Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 16, 2022 at 01:45:17PM +0200, Peter Zijlstra wrote:
> On Mon, May 16, 2022 at 11:54:33AM +0200, Peter Zijlstra wrote:
> > On Fri, May 13, 2022 at 01:21:58PM -0700, Sami Tolvanen wrote:

> > >   ; indirect call check
> > >     cmpl    <id>, -6(%r11)
> > >     je      .Ltmp1
> > >     ud2
> > >   .Ltmp1:
> > >     call    __x86_indirect_thunk_r11
> > 
> > The first one I try and find looks like:
> > 
> > 26:       41 81 7b fa a6 96 9e 38         cmpl   $0x389e96a6,-0x6(%r11)
> > 2e:       74 02                   je     32 <__traceiter_sched_kthread_stop+0x29>
> > 30:       0f 0b                   ud2
> > 32:       4c 89 f6                mov    %r14,%rsi
> > 35:       e8 00 00 00 00          call   3a <__traceiter_sched_kthread_stop+0x31> 36: R_X86_64_PLT32      __x86_indirect_thunk_r11-0x4
> > 
> > This must not be. If I'm to rewrite that lot to:
> > 
> >   movl	$\hash, %r10d
> >   sub	$9, %r11
> >   call	*%r11
> >   .nop  4
> > 
> > Then there must not be spurious instruction in between the ud2 and the
> > indirect call/retpoline thing.
> 
> Hmmm.. when I replace it with:
> 
>    movl	$\hash, %r10d
>    sub	$9, %r11
>    .nops 2
> 
> That would work, that has the added benefit of nicely co-existing with
> the current retpoline patching.
> 
> The only remaining problem is how to find this; the .retpoline_sites is
> fairly concenient, but if the compiler can put arbitrary amounts of code
> in between this is going to be somewhat tedious.
> 

Something like the *completely* untested below. It compiles, but because
the above code-gen issue it *cannot* work.

(also, it lacks module support)

@willy, how horribly broken is this xarray usage?

---
 arch/x86/include/asm/traps.h            |   1 +
 arch/x86/kernel/alternative.c           | 316 ++++++++++++++++++++++++++++++++
 arch/x86/kernel/cpu/common.c            |   5 +
 arch/x86/kernel/vmlinux.lds.S           |   9 +
 tools/objtool/check.c                   |  67 ++++++-
 tools/objtool/include/objtool/objtool.h |   1 +
 tools/objtool/objtool.c                 |   1 +
 7 files changed, 399 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 35317c5c551d..a423343cffbc 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -19,6 +19,7 @@ asmlinkage __visible noinstr struct pt_regs *vc_switch_off_ist(struct pt_regs *e
 #endif
 
 extern bool ibt_selftest(void);
+extern bool ibt_broken;
 
 #ifdef CONFIG_X86_F00F_BUG
 /* For handling the FOOF bug */
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d374cb3cf024..abce4e78a1e0 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -18,6 +18,9 @@
 #include <linux/mmu_context.h>
 #include <linux/bsearch.h>
 #include <linux/sync_core.h>
+#include <linux/moduleloader.h>
+#include <linux/xarray.h>
+#include <linux/set_memory.h>
 #include <asm/text-patching.h>
 #include <asm/alternative.h>
 #include <asm/sections.h>
@@ -115,6 +118,7 @@ static void __init_or_module add_nops(void *insns, unsigned int len)
 }
 
 extern s32 __retpoline_sites[], __retpoline_sites_end[];
+extern s32 __cfi_sites[], __cfi_sites_end[];
 extern s32 __ibt_endbr_seal[], __ibt_endbr_seal_end[];
 extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
 extern s32 __smp_locks[], __smp_locks_end[];
@@ -549,6 +553,315 @@ void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { }
 
