6.6-stable review patch. If anyone has any objections, please let me know.
------------------
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: Pawan Gupta pawan.kumar.gupta@linux.intel.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Reviewed-by: Alexandre Chartre alexandre.chartre@oracle.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/alternative.h | 10 ++ arch/x86/kernel/alternative.c | 132 ++++++++++++++++++++++++++++++++++++- arch/x86/kernel/module.c | 7 + include/linux/module.h | 5 + 4 files changed, 151 insertions(+), 3 deletions(-)
--- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -130,6 +130,16 @@ static __always_inline int x86_call_dept } #endif
+#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_OBJTOOL) extern bool cpu_wants_rethunk(void); extern bool cpu_wants_rethunk_at(void *addr); --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -18,6 +18,8 @@ #include <linux/mmu_context.h> #include <linux/bsearch.h> #include <linux/sync_core.h> +#include <linux/moduleloader.h> +#include <linux/cleanup.h> #include <asm/text-patching.h> #include <asm/alternative.h> #include <asm/sections.h> @@ -31,6 +33,7 @@ #include <asm/paravirt.h> #include <asm/asm-prototypes.h> #include <asm/cfi.h> +#include <asm/set_memory.h>
int __read_mostly alternatives_patched;
@@ -124,6 +127,125 @@ const unsigned char * const x86_nops[ASM #endif };
+#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) +{ + 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 (int i = 0; i < mod->its_num_pages; i++) { + void *page = mod->its_page_array[i]; + set_memory_rox((unsigned long)page, 1); + } +} + +void its_free_mod(struct module *mod) +{ + if (!cpu_feature_enabled(X86_FEATURE_INDIRECT_THUNK_ITS)) + return; + + for (int i = 0; i < mod->its_num_pages; i++) { + void *page = mod->its_page_array[i]; + module_memfree(page); + } + kfree(mod->its_page_array); +} + +DEFINE_FREE(its_execmem, void *, if (_T) module_memfree(_T)); + +static void *its_alloc(void) +{ + void *page __free(its_execmem) = 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) + return NULL; + + its_mod->its_page_array = tmp; + its_mod->its_page_array[its_mod->its_num_pages++] = page; + } + + return no_free_ptr(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_rox((unsigned long)its_page, 1); + + return thunk; +} + +#endif + /* * Fill the buffer with a single effective instruction of size @len. * @@ -577,9 +699,13 @@ static int emit_call_track_retpoline(voi #ifdef CONFIG_MITIGATION_ITS 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 */ --- a/arch/x86/kernel/module.c +++ b/arch/x86/kernel/module.c @@ -312,6 +312,9 @@ int module_finalize(const Elf_Ehdr *hdr, void *pseg = (void *)para->sh_addr; apply_paravirt(pseg, pseg + para->sh_size); } + + its_init_mod(me); + if (retpolines || cfi) { void *rseg = NULL, *cseg = NULL; unsigned int rsize = 0, csize = 0; @@ -332,6 +335,9 @@ int module_finalize(const Elf_Ehdr *hdr, 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); @@ -379,4 +385,5 @@ int module_finalize(const Elf_Ehdr *hdr, void module_arch_cleanup(struct module *mod) { alternatives_smp_module_del(mod); + its_free_mod(mod); } --- a/include/linux/module.h +++ b/include/linux/module.h @@ -572,6 +572,11 @@ 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;