From: Peter Zijlstra peterz@infradead.org
commit 872df34d7c51a79523820ea6a14860398c639b87 upstream.
ITS mitigation moves the unsafe indirect branches to a safe thunk. This could degrade the prediction accuracy as the source address of indirect branches becomes same for different execution paths.
To improve the predictions, and hence the performance, assign a separate thunk for each indirect callsite. This is also a defense-in-depth measure to avoid indirect branches aliasing with each other.
As an example, 5000 dynamic thunks would utilize around 16 bits of the address space, thereby gaining entropy. For a BTB that uses 32 bits for indexing, dynamic thunks could provide better prediction accuracy over fixed thunks.
Have ITS thunks be variable sized and use EXECMEM_MODULE_TEXT such that they are both more flexible (got to extend them later) and live in 2M TLBs, just like kernel code, avoiding undue TLB pressure.
[ pawan: CONFIG_EXECMEM and CONFIG_EXECMEM_ROX are not supported on backport kernel, made changes to use module_alloc() and set_memory_*() for dynamic thunks. ]
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Reviewed-by: Alexandre Chartre alexandre.chartre@oracle.com Signed-off-by: Pawan Gupta pawan.kumar.gupta@linux.intel.com --- arch/x86/include/asm/alternative.h | 10 +++ arch/x86/kernel/alternative.c | 133 ++++++++++++++++++++++++++++++++++++- arch/x86/kernel/module.c | 6 ++ include/linux/module.h | 5 ++ 4 files changed, 151 insertions(+), 3 deletions(-)
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h index 4b9a2842f90e..4ab021fd3887 100644 --- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -80,6 +80,16 @@ extern void apply_returns(s32 *start, s32 *end);
struct module;
+#ifdef CONFIG_MITIGATION_ITS +extern void its_init_mod(struct module *mod); +extern void its_fini_mod(struct module *mod); +extern void its_free_mod(struct module *mod); +#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) { } +#endif + #if defined(CONFIG_RETHUNK) && defined(CONFIG_STACK_VALIDATION) extern bool cpu_wants_rethunk(void); extern bool cpu_wants_rethunk_at(void *addr); diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index ae4a6bc25b29..33d17fe84182 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -18,6 +18,7 @@ #include <linux/mmu_context.h> #include <linux/bsearch.h> #include <linux/sync_core.h> +#include <linux/moduleloader.h> #include <asm/text-patching.h> #include <asm/alternative.h> #include <asm/sections.h> @@ -29,6 +30,7 @@ #include <asm/io.h> #include <asm/fixmap.h> #include <asm/asm-prototypes.h> +#include <asm/set_memory.h>
int __read_mostly alternatives_patched;
@@ -552,6 +554,127 @@ static int emit_indirect(int op, int reg, u8 *bytes)
#ifdef CONFIG_MITIGATION_ITS
+static struct module *its_mod; +static void *its_page; +static unsigned int its_offset; + +/* Initialize a thunk with the "jmp *reg; int3" instructions. */ +static void *its_init_thunk(void *thunk, int reg) +{ + u8 *bytes = thunk; + int i = 0; + + if (reg >= 8) { + bytes[i++] = 0x41; /* REX.B prefix */ + reg -= 8; + } + bytes[i++] = 0xff; + bytes[i++] = 0xe0 + reg; /* jmp *reg */ + bytes[i++] = 0xcc; + + return thunk; +} + +void its_init_mod(struct module *mod) +{ + if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) + return; + + mutex_lock(&text_mutex); + its_mod = mod; + its_page = NULL; +} + +void its_fini_mod(struct module *mod) +{ + int i; + + if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) + return; + + WARN_ON_ONCE(its_mod != mod); + + its_mod = NULL; + its_page = NULL; + mutex_unlock(&text_mutex); + + for (i = 0; i < mod->its_num_pages; i++) { + void *page = mod->its_page_array[i]; + set_memory_ro((unsigned long)page, 1); + set_memory_x((unsigned long)page, 1); + } +} + +void its_free_mod(struct module *mod) +{ + int i; + + if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) + return; + + for (i = 0; i < mod->its_num_pages; i++) { + void *page = mod->its_page_array[i]; + module_memfree(page); + } + kfree(mod->its_page_array); +} + +static void *its_alloc(void) +{ + void *page = module_alloc(PAGE_SIZE); + + if (!page) + return NULL; + + if (its_mod) { + void *tmp = krealloc(its_mod->its_page_array, + (its_mod->its_num_pages+1) * sizeof(void *), + GFP_KERNEL); + if (!tmp) { + module_memfree(page); + return NULL; + } + + its_mod->its_page_array = tmp; + its_mod->its_page_array[its_mod->its_num_pages++] = page; + } + + return page; +} + +static void *its_allocate_thunk(int reg) +{ + int size = 3 + (reg / 8); + void *thunk; + + if (!its_page || (its_offset + size - 1) >= PAGE_SIZE) { + its_page = its_alloc(); + if (!its_page) { + pr_err("ITS page allocation failed\n"); + return NULL; + } + memset(its_page, INT3_INSN_OPCODE, PAGE_SIZE); + its_offset = 32; + } + + /* + * If the indirect branch instruction will be in the lower half + * of a cacheline, then update the offset to reach the upper half. + */ + if ((its_offset + size - 1) % 64 < 32) + its_offset = ((its_offset - 1) | 0x3F) + 33; + + thunk = its_page + its_offset; + its_offset += size; + + set_memory_rw((unsigned long)its_page, 1); + thunk = its_init_thunk(thunk, reg); + set_memory_ro((unsigned long)its_page, 1); + set_memory_x((unsigned long)its_page, 1); + + return thunk; +} + static int __emit_trampoline(void *addr, struct insn *insn, u8 *bytes, void *call_dest, void *jmp_dest) { @@ -599,9 +722,13 @@ static int __emit_trampoline(void *addr, struct insn *insn, u8 *bytes,
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]); + u8 *thunk = __x86_indirect_its_thunk_array[reg]; + u8 *tmp = its_allocate_thunk(reg); + + if (tmp) + thunk = tmp; + + return __emit_trampoline(addr, insn, bytes, thunk, thunk); }
/* Check if an indirect branch is at ITS-unsafe address */ diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c index e28c701e8f8b..1ab8e583c795 100644 --- a/arch/x86/kernel/module.c +++ b/arch/x86/kernel/module.c @@ -274,10 +274,15 @@ int module_finalize(const Elf_Ehdr *hdr, returns = s; }
+ its_init_mod(me); + if (retpolines) { void *rseg = (void *)retpolines->sh_addr; apply_retpolines(rseg, rseg + retpolines->sh_size); } + + its_fini_mod(me); + if (returns) { void *rseg = (void *)returns->sh_addr; apply_returns(rseg, rseg + returns->sh_size); @@ -313,4 +318,5 @@ int module_finalize(const Elf_Ehdr *hdr, void module_arch_cleanup(struct module *mod) { alternatives_smp_module_del(mod); + its_free_mod(mod); } diff --git a/include/linux/module.h b/include/linux/module.h index 63fe94e6ae6f..f5a150c42918 100644 --- a/include/linux/module.h +++ b/include/linux/module.h @@ -524,6 +524,11 @@ struct module { atomic_t refcnt; #endif
+#ifdef CONFIG_MITIGATION_ITS + int its_num_pages; + void **its_page_array; +#endif + #ifdef CONFIG_CONSTRUCTORS /* Constructor functions. */ ctor_fn_t *ctors;