 #endif /* CONFIG_X86_KERNEL_IBT */
 
+#ifdef CONFIG_CFI_CLANG
+/*
+ *  FineIBT                                         kCFI
+ *
+ *  __fineibt_\hash:
+ *    xor     \hash, %r10     # 7
+ *    jz      1f              # 2
+ *    ud2                     # 2
+ *  1:ret                     # 1
+ *    int3                    # 1
+ *
+ *
+ *  __cfi_\sym:                                     __cfi_\sym:
+ *                                                    int3; int3                              # 2
+ *    endbr                   # 4                     mov     \hash, %eax                     # 5
+ *    call    __fineibt_\hash # 5                     int3; int3                              # 2
+ *  \sym:                                           \sym:
+ *  ...                                             ...
+ *
+ *
+ *  caller:                                         caller:
+ *    movl    \hash, %r10d    # 6                     cmpl    \hash, -6(%r11)                 # 8
+ *    sub     $9, %r11        # 4                     je      1f                              # 2
+ *    nop2                    # 2                     ud2                                     # 2
+ *
+ *    call    *%r11           # 3                     call    __x86_indirect_thunk_r11        # 5
+ *    nop2                    # 2
+ */
+
+static DEFINE_XARRAY(cfi_hashes);
+static int nr_cfi_hashes;
+
+static u32 decode_cfi_preamble(void *addr)
+{
+	u8 *p = addr;
+
+	if (p[0] == 0xcc && p[1] == 0xcc &&
+	    p[2] == 0xb8 &&
+	    p[7] == 0xcc && p[8] == 0xcc)
+		return *(u32 *)(addr + 3);
+
+	return 0; /* invalid hash value */
+}
+
+static u32 decode_cfi_caller(void *addr)
+{
+	u8 *p = addr;
+
+	if (((p[0] == 0x41 && p[1] == 0x81) ||
+	     (p[0] == 0xeb && p[1] == 0x0a)) && p[2] == 0x7b &&
+	    p[8] == 0x74 && p[9] == 0x02 &&
+	    p[10] == 0x0f && p[11] == 0x0b)
+		return *(u32 *)(addr + 4);
+
+	return 0; /* invalid hash value */
+}
+
+// .cfi_sites
+static int cfi_index_hashes(s32 *start, s32 *end)
+{
+	s32 *s;
+
+	for (s = start; s < end; s++) {
+		void *addr = (void *)s + *s;
+		void *xa;
+		u32 hash;
+
+		hash = decode_cfi_preamble(addr);
+		if (!hash) {
+			//WARN();
+			return -EINVAL;
+		}
+
+		xa = xa_store(&cfi_hashes, hash, NULL, GFP_KERNEL);
+		if (xa_is_err(xa)) {
+			//WARN();
+			return xa_err(xa);
+		}
+		nr_cfi_hashes++;
+	}
+
+	return 0;
+}
+
+asm (	".pushsection .rodata\n"
+	"fineibt_template_start:\n"
+	"	xorl	$0x12345678, %r10d\n"	// 7
+	"	je	1f\n"			// 2
+	"	ud2\n"				// 2
+	"1:	ret\n"				// 1
+	"	int3\n"
+	"	int3\n"
+	"	int3\n"
+	"	int3\n"				// 4
+	"fineibt_template_end:\n"
+	".popsection\n"
+    );
+
+extern u8 fineibt_template_start[];
+extern u8 fineibt_template_end[];
+
+static int cfi_create_fineibt_stubs(void)
+{
+	size_t size = 16 * nr_cfi_hashes;
+	int pages = 1 + ((size - 1) >> PAGE_SHIFT);
+	void *text, *entry, *xa;
+	unsigned long hash;
+	int err = -ENOMEM;
+
+	text = module_alloc(size);
+	if (!text)
+		return err;
+
+	entry = text;
+	xa_for_each(&cfi_hashes, hash, xa) {
+
+		memcpy(entry, fineibt_template_start, 16);
+		*(u32 *)(entry + 3) = hash;
+
+		xa = xa_store(&cfi_hashes, hash, entry, GFP_KERNEL);
+		if (xa_is_err(xa)) {
+			err = xa_err(xa);
+			goto err_alloc;
+		}
+		if (xa) {
+			err = -EINVAL;
+			goto err_alloc;
+		}
+
+		entry += 16;
+	}
+
+	set_memory_ro((unsigned long)text, pages);
+	set_memory_x((unsigned long)text, pages);
+
+	return 0;
+
+err_alloc:
+	module_memfree(text);
+	return -EINVAL;
+}
+
+// .retpoline_sites
+static int cfi_disable_callers(s32 *start, s32 *end)
+{
+	/*
