6.1-stable review patch. If anyone has any objections, please let me know.
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From: Pawan Gupta pawan.kumar.gupta@linux.intel.com
commit 8754e67ad4ac692c67ff1f99c0d07156f04ae40c upstream.
Due to ITS, indirect branches in the lower half of a cacheline may be vulnerable to branch target injection attack.
Introduce ITS-safe thunks to patch indirect branches in the lower half of cacheline with the thunk. Also thunk any eBPF generated indirect branches in emit_indirect_jump().
Below category of indirect branches are not mitigated:
- Indirect branches in the .init section are not mitigated because they are discarded after boot. - Indirect branches that are explicitly marked retpoline-safe.
Note that retpoline also mitigates the indirect branches against ITS. This is because the retpoline sequence fills an RSB entry before RET, and it does not suffer from RSB-underflow part of the ITS.
Signed-off-by: Pawan Gupta pawan.kumar.gupta@linux.intel.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Reviewed-by: Josh Poimboeuf jpoimboe@kernel.org Reviewed-by: Alexandre Chartre alexandre.chartre@oracle.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/Kconfig | 11 +++++ arch/x86/include/asm/cpufeatures.h | 1 arch/x86/include/asm/nospec-branch.h | 4 + arch/x86/kernel/alternative.c | 77 +++++++++++++++++++++++++++++++++++ arch/x86/kernel/vmlinux.lds.S | 6 ++ arch/x86/lib/retpoline.S | 28 ++++++++++++ arch/x86/net/bpf_jit_comp.c | 6 ++ 7 files changed, 132 insertions(+), 1 deletion(-)
--- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -2575,6 +2575,17 @@ config MITIGATION_SPECTRE_BHI indirect branches. See file:Documentation/admin-guide/hw-vuln/spectre.rst
+config MITIGATION_ITS + bool "Enable Indirect Target Selection mitigation" + depends on CPU_SUP_INTEL && X86_64 + depends on RETPOLINE && RETHUNK + default y + help + Enable Indirect Target Selection (ITS) mitigation. ITS is a bug in + BPU on some Intel CPUs that may allow Spectre V2 style attacks. If + disabled, mitigation cannot be enabled via cmdline. + See file:Documentation/admin-guide/hw-vuln/indirect-target-selection.rst + endif
config ARCH_HAS_ADD_PAGES --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -445,6 +445,7 @@ #define X86_FEATURE_BHI_CTRL (21*32+ 2) /* "" BHI_DIS_S HW control available */ #define X86_FEATURE_CLEAR_BHB_HW (21*32+ 3) /* "" BHI_DIS_S HW control enabled */ #define X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT (21*32+ 4) /* "" Clear branch history at vmexit using SW loop */ +#define X86_FEATURE_INDIRECT_THUNK_ITS (21*32 + 5) /* "" Use thunk for indirect branches in lower half of cacheline */
/* * BUG word(s) --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -244,8 +244,12 @@ _ASM_PTR " 999b\n\t" \ ".popsection\n\t"
+#define ITS_THUNK_SIZE 64 + typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE]; +typedef u8 its_thunk_t[ITS_THUNK_SIZE]; extern retpoline_thunk_t __x86_indirect_thunk_array[]; +extern its_thunk_t __x86_indirect_its_thunk_array[];
#ifdef CONFIG_RETHUNK extern void __x86_return_thunk(void); --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -396,6 +396,74 @@ static int emit_indirect(int op, int reg return i; }
