x86/speculation: Include unprivileged eBPF status in Spectre v2 mitigation reporting

With unprivileged eBPF enabled, eIBRS (without retpoline) is vulnerable
to Spectre v2 BHB-based attacks.

When both are enabled, print a warning message and report it in the
'spectre_v2' sysfs vulnerabilities file.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/cpu/bugs.c

@@ -16,6 +16,7 @@
 #include <linux/prctl.h>
 #include <linux/sched/smt.h>
 #include <linux/pgtable.h>
+#include <linux/bpf.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
@@ -650,6 +651,16 @@ static inline const char *spectre_v2_module_string(void)
 static inline const char *spectre_v2_module_string(void) { return ""; }
 #endif
 
+#define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
+
+#ifdef CONFIG_BPF_SYSCALL
+void unpriv_ebpf_notify(int new_state)
+{
+	if (spectre_v2_enabled == SPECTRE_V2_EIBRS && !new_state)
+		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
+}
+#endif
+
 static inline bool match_option(const char *arg, int arglen, const char *opt)
 {
 	int len = strlen(opt);
@@ -994,6 +1005,9 @@ static void __init spectre_v2_select_mitigation(void)
 		break;
 	}
 
+	if (mode == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
+		pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
+
 	if (spectre_v2_in_eibrs_mode(mode)) {
 		/* Force it so VMEXIT will restore correctly */
 		x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
@@ -1780,6 +1794,20 @@ static char *ibpb_state(void)
 	return "";
 }
 
+static ssize_t spectre_v2_show_state(char *buf)
+{
+	if (spectre_v2_enabled == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
+		return sprintf(buf, "Vulnerable: Unprivileged eBPF enabled\n");
+
+	return sprintf(buf, "%s%s%s%s%s%s\n",
+		       spectre_v2_strings[spectre_v2_enabled],
+		       ibpb_state(),
+		       boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
+		       stibp_state(),
+		       boot_cpu_has(X86_FEATURE_RSB_CTXSW) ? ", RSB filling" : "",
+		       spectre_v2_module_string());
+}
+
 static ssize_t srbds_show_state(char *buf)
 {
 	return sprintf(buf, "%s\n", srbds_strings[srbds_mitigation]);
@@ -1805,12 +1833,7 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
 		return sprintf(buf, "%s\n", spectre_v1_strings[spectre_v1_mitigation]);
 
 	case X86_BUG_SPECTRE_V2:
-		return sprintf(buf, "%s%s%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
-			       ibpb_state(),
-			       boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
-			       stibp_state(),
-			       boot_cpu_has(X86_FEATURE_RSB_CTXSW) ? ", RSB filling" : "",
-			       spectre_v2_module_string());
+		return spectre_v2_show_state(buf);
 
 	case X86_BUG_SPEC_STORE_BYPASS:
 		return sprintf(buf, "%s\n", ssb_strings[ssb_mode]);

include/linux/bpf.h

@@ -1793,6 +1793,11 @@ struct bpf_core_ctx {
 int bpf_core_apply(struct bpf_core_ctx *ctx, const struct bpf_core_relo *relo,
 		   int relo_idx, void *insn);
 
+static inline bool unprivileged_ebpf_enabled(void)
+{
+	return !sysctl_unprivileged_bpf_disabled;
+}
+
 #else /* !CONFIG_BPF_SYSCALL */
 static inline struct bpf_prog *bpf_prog_get(u32 ufd)
 {
@@ -2012,6 +2017,12 @@ bpf_jit_find_kfunc_model(const struct bpf_prog *prog,
 {
 	return NULL;
 }
+
+static inline bool unprivileged_ebpf_enabled(void)
+{
+	return false;
+}
+
 #endif /* CONFIG_BPF_SYSCALL */
 
 void __bpf_free_used_btfs(struct bpf_prog_aux *aux,

kernel/sysctl.c

@@ -180,6 +180,10 @@ static int bpf_stats_handler(struct ctl_table *table, int write,
 	return ret;
 }
 
+void __weak unpriv_ebpf_notify(int new_state)
+{
+}
+
 static int bpf_unpriv_handler(struct ctl_table *table, int write,
 			      void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -197,6 +201,9 @@ static int bpf_unpriv_handler(struct ctl_table *table, int write,
 			return -EPERM;
 		*(int *)table->data = unpriv_enable;
 	}
+
+	unpriv_ebpf_notify(unpriv_enable);
+
 	return ret;
 }
 #endif /* CONFIG_BPF_SYSCALL && CONFIG_SYSCTL */