+	 * Disable CFI by patching in a 2 byte JMP, this leaves the hash in
+	 * tact for later usage. Also see decode_cfi_caller() and
+	 * cfu_rewrite_callers().
+	 */
+	const u8 jmp12[] = { 0xeb, 0x0a };
+	s32 *s;
+
+	for (s = start; s < end; s++) {
+		void *addr = (void *)s + *s;
+		u32 hash;
+
+		hash = decode_cfi_caller(addr - 12);
+		if (!hash) {
+			// WARN();
+			return -EINVAL;
+		}
+
+		text_poke_early(addr - 12, jmp12, 2);
+	}
+
+	return 0;
+}
+
+asm (	".pushsection .rodata\n"
+	"fineibt_cfi_start:\n"
+	"	endbr64\n"
+	"	call fineibt_caller_start\n"
+	"fineibt_cfi_end:"
+	".popsection\n"
+    );
+
+extern u8 fineibt_cfi_start[];
+extern u8 fineibt_cfi_end[];
+
+// .cfi_sites
+static int cfi_rewrite_cfi(s32 *start, s32 *end)
+{
+	s32 *s;
+
+	for (s = start; s < end; s++) {
+		void *dest, *addr = (void *)s + *s;
+		unsigned long index;
+		u32 hash;
+
+		index = hash = decode_cfi_preamble(addr);
+		dest = xa_find(&cfi_hashes, &index, hash, XA_PRESENT);
+
+		if (WARN_ON_ONCE(index != hash || !dest))
+			return -EINVAL;
+
+		text_poke_early(addr, fineibt_cfi_start,
+				(fineibt_cfi_end - fineibt_cfi_start));
+
+		__text_gen_insn(addr + 4,
+				CALL_INSN_OPCODE, addr + 4,
+				dest, CALL_INSN_SIZE);
+	}
+
+	return 0;
+}
+
+asm (	".pushsection .rodata\n"
+	"fineibt_caller_start:\n"
+	"	movl	$0x12345678, %r10d\n"
+	"	sub	$9, %r11\n"
+	"	.nops	2\n"
+	"fineibt_caller_end:"
+	".popsection\n"
+    );
+
+extern u8 fineibt_caller_start[];
+extern u8 fineibt_caller_end[];
+
+// .retpoline_sites
+static int cfi_rewrite_callers(s32 *start, s32 *end)
+{
+	s32 *s;
+
+	for (s = start; s < end; s++) {
+		void *addr = (void *)s + *s;
+		u32 hash;
+
+		hash = decode_cfi_caller(addr - 12);
+
+		if (WARN_ON_ONCE(!hash))
+			return -EINVAL;
+
+		text_poke_early(addr - 12, fineibt_caller_start,
+				(fineibt_caller_end - fineibt_caller_end));
+
+		*(u32 *)(addr - 12 + 2) = hash;
+
+		/* rely on apply_retpolines() to rewrite the actual call */
+	}
+
+	return 0;
+}
+
+bool __ro_after_init ibt_broken = false;
+
+static void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
+			  s32 *start_cfi, s32 *end_cfi)
+{
+	int ret;
+
+	/* If IBT, use FineIBT */
+	if (!HAS_KERNEL_IBT || !cpu_feature_enabled(X86_FEATURE_IBT))
+		return;
+
+	/*
+	 * Find and count all unique hash values.
+	 */
+	ret = cfi_index_hashes(start_cfi, end_cfi);
+	if (ret)
+		goto err;
+
+	/*
+	 * Allocate module memory and write FineIBT stubs.
+	 */
+	ret = cfi_create_fineibt_stubs();
+	if (ret)
+		goto err;
+
+	/*
+	 * Rewrite the callers to not use the __cfi_ stubs, such that we might
+	 * rewrite them. Disables all CFI. If this succeeds but any of the
+	 * later stages fails, we're CFI-less.
+	 */
+	ret = cfi_disable_callers(start_retpoline, end_retpoline);
+	if (ret)
+		goto err;
+
+	/*
+	 * Rewrite the __cfi_ stubs from kCFI to FineIBT.
+	 */
+	ret = cfi_rewrite_cfi(start_cfi, end_cfi);
+	if (ret)
+		goto err;
+
+	/*
+	 * Now that everything is in place; rewrite the callers to FineIBT.
+	 */
+	ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
+	if (ret)
+		goto err;
+
+	return;
+
+err:
+	pr_err("Something went horribly wrong trying to rewrite the CFI implementation.\n");
+	/* must *NOT* enable IBT */
+	ibt_broken = true;
+}
+
+#else
+
+static void apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
+			  s32 *start_cfi, s32 *end_cfi)
+{
+}
+
+#endif
+
 #ifdef CONFIG_SMP
 static void alternatives_smp_lock(const s32 *start, const s32 *end,
 				  u8 *text, u8 *text_end)
@@ -855,6 +1168,9 @@ void __init alternative_instructions(void)
 	 */
 	apply_paravirt(__parainstructions, __parainstructions_end);
 
