x86/bhi: Add support for clearing branch history at syscall entry
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@linux.intel.com> Co-developed-by: Daniel Sneddon <daniel.sneddon@linux.intel.com> Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com> Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Pawan Gupta
Thomas Gleixner
parent
1e3ad78334
commit
7390db8aea
@ -189,7 +189,7 @@ static __always_inline bool int80_is_external(void)
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}
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/**
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* int80_emulation - 32-bit legacy syscall entry
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* do_int80_emulation - 32-bit legacy syscall C entry from asm
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*
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* This entry point can be used by 32-bit and 64-bit programs to perform
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* 32-bit system calls. Instances of INT $0x80 can be found inline in
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@ -207,7 +207,7 @@ static __always_inline bool int80_is_external(void)
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* eax: system call number
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* ebx, ecx, edx, esi, edi, ebp: arg1 - arg 6
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*/
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DEFINE_IDTENTRY_RAW(int80_emulation)
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__visible noinstr void do_int80_emulation(struct pt_regs *regs)
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{
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int nr;
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@ -116,6 +116,7 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
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/* clobbers %rax, make sure it is after saving the syscall nr */
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IBRS_ENTER
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UNTRAIN_RET
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CLEAR_BRANCH_HISTORY
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call do_syscall_64 /* returns with IRQs disabled */
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@ -1491,3 +1492,63 @@ SYM_CODE_START_NOALIGN(rewind_stack_and_make_dead)
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call make_task_dead
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SYM_CODE_END(rewind_stack_and_make_dead)
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.popsection
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/*
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* This sequence executes branches in order to remove user branch information
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* from the branch history tracker in the Branch Predictor, therefore removing
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* user influence on subsequent BTB lookups.
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*
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* It should be used on parts prior to Alder Lake. Newer parts should use the
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* BHI_DIS_S hardware control instead. If a pre-Alder Lake part is being
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* virtualized on newer hardware the VMM should protect against BHI attacks by
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* setting BHI_DIS_S for the guests.
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*
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* CALLs/RETs are necessary to prevent Loop Stream Detector(LSD) from engaging
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* and not clearing the branch history. The call tree looks like:
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*
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* call 1
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* call 2
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* call 2
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* call 2
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* call 2
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* call 2
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* ret
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* ret
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* ret
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* ret
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* ret
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* ret
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*
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* This means that the stack is non-constant and ORC can't unwind it with %rsp
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* alone. Therefore we unconditionally set up the frame pointer, which allows
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* ORC to unwind properly.
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*
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* The alignment is for performance and not for safety, and may be safely
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* refactored in the future if needed.
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*/
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SYM_FUNC_START(clear_bhb_loop)
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push %rbp
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mov %rsp, %rbp
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movl $5, %ecx
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ANNOTATE_INTRA_FUNCTION_CALL
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call 1f
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jmp 5f
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.align 64, 0xcc
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ANNOTATE_INTRA_FUNCTION_CALL
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1: call 2f
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RET
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.align 64, 0xcc
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2: movl $5, %eax
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3: jmp 4f
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nop
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4: sub $1, %eax
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jnz 3b
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sub $1, %ecx
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jnz 1b
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RET
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5: lfence
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pop %rbp
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RET
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SYM_FUNC_END(clear_bhb_loop)
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EXPORT_SYMBOL_GPL(clear_bhb_loop)
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STACK_FRAME_NON_STANDARD(clear_bhb_loop)
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@ -92,6 +92,7 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
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IBRS_ENTER
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UNTRAIN_RET
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CLEAR_BRANCH_HISTORY
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/*
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* SYSENTER doesn't filter flags, so we need to clear NT and AC
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@ -206,6 +207,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
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IBRS_ENTER
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UNTRAIN_RET
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CLEAR_BRANCH_HISTORY
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movq %rsp, %rdi
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call do_fast_syscall_32
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@ -276,3 +278,17 @@ SYM_INNER_LABEL(entry_SYSRETL_compat_end, SYM_L_GLOBAL)
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ANNOTATE_NOENDBR
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int3
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SYM_CODE_END(entry_SYSCALL_compat)
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/*
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* int 0x80 is used by 32 bit mode as a system call entry. Normally idt entries
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* point to C routines, however since this is a system call interface the branch
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* history needs to be scrubbed to protect against BHI attacks, and that
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* scrubbing needs to take place in assembly code prior to entering any C
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* routines.
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*/
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SYM_CODE_START(int80_emulation)
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ANNOTATE_NOENDBR
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UNWIND_HINT_FUNC
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CLEAR_BRANCH_HISTORY
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jmp do_int80_emulation
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SYM_CODE_END(int80_emulation)
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@ -461,11 +461,12 @@
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/*
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* Extended auxiliary flags: Linux defined - for features scattered in various
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* CPUID levels like 0x80000022, etc.
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* CPUID levels like 0x80000022, etc and Linux defined features.
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*
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* Reuse free bits when adding new feature flags!
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*/
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#define X86_FEATURE_AMD_LBR_PMC_FREEZE (21*32+ 0) /* AMD LBR and PMC Freeze */
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#define X86_FEATURE_CLEAR_BHB_LOOP (21*32+ 1) /* "" Clear branch history at syscall entry using SW loop */
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/*
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* BUG word(s)
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@ -326,6 +326,14 @@
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ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
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.endm
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#ifdef CONFIG_X86_64
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.macro CLEAR_BRANCH_HISTORY
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ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP
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.endm
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#else
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#define CLEAR_BRANCH_HISTORY
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#endif
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#else /* __ASSEMBLY__ */
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#define ANNOTATE_RETPOLINE_SAFE \
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@ -368,6 +376,10 @@ extern void srso_alias_return_thunk(void);
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extern void entry_untrain_ret(void);
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extern void entry_ibpb(void);
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#ifdef CONFIG_X86_64
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extern void clear_bhb_loop(void);
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#endif
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extern void (*x86_return_thunk)(void);
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extern void __warn_thunk(void);
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@ -125,6 +125,7 @@ static inline int syscall_get_arch(struct task_struct *task)
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}
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bool do_syscall_64(struct pt_regs *regs, int nr);
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void do_int80_emulation(struct pt_regs *regs);
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#endif /* CONFIG_X86_32 */
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@ -275,6 +275,8 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_GLOBAL)
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call vmx_spec_ctrl_restore_host
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CLEAR_BRANCH_HISTORY
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/* Put return value in AX */
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mov %_ASM_BX, %_ASM_AX
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