branch history at syscall entry
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
commit 7390db8aea0d64e9deb28b8e1ce716f5020c7ee5 upstream.
Branch History Injection (BHI) attacks may allow a malicious application to
influence indirect branch prediction in kernel by poisoning the branch
history. eIBRS isolates indirect branch targets in ring0. The BHB can
still influence the choice of indirect branch predictor entry, and although
branch predictor entries are isolated between modes when eIBRS is enabled,
the BHB itself is not isolated between modes.
Alder Lake and new processors supports a hardware control BHI_DIS_S to
mitigate BHI. For older processors Intel has released a software sequence
to clear the branch history on parts that don't support BHI_DIS_S. Add
support to execute the software sequence at syscall entry and VMexit to
overwrite the branch history.
For now, branch history is not cleared at interrupt entry, as malicious
applications are not believed to have sufficient control over the
registers, since previous register state is cleared at interrupt
entry. Researchers continue to poke at this area and it may become
necessary to clear at interrupt entry as well in the future.
This mitigation is only defined here. It is enabled later.
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Co-developed-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/entry/common.c | 4 +-
arch/x86/entry/entry_64.S | 61 +++++++++++++++++++++++++++++++++++
arch/x86/entry/entry_64_compat.S | 16 +++++++++
arch/x86/include/asm/cpufeatures.h | 8 ++++
arch/x86/include/asm/nospec-branch.h | 12 ++++++
arch/x86/include/asm/syscall.h | 1
arch/x86/kvm/vmx/vmenter.S | 2 +
7 files changed, 102 insertions(+), 2 deletions(-)
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -189,7 +189,7 @@ static __always_inline bool int80_is_ext
}
/**
- * int80_emulation - 32-bit legacy syscall entry
+ * do_int80_emulation - 32-bit legacy syscall C entry from asm
*
* This entry point can be used by 32-bit and 64-bit programs to perform
* 32-bit system calls. Instances of INT $0x80 can be found inline in
@@ -207,7 +207,7 @@ static __always_inline bool int80_is_ext
* eax: system call number
* ebx, ecx, edx, esi, edi, ebp: arg1 - arg 6
*/
-DEFINE_IDTENTRY_RAW(int80_emulation)
+__visible noinstr void do_int80_emulation(struct pt_regs *regs)
{
int nr;
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -116,6 +116,7 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_h
/* clobbers %rax, make sure it is after saving the syscall nr */
IBRS_ENTER
UNTRAIN_RET
+ CLEAR_BRANCH_HISTORY
call do_syscall_64 /* returns with IRQs disabled */
@@ -1494,3 +1495,63 @@ SYM_CODE_START_NOALIGN(rewind_stack_and_
call make_task_dead
SYM_CODE_END(rewind_stack_and_make_dead)
.popsection
+
+/*
+ * This sequence executes branches in order to remove user branch information
+ * from the branch history tracker in the Branch Predictor, therefore removing
+ * user influence on subsequent BTB lookups.
+ *
+ * It should be used on parts prior to Alder Lake. Newer parts should use the
+ * BHI_DIS_S hardware control instead. If a pre-Alder Lake part is being
+ * virtualized on newer hardware the VMM should protect against BHI attacks by
+ * setting BHI_DIS_S for the guests.
+ *
+ * CALLs/RETs are necessary to prevent Loop Stream Detector(LSD) from engaging
+ * and not clearing the branch history. The call tree looks like:
+ *
+ * call 1
+ * call 2
+ * call 2
+ * call 2
+ * call 2
+ * call 2
+ * ret
+ * ret
+ * ret
+ * ret
+ * ret
+ * ret
+ *
+ * This means that the stack is non-constant and ORC can't unwind it with %rsp
+ * alone. Therefore we unconditionally set up the frame pointer, which allows
+ * ORC to unwind properly.
+ *
+ * The alignment is for performance and not for safety, and may be safely
+ * refactored in the future if needed.
