prctl: cfi: change the branch landing pad prctl()s to be more descriptive

Per Linus' comments requesting the replacement of "INDIR_BR_LP" in the
indirect branch tracking prctl()s with something more readable, and
suggesting the use of the speculation control prctl()s as an exemplar,
reimplement the prctl()s and related constants that control per-task
forward-edge control flow integrity.

This primarily involves two changes. First, the prctls are
restructured to resemble the style of the speculative execution
workaround control prctls PR_{GET,SET}_SPECULATION_CTRL, to make them
easier to extend in the future. Second, the "indir_br_lp" abbrevation
is expanded to "branch_landing_pads" to be less telegraphic. The
kselftest and documentation is adjusted accordingly.

Link: https://lore.kernel.org/linux-riscv/CAHk-=[e-mail]/
Cc: Deepak Gupta <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Mark Brown <[e-mail]>
Signed-off-by: Paul Walmsley <[e-mail]>

Documentation/arch/riscv/zicfilp.rst

···
 4. prctl() enabling
 --------------------
 
-:c:macro:`PR_SET_INDIR_BR_LP_STATUS` / :c:macro:`PR_GET_INDIR_BR_LP_STATUS` /
-:c:macro:`PR_LOCK_INDIR_BR_LP_STATUS` are three prctls added to manage indirect
-branch tracking.  These prctls are architecture-agnostic and return -EINVAL if
-the underlying functionality is not supported.
+Per-task indirect branch tracking state can be monitored and
+controlled via the :c:macro:`PR_GET_CFI` and :c:macro:`PR_SET_CFI`
+``prctl()` arguments (respectively), by supplying
+:c:macro:`PR_CFI_BRANCH_LANDING_PADS` as the second argument.  These
+are architecture-agnostic, and will return -EINVAL if the underlying
+functionality is not supported.
 
-* prctl(PR_SET_INDIR_BR_LP_STATUS, unsigned long arg)
+* prctl(:c:macro:`PR_SET_CFI`, :c:macro:`PR_CFI_BRANCH_LANDING_PADS`, unsigned long arg)
 
-If arg1 is :c:macro:`PR_INDIR_BR_LP_ENABLE` and if CPU supports
-``zicfilp`` then the kernel will enable indirect branch tracking for the
-task.  The dynamic loader can issue this :c:macro:`prctl` once it has
+arg is a bitmask.
+
+If :c:macro:`PR_CFI_ENABLE` is set in arg, and the CPU supports
+``zicfilp``, then the kernel will enable indirect branch tracking for
+the task.  The dynamic loader can issue this ``prctl()`` once it has
 determined that all the objects loaded in the address space support
-indirect branch tracking.  Additionally, if there is a `dlopen` to an
-object which wasn't compiled with ``zicfilp``, the dynamic loader can
-issue this prctl with arg1 set to 0 (i.e. :c:macro:`PR_INDIR_BR_LP_ENABLE`
-cleared).
+indirect branch tracking.
 
-* prctl(PR_GET_INDIR_BR_LP_STATUS, unsigned long * arg)
+Indirect branch tracking state can also be locked once enabled.  This
+prevents the task from subsequently disabling it.  This is done by
+setting the bit :c:macro:`PR_CFI_LOCK` in arg.  Either indirect branch
+tracking must already be enabled for the task, or the bit
+:c:macro:`PR_CFI_ENABLE` must also be set in arg.  This is intended
+for environments that wish to run with a strict security posture that
+do not wish to load objects without ``zicfilp`` support.
 
-Returns the current status of indirect branch tracking. If enabled
-it'll return :c:macro:`PR_INDIR_BR_LP_ENABLE`
+Indirect branch tracking can also be disabled for the task, assuming
+that it has not previously been enabled and locked.  If there is a
+``dlopen()`` to an object which wasn't compiled with ``zicfilp``, the
+dynamic loader can issue this ``prctl()`` with arg set to
+:c:macro:`PR_CFI_DISABLE`.  Disabling indirect branch tracking for the
+task is not possible if it has previously been enabled and locked.
 
