From cfcb2a5affbe8a4bd3de6890e1c8ad0184466cc2 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Fri, 16 May 2025 17:03:37 -0700 Subject: [PATCH] x86/its: FineIBT-paranoid vs ITS MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit commit e52c1dc7455d32c8a55f9949d300e5e87d011fa6 upstream. FineIBT-paranoid was using the retpoline bytes for the paranoid check, disabling retpolines, because all parts that have IBT also have eIBRS and thus don't need no stinking retpolines. Except... ITS needs the retpolines for indirect calls must not be in the first half of a cacheline :-/ So what was the paranoid call sequence: : 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d 6: 45 3b 53 f7 cmp -0x9(%r11), %r10d a: 4d 8d 5b lea -0x10(%r11), %r11 e: 75 fd jne d 10: 41 ff d3 call *%r11 13: 90 nop Now becomes: : 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d 6: 45 3b 53 f7 cmp -0x9(%r11), %r10d a: 4d 8d 5b f0 lea -0x10(%r11), %r11 e: 2e e8 XX XX XX XX cs call __x86_indirect_paranoid_thunk_r11 Where the paranoid_thunk looks like: 1d: (bad) __x86_indirect_paranoid_thunk_r11: 1e: 75 fd jne 1d __x86_indirect_its_thunk_r11: 20: 41 ff eb jmp *%r11 23: cc int3 [ dhansen: remove initialization to false ] Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Alexandre Chartre [ Just a portion of the original commit, in order to fix a build issue in stable kernels due to backports ] Tested-by: Holger Hoffstätte Link: https://lore.kernel.org/r/20250514113952.GB16434@noisy.programming.kicks-ass.net Signed-off-by: Greg Kroah-Hartman Signed-off-by: Greg Kroah-Hartman --- arch/x86/include/asm/alternative.h | 8 ++++++++ arch/x86/kernel/alternative.c | 8 ++++++++ arch/x86/net/bpf_jit_comp.c | 2 +- 3 files changed, 17 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h index aa7b155b6173..1797f80c10de 100644 --- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -5,6 +5,7 @@ #include #include #include +#include #define ALTINSTR_FLAG_INV (1 << 15) #define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV) @@ -84,10 +85,17 @@ struct module; extern void its_init_mod(struct module *mod); extern void its_fini_mod(struct module *mod); extern void its_free_mod(struct module *mod); +extern u8 *its_static_thunk(int reg); #else /* CONFIG_MITIGATION_ITS */ static inline void its_init_mod(struct module *mod) { } static inline void its_fini_mod(struct module *mod) { } static inline void its_free_mod(struct module *mod) { } +static inline u8 *its_static_thunk(int reg) +{ + WARN_ONCE(1, "ITS not compiled in"); + + return NULL; +} #endif #ifdef CONFIG_RETHUNK diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 3c08f0e25df7..43ec73da66d8 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -597,6 +597,14 @@ static bool cpu_wants_indirect_its_thunk_at(unsigned long addr, int reg) /* Lower-half of the cacheline? */ return !(addr & 0x20); } + +u8 *its_static_thunk(int reg) +{ + u8 *thunk = __x86_indirect_its_thunk_array[reg]; + + return thunk; +} + #endif /* diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 8be74c2aa7b8..37a005df0b95 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -449,7 +449,7 @@ static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip) 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); + emit_jump(&prog, its_static_thunk(reg), ip); } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) { EMIT_LFENCE(); EMIT2(0xFF, 0xE0 + reg);