+ */
+SYM_FUNC_START(clear_bhb_loop)
+ push %rbp
+ mov %rsp, %rbp
+ movl $5, %ecx
+ ANNOTATE_INTRA_FUNCTION_CALL
+ call 1f
+ jmp 5f
+ .align 64, 0xcc
+ ANNOTATE_INTRA_FUNCTION_CALL
+1: call 2f
+ RET
+ .align 64, 0xcc
+2: movl $5, %eax
+3: jmp 4f
+ nop
+4: sub $1, %eax
+ jnz 3b
+ sub $1, %ecx
+ jnz 1b
+ RET
+5: lfence
+ pop %rbp
+ RET
+SYM_FUNC_END(clear_bhb_loop)
+EXPORT_SYMBOL_GPL(clear_bhb_loop)
+STACK_FRAME_NON_STANDARD(clear_bhb_loop)
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -92,6 +92,7 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_af
IBRS_ENTER
UNTRAIN_RET
+ CLEAR_BRANCH_HISTORY
/*
* SYSENTER doesn't filter flags, so we need to clear NT and AC
@@ -206,6 +207,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
IBRS_ENTER
UNTRAIN_RET
+ CLEAR_BRANCH_HISTORY
movq %rsp, %rdi
call do_fast_syscall_32
@@ -276,3 +278,17 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_end
ANNOTATE_NOENDBR
int3
SYM_CODE_END(entry_SYSCALL_compat)
+
+/*
+ * int 0x80 is used by 32 bit mode as a system call entry. Normally idt entries
+ * point to C routines, however since this is a system call interface the branch
+ * history needs to be scrubbed to protect against BHI attacks, and that
+ * scrubbing needs to take place in assembly code prior to entering any C
+ * routines.
+ */
+SYM_CODE_START(int80_emulation)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ CLEAR_BRANCH_HISTORY
+ jmp do_int80_emulation
+SYM_CODE_END(int80_emulation)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -457,6 +457,14 @@
#define X86_FEATURE_SRSO_NO (20*32+29) /* "" CPU is not affected by SRSO */
/*
+ * Extended auxiliary flags: Linux defined - for features scattered in various
+ * CPUID levels like 0x80000022, etc and Linux defined features.
+ *
+ * Reuse free bits when adding new feature flags!
+ */
+#define X86_FEATURE_CLEAR_BHB_LOOP (21*32+ 1) /* "" Clear branch history at syscall entry using SW loop */
+
+/*
* BUG word(s)
*/
#define X86_BUG(x) (NCAPINTS*32 + (x))
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -335,6 +335,14 @@
ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
.endm
+#ifdef CONFIG_X86_64
+.macro CLEAR_BRANCH_HISTORY
+ ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP
+.endm
+#else
+#define CLEAR_BRANCH_HISTORY
+#endif
+
#else /* __ASSEMBLY__ */
#define ANNOTATE_RETPOLINE_SAFE \
@@ -377,6 +385,10 @@ extern void srso_alias_return_thunk(void
extern void entry_untrain_ret(void);
extern void entry_ibpb(void);
+#ifdef CONFIG_X86_64
+extern void clear_bhb_loop(void);
+#endif
+
extern void (*x86_return_thunk)(void);
#ifdef CONFIG_CALL_DEPTH_TRACKING
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -125,6 +125,7 @@ static inline int syscall_get_arch(struc
}
bool do_syscall_64(struct pt_regs *regs, int nr);
+void do_int80_emulation(struct pt_regs *regs);
#endif /* CONFIG_X86_32 */
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -275,6 +275,8 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_
call vmx_spec_ctrl_restore_host
+ CLEAR_BRANCH_HISTORY
+
/* Put return value in AX */
mov %_ASM_BX, %_ASM_AX
Patches currently in stable-queue which might be from pawan.kumar.gupta@xxxxxxxxxxxxxxx are
queue-6.8/x86-bhi-define-spec_ctrl_bhi_dis_s.patch
queue-6.8/x86-bhi-add-support-for-clearing-branch-history-at-syscall-entry.patch
queue-6.8/x86-bhi-mitigate-kvm-by-default.patch
queue-6.8/x86-bhi-enumerate-branch-history-injection-bhi-bug.patch
queue-6.8/x86-bhi-add-bhi-mitigation-knob.patch
queue-6.8/kvm-x86-add-bhi_no.patch