-* prctl(PR_LOCK_INDIR_BR_LP_STATUS, unsigned long arg)
 
-Locks the current status of indirect branch tracking on the task. User
-space may want to run with a strict security posture and wouldn't want
-loading of objects without ``zicfilp`` support in them, to disallow
-disabling of indirect branch tracking. In this case, user space can
-use this prctl to lock the current settings.
+* prctl(:c:macro:`PR_GET_CFI`, :c:macro:`PR_CFI_BRANCH_LANDING_PADS`, unsigned long * arg)
+
+Returns the current status of indirect branch tracking into a bitmask
+stored into the memory location pointed to by arg.  The bitmask will
+have the :c:macro:`PR_CFI_ENABLE` bit set if indirect branch tracking
+is currently enabled for the task, and if it is locked, will
+additionally have the :c:macro:`PR_CFI_LOCK` bit set.  If indirect
+branch tracking is currently disabled for the task, the
+:c:macro:`PR_CFI_DISABLE` bit will be set.
+
 
 5. violations related to indirect branch tracking
 --------------------------------------------------

arch/riscv/kernel/usercfi.c

···
 	if (!is_user_lpad_enabled())
 		return -EINVAL;
 
-	/* indirect branch tracking is enabled on the task or not */
-	fcfi_status |= (is_indir_lp_enabled(t) ? PR_INDIR_BR_LP_ENABLE : 0);
+	fcfi_status = (is_indir_lp_enabled(t) ? PR_CFI_ENABLE : PR_CFI_DISABLE);
+	fcfi_status |= (is_indir_lp_locked(t) ? PR_CFI_LOCK : 0);
 
 	return copy_to_user(state, &fcfi_status, sizeof(fcfi_status)) ? -EFAULT : 0;
 }
 
 int arch_prctl_set_branch_landing_pad_state(struct task_struct *t, unsigned long state)
 {
-	bool enable_indir_lp = false;
-
 	if (!is_user_lpad_enabled())
 		return -EINVAL;
 
···
 	if (is_indir_lp_locked(t))
 		return -EINVAL;
 
-	/* Reject unknown flags */
-	if (state & ~PR_INDIR_BR_LP_ENABLE)
+	if (!(state & (PR_CFI_ENABLE | PR_CFI_DISABLE)))
 		return -EINVAL;
 
-	enable_indir_lp = (state & PR_INDIR_BR_LP_ENABLE);
-	set_indir_lp_status(t, enable_indir_lp);
+	if (state & PR_CFI_ENABLE && state & PR_CFI_DISABLE)
+		return -EINVAL;
+
+	set_indir_lp_status(t, !!(state & PR_CFI_ENABLE));
 
 	return 0;
 }

include/uapi/linux/prctl.h

···
 # define PR_RSEQ_SLICE_EXT_ENABLE		0x01
 
 /*
- * Get the current indirect branch tracking configuration for the current
- * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS.
+ * Get or set the control flow integrity (CFI) configuration for the
+ * current thread.
+ *
+ * Some per-thread control flow integrity settings are not yet
+ * controlled through this prctl(); see for example
+ * PR_{GET,SET,LOCK}_SHADOW_STACK_STATUS
  */
-#define PR_GET_INDIR_BR_LP_STATUS      80
-
+#define PR_GET_CFI	80
+#define PR_SET_CFI	81
 /*
- * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will
- * enable cpu feature for user thread, to track all indirect branches and ensure
- * they land on arch defined landing pad instruction.
- * x86 - If enabled, an indirect branch must land on an ENDBRANCH instruction.
- * arch64 - If enabled, an indirect branch must land on a BTI instruction.
- * riscv - If enabled, an indirect branch must land on an lpad instruction.
- * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect
- * branches will no more be tracked by cpu to land on arch defined landing pad
- * instruction.
+ * Forward-edge CFI variants (excluding ARM64 BTI, which has its own
+ * prctl()s).
  */
-#define PR_SET_INDIR_BR_LP_STATUS      81
-# define PR_INDIR_BR_LP_ENABLE		   (1UL << 0)
-
-/*
- * Prevent further changes to the specified indirect branch tracking
- * configuration.  All bits may be locked via this call, including
- * undefined bits.
- */
-#define PR_LOCK_INDIR_BR_LP_STATUS      82
+#define PR_CFI_BRANCH_LANDING_PADS	0
+/* Return and control values for PR_{GET,SET}_CFI */
+# define PR_CFI_ENABLE		_BITUL(0)
+# define PR_CFI_DISABLE		_BITUL(1)
+# define PR_CFI_LOCK		_BITUL(2)
 