+	apply_fineibt(__retpoline_sites, __retpoline_sites_end,
+		      __cfi_sites, __cfi_sites_end);
+
 	/*
 	 * Rewrite the retpolines, must be done before alternatives since
 	 * those can rewrite the retpoline thunks.
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index e342ae4db3c4..e4377256b952 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -630,6 +630,11 @@ static __always_inline void setup_cet(struct cpuinfo_x86 *c)
 	    !cpu_feature_enabled(X86_FEATURE_IBT))
 		return;
 
+#ifdef CONFIG_CFI_CLANG
+	if (ibt_broken)
+		return;
+#endif
+
 	wrmsrl(MSR_IA32_S_CET, msr);
 	cr4_set_bits(X86_CR4_CET);
 
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 7fda7f27e762..72ffc91ddd20 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -294,6 +294,15 @@ SECTIONS
 	}
 #endif
 
+#if defined(CONFIG_CFI_CLANG) && defined(CONFIG_RETPOLINE) && defined(CONFIG_X86_KERNEL_IBT)
+	. = ALIGN(8);
+	.cfi_sites : AT(ADDR(.cfi_sites) - LOAD_OFFSET) {
+		__cfi_sites = .;
+		*(.cfi_sites)
+		__cfi_sites_end = .;
+	}
+#endif
+
 	/*
 	 * struct alt_inst entries. From the header (alternative.h):
 	 * "Alternative instructions for different CPU types or capabilities"
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 88f005ae6dcc..edc8aecf229c 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -797,6 +797,52 @@ static int create_ibt_endbr_seal_sections(struct objtool_file *file)
 	return 0;
 }
 
+static int create_cfi_sections(struct objtool_file *file)
+{
+	struct instruction *insn;
+	struct section *sec;
+	int idx;
+
+	sec = find_section_by_name(file->elf, ".cfi_sites");
+	if (sec) {
+		WARN("file already has .cfi_sites, skipping");
+		return 0;
+	}
+
+	idx = 0;
+	list_for_each_entry(insn, &file->cfi_list, call_node)
+		idx++;
+
+	if (!idx)
+		return 0;
+
+	sec = elf_create_section(file->elf, ".cfi_sites", 0,
+				 sizeof(int), idx);
+	if (!sec) {
+		WARN("elf_create_section: .cfi_sites");
+		return -1;
+	}
+
+	idx = 0;
+	list_for_each_entry(insn, &file->cfi_list, call_node) {
+
+		int *site = (int *)sec->data->d_buf + idx;
+		*site = 0;
+
+		if (elf_add_reloc_to_insn(file->elf, sec,
+					  idx * sizeof(int),
+					  R_X86_64_PC32,
+					  insn->sec, insn->offset)) {
+			WARN("elf_add_reloc_to_insn: .cfi_sites");
+			return -1;
+		}
+
+		idx++;
+	}
+
+	return 0;
+}
+
 static int create_mcount_loc_sections(struct objtool_file *file)
 {
 	struct section *sec;
@@ -3301,6 +3347,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 {
 	struct alternative *alt;
 	struct instruction *next_insn, *prev_insn = NULL;
+	struct instruction *first_insn = insn;
 	struct section *sec;
 	u8 visited;
 	int ret;
@@ -3312,8 +3359,19 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		if (func && insn->func && func != insn->func->pfunc) {
 			/* Ignore KCFI type preambles, which always fall through */
-			if (!strncmp(func->name, "__cfi_", 6))
+			if (!strncmp(func->name, "__cfi_", 6)) {
+				/*
+				 * If the function has a __cfi_ preamble, the endbr
+				 * will live in there.
+				 */
+				insn->noendbr = true;
+				/*
+				 * The preamble starts with INSN_TRAP,
+				 * call_node cannot be used.
+				 */
+				list_add_tail(&first_insn->call_node, &file->cfi_list);
 				return 0;
+			}
 
 			WARN("%s() falls through to next function %s()",
 			     func->name, insn->func->name);
@@ -3953,6 +4011,13 @@ int check(struct objtool_file *file)
 		warnings += ret;
 	}
 
+	if (ibt && retpoline) {
+		ret = create_cfi_sections(file);
+		if (ret < 0)
+			goto out;
+		warnings += ret;
+	}
+
 	if (stats) {
 		printf("nr_insns_visited: %ld\n", nr_insns_visited);
 		printf("nr_cfi: %ld\n", nr_cfi);
diff --git a/tools/objtool/include/objtool/objtool.h b/tools/objtool/include/objtool/objtool.h
index a6e72d916807..93f52e275fa6 100644
--- a/tools/objtool/include/objtool/objtool.h
+++ b/tools/objtool/include/objtool/objtool.h
@@ -27,6 +27,7 @@ struct objtool_file {
 	struct list_head static_call_list;
 	struct list_head mcount_loc_list;
 	struct list_head endbr_list;
+	struct list_head cfi_list;
 	bool ignore_unreachables, hints, rodata;
 
 	unsigned int nr_endbr;
diff --git a/tools/objtool/objtool.c b/tools/objtool/objtool.c
index 843ff3c2f28e..16ed3613b0e2 100644
--- a/tools/objtool/objtool.c
+++ b/tools/objtool/objtool.c
@@ -129,6 +129,7 @@ struct objtool_file *objtool_open_read(const char *_objname)
 	INIT_LIST_HEAD(&file.static_call_list);
 	INIT_LIST_HEAD(&file.mcount_loc_list);
 	INIT_LIST_HEAD(&file.endbr_list);
+	INIT_LIST_HEAD(&file.cfi_list);
 	file.ignore_unreachables = no_unreachable;
 	file.hints = false;