+#ifdef CONFIG_MITIGATION_ITS + +static int __emit_trampoline(void *addr, struct insn *insn, u8 *bytes, + void *call_dest, void *jmp_dest) +{ + u8 op = insn->opcode.bytes[0]; + int i = 0; + + /* + * Clang does 'weird' Jcc __x86_indirect_thunk_r11 conditional + * tail-calls. Deal with them. + */ + if (is_jcc32(insn)) { + bytes[i++] = op; + op = insn->opcode.bytes[1]; + goto clang_jcc; + } + + if (insn->length == 6) + bytes[i++] = 0x2e; /* CS-prefix */ + + switch (op) { + case CALL_INSN_OPCODE: + __text_gen_insn(bytes+i, op, addr+i, + call_dest, + CALL_INSN_SIZE); + i += CALL_INSN_SIZE; + break; + + case JMP32_INSN_OPCODE: +clang_jcc: + __text_gen_insn(bytes+i, op, addr+i, + jmp_dest, + JMP32_INSN_SIZE); + i += JMP32_INSN_SIZE; + break; + + default: + WARN(1, "%pS %px %*ph\n", addr, addr, 6, addr); + return -1; + } + + WARN_ON_ONCE(i != insn->length); + + return i; +} + +static int emit_its_trampoline(void *addr, struct insn *insn, int reg, u8 *bytes) +{ + return __emit_trampoline(addr, insn, bytes, + __x86_indirect_its_thunk_array[reg], + __x86_indirect_its_thunk_array[reg]); +} + +/* Check if an indirect branch is at ITS-unsafe address */ +static bool cpu_wants_indirect_its_thunk_at(unsigned long addr, int reg) +{ + if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) + return false; + + /* Indirect branch opcode is 2 or 3 bytes depending on reg */ + addr += 1 + reg / 8; + + /* Lower-half of the cacheline? */ + return !(addr & 0x20); +} +#endif + /* * Rewrite the compiler generated retpoline thunk calls. * @@ -466,6 +534,15 @@ static int patch_retpoline(void *addr, s bytes[i++] = 0xe8; /* LFENCE */ }
+#ifdef CONFIG_MITIGATION_ITS + /* + * Check if the address of last byte of emitted-indirect is in + * lower-half of the cacheline. Such branches need ITS mitigation. + */ + if (cpu_wants_indirect_its_thunk_at((unsigned long)addr + i, reg)) + return emit_its_trampoline(addr, insn, reg, bytes); +#endif + ret = emit_indirect(op, reg, bytes + i); if (ret < 0) return ret; --- a/arch/x86/kernel/vmlinux.lds.S +++ b/arch/x86/kernel/vmlinux.lds.S @@ -528,6 +528,12 @@ INIT_PER_CPU(irq_stack_backing_store); "SRSO function pair won't alias"); #endif
+#if defined(CONFIG_MITIGATION_ITS) && !defined(CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B) +. = ASSERT(__x86_indirect_its_thunk_rax & 0x20, "__x86_indirect_thunk_rax not in second half of cacheline"); +. = ASSERT(((__x86_indirect_its_thunk_rcx - __x86_indirect_its_thunk_rax) % 64) == 0, "Indirect thunks are not cacheline apart"); +. = ASSERT(__x86_indirect_its_thunk_array == __x86_indirect_its_thunk_rax, "Gap in ITS thunk array"); +#endif + #endif /* CONFIG_X86_64 */
#ifdef CONFIG_KEXEC_CORE --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -258,6 +258,34 @@ SYM_FUNC_START(entry_untrain_ret) SYM_FUNC_END(entry_untrain_ret) __EXPORT_THUNK(entry_untrain_ret)
+#ifdef CONFIG_MITIGATION_ITS + +.macro ITS_THUNK reg + +SYM_INNER_LABEL(__x86_indirect_its_thunk_\reg, SYM_L_GLOBAL) + UNWIND_HINT_EMPTY + ANNOTATE_NOENDBR + ANNOTATE_RETPOLINE_SAFE + jmp *%\reg + int3 + .align 32, 0xcc /* fill to the end of the line */ + .skip 32, 0xcc /* skip to the next upper half */ +.endm + +/* ITS mitigation requires thunks be aligned to upper half of cacheline */ +.align 64, 0xcc +.skip 32, 0xcc +SYM_CODE_START(__x86_indirect_its_thunk_array) + +#define GEN(reg) ITS_THUNK reg +#include <asm/GEN-for-each-reg.h> +#undef GEN + + .align 64, 0xcc +SYM_CODE_END(__x86_indirect_its_thunk_array) + +#endif + SYM_CODE_START(__x86_return_thunk) UNWIND_HINT_FUNC ANNOTATE_NOENDBR --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -464,7 +464,11 @@ static void emit_indirect_jump(u8 **ppro { u8 *prog = *pprog;
- if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) { + if (IS_ENABLED(CONFIG_MITIGATION_ITS) && + cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) { + OPTIMIZER_HIDE_VAR(reg); + emit_jump(&prog, &__x86_indirect_its_thunk_array[reg], ip); + } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) { EMIT_LFENCE(); EMIT2(0xFF, 0xE0 + reg); } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {