Re: [RFC PATCH v2 22/27] x86/cet/ibt: User-mode indirect branch tracking support

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index d9ae3d86cdd7..71da2cccba16 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h

@@ -12,7 +12,10 @@ struct task_struct;
 struct cet_status {
 	unsigned long	shstk_base;
 	unsigned long	shstk_size;
+	unsigned long	ibt_bitmap_addr;
+	unsigned long	ibt_bitmap_size;
 	unsigned int	shstk_enabled:1;
+	unsigned int	ibt_enabled:1;
 };

+static unsigned long ibt_mmap(unsigned long addr, unsigned long len)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long populate;
+
+	down_write(&mm->mmap_sem);
+	addr = do_mmap(NULL, addr, len, PROT_READ | PROT_WRITE,
+		       MAP_ANONYMOUS | MAP_PRIVATE,
+		       VM_DONTDUMP, 0, &populate, NULL);
+	up_write(&mm->mmap_sem);
+
+	if (populate)
+		mm_populate(addr, populate);
+
+	return addr;
+}

+int cet_setup_ibt_bitmap(void)
+{
+	u64 r;
+	unsigned long bitmap;
+	unsigned long size;
+
+	if (!cpu_feature_enabled(X86_FEATURE_IBT))
+		return -EOPNOTSUPP;
+
+	size = TASK_SIZE_MAX / PAGE_SIZE / BITS_PER_BYTE;

+	bitmap = ibt_mmap(0, size);
+
+	if (bitmap >= TASK_SIZE_MAX)
+		return -ENOMEM;
+
+	bitmap &= PAGE_MASK;

+	rdmsrl(MSR_IA32_U_CET, r);
+	r |= (MSR_IA32_CET_LEG_IW_EN | bitmap);
+	wrmsrl(MSR_IA32_U_CET, r);

+	current->thread.cet.ibt_bitmap_addr = bitmap;
+	current->thread.cet.ibt_bitmap_size = size;
+	return 0;
+}
+
+void cet_disable_ibt(void)
+{
+	u64 r;
+
+	if (!cpu_feature_enabled(X86_FEATURE_IBT))
+		return;

+	rdmsrl(MSR_IA32_U_CET, r);
+	r &= ~(MSR_IA32_CET_ENDBR_EN | MSR_IA32_CET_LEG_IW_EN |
+	       MSR_IA32_CET_NO_TRACK_EN);
+	wrmsrl(MSR_IA32_U_CET, r);
+	current->thread.cet.ibt_enabled = 0;
+}

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 705467839ce8..c609c9ce5691 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c

@@ -413,7 +413,8 @@ __setup("nopku", setup_disable_pku);
 
 static __always_inline void setup_cet(struct cpuinfo_x86 *c)
 {
-	if (cpu_feature_enabled(X86_FEATURE_SHSTK))
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+	    cpu_feature_enabled(X86_FEATURE_IBT))
 		cr4_set_bits(X86_CR4_CET);
 }
 

@@ -434,6 +435,23 @@ static __init int setup_disable_shstk(char *s)
 __setup("no_cet_shstk", setup_disable_shstk);
 #endif
 
+#ifdef CONFIG_X86_INTEL_BRANCH_TRACKING_USER
+static __init int setup_disable_ibt(char *s)
+{
+	/* require an exact match without trailing characters */
+	if (strlen(s))
+		return 0;
+
+	if (!boot_cpu_has(X86_FEATURE_IBT))
+		return 1;
+
+	setup_clear_cpu_cap(X86_FEATURE_IBT);
+	pr_info("x86: 'no_cet_ibt' specified, disabling Branch Tracking\n");
+	return 1;
+}
+__setup("no_cet_ibt", setup_disable_ibt);
+#endif
 /*
  * Some CPU features depend on higher CPUID levels, which may not always
  * be available due to CPUID level capping or broken virtualization

diff --git a/arch/x86/kernel/elf.c b/arch/x86/kernel/elf.c
index 233f6dad9c1f..42e08d3b573e 100644
--- a/arch/x86/kernel/elf.c
+++ b/arch/x86/kernel/elf.c

@@ -15,6 +15,7 @@
 #include <linux/fs.h>
 #include <linux/uaccess.h>
 #include <linux/string.h>
+#include <linux/compat.h>
 
 /*
  * The .note.gnu.property layout:

@@ -222,7 +223,8 @@ int arch_setup_features(void *ehdr_p, void *phdr_p,
 
 	struct elf64_hdr *ehdr64 = ehdr_p;
 
-	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+	if (!cpu_feature_enabled(X86_FEATURE_SHSTK) &&
+	    !cpu_feature_enabled(X86_FEATURE_IBT))
 		return 0;
 
 	if (ehdr64->e_ident[EI_CLASS] == ELFCLASS64) {

@@ -250,6 +252,9 @@ int arch_setup_features(void *ehdr_p, void *phdr_p,
 	current->thread.cet.shstk_enabled = 0;
 	current->thread.cet.shstk_base = 0;
 	current->thread.cet.shstk_size = 0;
+	current->thread.cet.ibt_enabled = 0;
+	current->thread.cet.ibt_bitmap_addr = 0;
+	current->thread.cet.ibt_bitmap_size = 0;
 	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
 		if (shstk) {
 			err = cet_setup_shstk();

@@ -257,6 +262,15 @@ int arch_setup_features(void *ehdr_p, void *phdr_p,
 				goto out;
 		}
 	}
+
+	if (cpu_feature_enabled(X86_FEATURE_IBT)) {
+		if (ibt) {
+			err = cet_setup_ibt();
+			if (err < 0)
+				goto out;
+		}
+	}