The following commit has been merged into the x86/urgent branch of tip: Commit-ID: 92dc47c57eac757c0987b0af0405d12192feff80 Gitweb: https://git.kernel.org/tip/92dc47c57eac757c0987b0af0405d12192feff80 Author: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> AuthorDate: Wed, 10 Apr 2024 22:40:49 -07:00 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitterDate: Thu, 11 Apr 2024 10:30:33 +02:00 x86/bugs: Only harden syscalls when needed Syscall hardening (i.e., converting the syscall indirect branch to a series of direct branches) may cause performance regressions in certain scenarios. Only use the syscall hardening when indirect branches are considered unsafe. Fixes: 1e3ad78334a6 ("x86/syscall: Don't force use of indirect calls for system calls") Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Nikolay Borisov <nik.borisov@xxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Sean Christopherson <seanjc@xxxxxxxxxx> Link: https://lore.kernel.org/r/97befd7c1e008797734dee05181c49056ff6de57.1712813475.git.jpoimboe@xxxxxxxxxx --- arch/x86/entry/common.c | 30 ++++++++++++++++++++++++--- arch/x86/entry/syscall_32.c | 11 +---------- arch/x86/entry/syscall_64.c | 8 +------- arch/x86/entry/syscall_x32.c | 7 +++++- arch/x86/include/asm/cpufeatures.h | 1 +- arch/x86/include/asm/syscall.h | 8 ++++++- arch/x86/kernel/cpu/bugs.c | 32 ++++++++++++++++++++++++++++- 7 files changed, 74 insertions(+), 23 deletions(-) diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 6de50b8..80d432d 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -39,6 +39,28 @@ #ifdef CONFIG_X86_64 +/* + * Do either a direct or an indirect call, depending on whether indirect calls + * are considered safe. + */ +#define __do_syscall(table, func_direct, nr, regs) \ +({ \ + unsigned long __rax, __rdi, __rsi; \ + \ + asm_inline volatile( \ + ALTERNATIVE("call " __stringify(func_direct) "\n\t", \ + ANNOTATE_RETPOLINE_SAFE \ + "call *%[func_ptr]\n\t", \ + X86_FEATURE_INDIRECT_SAFE) \ + : "=D" (__rdi), "=S" (__rsi), "=a" (__rax), \ + ASM_CALL_CONSTRAINT \ + : "0" (regs), "1" (nr), [func_ptr] "r" (table[nr]) \ + : "rdx", "rcx", "r8", "r9", "r10", "r11", \ + "cc", "memory"); \ + \ + __rax; \ +}) + static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr) { /* @@ -49,7 +71,7 @@ static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr) if (likely(unr < NR_syscalls)) { unr = array_index_nospec(unr, NR_syscalls); - regs->ax = x64_sys_call(regs, unr); + regs->ax = __do_syscall(sys_call_table, x64_sys_call, unr, regs); return true; } return false; @@ -66,7 +88,7 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr) if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) { xnr = array_index_nospec(xnr, X32_NR_syscalls); - regs->ax = x32_sys_call(regs, xnr); + regs->ax = __do_syscall(x32_sys_call_table, x32_sys_call, xnr, regs); return true; } return false; @@ -147,6 +169,8 @@ static int ia32_emulation_override_cmdline(char *arg) return kstrtobool(arg, &__ia32_enabled); } early_param("ia32_emulation", ia32_emulation_override_cmdline); +#else +#define __do_syscall(table, func_direct, nr, regs) table[nr](regs) #endif /* @@ -162,7 +186,7 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr) if (likely(unr < IA32_NR_syscalls)) { unr = array_index_nospec(unr, IA32_NR_syscalls); - regs->ax = ia32_sys_call(regs, unr); + regs->ax = __do_syscall(ia32_sys_call_table, ia32_sys_call, unr, regs); } else if (nr != -1) { regs->ax = __ia32_sys_ni_syscall(regs); } diff --git a/arch/x86/entry/syscall_32.c b/arch/x86/entry/syscall_32.c index c2235ba..9185870 100644 --- a/arch/x86/entry/syscall_32.c +++ b/arch/x86/entry/syscall_32.c @@ -14,25 +14,16 @@ #endif #define __SYSCALL(nr, sym) extern long __ia32_##sym(const struct pt_regs *); - #include <asm/syscalls_32.h> #undef __SYSCALL -/* - * The sys_call_table[] is no longer used for system calls, but - * kernel/trace/trace_syscalls.c still wants to know the system - * call address. - */ -#ifdef CONFIG_X86_32 #define __SYSCALL(nr, sym) __ia32_##sym, -const sys_call_ptr_t sys_call_table[] = { +__visible const sys_call_ptr_t ia32_sys_call_table[] = { #include <asm/syscalls_32.h> }; #undef __SYSCALL -#endif #define __SYSCALL(nr, sym) case nr: return __ia32_##sym(regs); - long ia32_sys_call(const struct pt_regs *regs, unsigned int nr) { switch (nr) { diff --git a/arch/x86/entry/syscall_64.c b/arch/x86/entry/syscall_64.c index 33b3f09..c368048 100644 --- a/arch/x86/entry/syscall_64.c +++ b/arch/x86/entry/syscall_64.c @@ -11,19 +11,13 @@ #include <asm/syscalls_64.h> #undef __SYSCALL -/* - * The sys_call_table[] is no longer used for system calls, but - * kernel/trace/trace_syscalls.c still wants to know the system - * call address. - */ #define __SYSCALL(nr, sym) __x64_##sym, -const sys_call_ptr_t sys_call_table[] = { +asmlinkage const sys_call_ptr_t sys_call_table[] = { #include <asm/syscalls_64.h> }; #undef __SYSCALL #define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs); - long x64_sys_call(const struct pt_regs *regs, unsigned int nr) { switch (nr) { diff --git a/arch/x86/entry/syscall_x32.c b/arch/x86/entry/syscall_x32.c index 03de4a9..89a7172 100644 --- a/arch/x86/entry/syscall_x32.c +++ b/arch/x86/entry/syscall_x32.c @@ -11,8 +11,13 @@ #include <asm/syscalls_x32.h> #undef __SYSCALL -#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs); +#define __SYSCALL(nr, sym) __x64_##sym, +asmlinkage const sys_call_ptr_t x32_sys_call_table[] = { +#include <asm/syscalls_x32.h> +}; +#undef __SYSCALL +#define __SYSCALL(nr, sym) case nr: return __x64_##sym(regs); long x32_sys_call(const struct pt_regs *regs, unsigned int nr) { switch (nr) { diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h index 3c74343..7c87fe8 100644 --- a/arch/x86/include/asm/cpufeatures.h +++ b/arch/x86/include/asm/cpufeatures.h @@ -470,6 +470,7 @@ #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_SAFE (21*32+ 4) /* "" Indirect branches aren't vulnerable to Spectre v2 */ /* * BUG word(s) diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h index 2fc7bc3..dfb5952 100644 --- a/arch/x86/include/asm/syscall.h +++ b/arch/x86/include/asm/syscall.h @@ -16,14 +16,20 @@ #include <asm/thread_info.h> /* for TS_COMPAT */ #include <asm/unistd.h> -/* This is used purely for kernel/trace/trace_syscalls.c */ typedef long (*sys_call_ptr_t)(const struct pt_regs *); extern const sys_call_ptr_t sys_call_table[]; +#if defined(CONFIG_X86_32) +#define ia32_sys_call_table sys_call_table +#else /* * These may not exist, but still put the prototypes in so we * can use IS_ENABLED(). */ +extern const sys_call_ptr_t ia32_sys_call_table[]; +extern const sys_call_ptr_t x32_sys_call_table[]; +#endif + extern long ia32_sys_call(const struct pt_regs *, unsigned int nr); extern long x32_sys_call(const struct pt_regs *, unsigned int nr); extern long x64_sys_call(const struct pt_regs *, unsigned int nr); diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 9eeb60f..b4ec0f0 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -1669,6 +1669,15 @@ static void __init bhi_select_mitigation(void) if (!IS_ENABLED(CONFIG_X86_64)) return; + /* + * There's no hardware mitigation in place, so mark indirect branches + * as unsafe. + * + * One could argue the SW loop makes indirect branches safe again, but + * Linus prefers it this way. + */ + setup_clear_cpu_cap(X86_FEATURE_INDIRECT_SAFE); + /* Mitigate KVM by default */ setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT); pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n"); @@ -1687,6 +1696,21 @@ static void __init spectre_v2_select_mitigation(void) enum spectre_v2_mitigation mode = SPECTRE_V2_NONE; /* + * X86_FEATURE_INDIRECT_SAFE indicates whether indirect calls can be + * considered safe. That means either: + * + * - the CPU isn't vulnerable to Spectre v2 or its variants; + * + * - a hardware mitigation is in place (e.g., IBRS, BHI_DIS_S); or + * + * - the user turned off mitigations altogether. + * + * Assume innocence until proven guilty: set the cap bit now, then + * clear it later if/when needed. + */ + setup_force_cpu_cap(X86_FEATURE_INDIRECT_SAFE); + + /* * If the CPU is not affected and the command line mode is NONE or AUTO * then nothing to do. */ @@ -1720,6 +1744,7 @@ static void __init spectre_v2_select_mitigation(void) case SPECTRE_V2_CMD_RETPOLINE_LFENCE: pr_err(SPECTRE_V2_LFENCE_MSG); + setup_clear_cpu_cap(X86_FEATURE_INDIRECT_SAFE); mode = SPECTRE_V2_LFENCE; break; @@ -1772,11 +1797,16 @@ static void __init spectre_v2_select_mitigation(void) break; case SPECTRE_V2_LFENCE: + setup_clear_cpu_cap(X86_FEATURE_INDIRECT_SAFE); + fallthrough; case SPECTRE_V2_EIBRS_LFENCE: setup_force_cpu_cap(X86_FEATURE_RETPOLINE_LFENCE); - fallthrough; + setup_force_cpu_cap(X86_FEATURE_RETPOLINE); + break; case SPECTRE_V2_RETPOLINE: + setup_clear_cpu_cap(X86_FEATURE_INDIRECT_SAFE); + fallthrough; case SPECTRE_V2_EIBRS_RETPOLINE: setup_force_cpu_cap(X86_FEATURE_RETPOLINE); break;