 #endif /* _LINUX_PRCTL_H */

kernel/sys.c

···
 			return -EINVAL;
 		error = rseq_slice_extension_prctl(arg2, arg3);
 		break;
-	case PR_GET_INDIR_BR_LP_STATUS:
-		if (arg3 || arg4 || arg5)
+	case PR_GET_CFI:
+		if (arg2 != PR_CFI_BRANCH_LANDING_PADS)
 			return -EINVAL;
-		error = arch_prctl_get_branch_landing_pad_state(me, (unsigned long __user *)arg2);
+		if (arg4 || arg5)
+			return -EINVAL;
+		error = arch_prctl_get_branch_landing_pad_state(me, (unsigned long __user *)arg3);
 		break;
-	case PR_SET_INDIR_BR_LP_STATUS:
-		if (arg3 || arg4 || arg5)
+	case PR_SET_CFI:
+		if (arg2 != PR_CFI_BRANCH_LANDING_PADS)
 			return -EINVAL;
-		error = arch_prctl_set_branch_landing_pad_state(me, arg2);
-		break;
-	case PR_LOCK_INDIR_BR_LP_STATUS:
-		if (arg2 || arg3 || arg4 || arg5)
+		if (arg4 || arg5)
 			return -EINVAL;
-		error = arch_prctl_lock_branch_landing_pad_state(me);
+		error = arch_prctl_set_branch_landing_pad_state(me, arg3);
+		if (error)
+			break;
+		if (arg3 & PR_CFI_LOCK && !(arg3 & PR_CFI_DISABLE))
+			error = arch_prctl_lock_branch_landing_pad_state(me);
 		break;
 	default:
 		trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5);

tools/perf/trace/beauty/include/uapi/linux/prctl.h

···
 # define PR_RSEQ_SLICE_EXT_ENABLE		0x01
 
 /*
- * Get the current indirect branch tracking configuration for the current
- * thread, this will be the value configured via PR_SET_INDIR_BR_LP_STATUS.
+ * Get or set the control flow integrity (CFI) configuration for the
+ * current thread.
+ *
+ * Some per-thread control flow integrity settings are not yet
+ * controlled through this prctl(); see for example
+ * PR_{GET,SET,LOCK}_SHADOW_STACK_STATUS
  */
-#define PR_GET_INDIR_BR_LP_STATUS      80
-
+#define PR_GET_CFI	80
+#define PR_SET_CFI	81
 /*
- * Set the indirect branch tracking configuration. PR_INDIR_BR_LP_ENABLE will
- * enable cpu feature for user thread, to track all indirect branches and ensure
- * they land on arch defined landing pad instruction.
- * x86 - If enabled, an indirect branch must land on an ENDBRANCH instruction.
- * arch64 - If enabled, an indirect branch must land on a BTI instruction.
- * riscv - If enabled, an indirect branch must land on an lpad instruction.
- * PR_INDIR_BR_LP_DISABLE will disable feature for user thread and indirect
- * branches will no more be tracked by cpu to land on arch defined landing pad
- * instruction.
+ * Forward-edge CFI variants (excluding ARM64 BTI, which has its own
+ * prctl()s).
  */
-#define PR_SET_INDIR_BR_LP_STATUS      81
-# define PR_INDIR_BR_LP_ENABLE		   (1UL << 0)
+#define PR_CFI_BRANCH_LANDING_PADS	0
+/* Return and control values for PR_{GET,SET}_CFI */
+# define PR_CFI_ENABLE		_BITUL(0)
+# define PR_CFI_DISABLE		_BITUL(1)
+# define PR_CFI_LOCK		_BITUL(2)
 
-/*
- * Prevent further changes to the specified indirect branch tracking
- * configuration.  All bits may be locked via this call, including
- * undefined bits.
- */
-#define PR_LOCK_INDIR_BR_LP_STATUS      82
 
 #endif /* _LINUX_PRCTL_H */

tools/testing/selftests/riscv/cfi/cfitests.c

···
 	 * pads for user mode except lighting up a bit in senvcfg via a prctl.
 	 * Enable landing pad support throughout the execution of the test binary.
 	 */
-	ret = my_syscall5(__NR_prctl, PR_GET_INDIR_BR_LP_STATUS, &lpad_status, 0, 0, 0);
+	ret = my_syscall5(__NR_prctl, PR_GET_CFI, PR_CFI_BRANCH_LANDING_PADS, &lpad_status, 0, 0);
 	if (ret)
 		ksft_exit_fail_msg("Get landing pad status failed with %d\n", ret);
 
-	if (!(lpad_status & PR_INDIR_BR_LP_ENABLE))
+	if (!(lpad_status & PR_CFI_ENABLE))
 		ksft_exit_fail_msg("Landing pad is not enabled, should be enabled via glibc\n");
 
 	ret = my_syscall5(__NR_prctl, PR_GET_SHADOW_STACK_STATUS, &ss_status, 0, 0, 0);