- For syscall user dispatch, separate ptctl operation from syscall

redirection range specification before the API has been made official in 5.11.
 
 - Ensure tasks using the generic syscall code do trap after returning
 from a syscall when single-stepping is requested.
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Merge tag 'core_urgent_for_v5.11_rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull syscall entry fixes from Borislav Petkov:

 - For syscall user dispatch, separate prctl operation from syscall
   redirection range specification before the API has been made official
   in 5.11.

 - Ensure tasks using the generic syscall code do trap after returning
   from a syscall when single-stepping is requested.

* tag 'core_urgent_for_v5.11_rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  entry: Use different define for selector variable in SUD
  entry: Ensure trap after single-step on system call return

Documentation/admin-guide/syscall-user-dispatch.rst

@@ -70,8 +70,8 @@ trampoline code on the vDSO, that trampoline is never intercepted.
 [selector] is a pointer to a char-sized region in the process memory
 region, that provides a quick way to enable disable syscall redirection
 thread-wide, without the need to invoke the kernel directly.  selector
-can be set to PR_SYS_DISPATCH_ON or PR_SYS_DISPATCH_OFF.  Any other
-value should terminate the program with a SIGSYS.
+can be set to SYSCALL_DISPATCH_FILTER_ALLOW or SYSCALL_DISPATCH_FILTER_BLOCK.
+Any other value should terminate the program with a SIGSYS.
 
 Security Notes
 --------------

arch/x86/include/asm/entry-common.h

@@ -43,8 +43,6 @@ static __always_inline void arch_check_user_regs(struct pt_regs *regs)
 }
 #define arch_check_user_regs arch_check_user_regs
 
-#define ARCH_SYSCALL_EXIT_WORK		(_TIF_SINGLESTEP)
-
 static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 						  unsigned long ti_work)
 {

arch/x86/kernel/step.c

@@ -127,12 +127,17 @@ static int enable_single_step(struct task_struct *child)
 		regs->flags |= X86_EFLAGS_TF;
 
 	/*
-	 * Always set TIF_SINGLESTEP - this guarantees that
-	 * we single-step system calls etc..  This will also
+	 * Always set TIF_SINGLESTEP.  This will also
 	 * cause us to set TF when returning to user mode.
 	 */
 	set_tsk_thread_flag(child, TIF_SINGLESTEP);
 
+	/*
+	 * Ensure that a trap is triggered once stepping out of a system
+	 * call prior to executing any user instruction.
+	 */
+	set_task_syscall_work(child, SYSCALL_EXIT_TRAP);
+
 	oflags = regs->flags;
 
 	/* Set TF on the kernel stack.. */
@@ -230,6 +235,7 @@ void user_disable_single_step(struct task_struct *child)
 
 	/* Always clear TIF_SINGLESTEP... */
 	clear_tsk_thread_flag(child, TIF_SINGLESTEP);
+	clear_task_syscall_work(child, SYSCALL_EXIT_TRAP);
 
 	/* But touch TF only if it was set by us.. */
 	if (test_and_clear_tsk_thread_flag(child, TIF_FORCED_TF))

include/linux/entry-common.h

@@ -46,6 +46,7 @@
 				 SYSCALL_WORK_SYSCALL_TRACE |		\
 				 SYSCALL_WORK_SYSCALL_AUDIT |		\
 				 SYSCALL_WORK_SYSCALL_USER_DISPATCH |	\
+				 SYSCALL_WORK_SYSCALL_EXIT_TRAP	|	\
 				 ARCH_SYSCALL_WORK_EXIT)
 
 /*

include/linux/thread_info.h

@@ -43,6 +43,7 @@ enum syscall_work_bit {
 	SYSCALL_WORK_BIT_SYSCALL_EMU,
 	SYSCALL_WORK_BIT_SYSCALL_AUDIT,
 	SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH,
+	SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP,
 };
 
 #define SYSCALL_WORK_SECCOMP		BIT(SYSCALL_WORK_BIT_SECCOMP)
@@ -51,6 +52,7 @@ enum syscall_work_bit {
 #define SYSCALL_WORK_SYSCALL_EMU	BIT(SYSCALL_WORK_BIT_SYSCALL_EMU)
 #define SYSCALL_WORK_SYSCALL_AUDIT	BIT(SYSCALL_WORK_BIT_SYSCALL_AUDIT)
 #define SYSCALL_WORK_SYSCALL_USER_DISPATCH BIT(SYSCALL_WORK_BIT_SYSCALL_USER_DISPATCH)
+#define SYSCALL_WORK_SYSCALL_EXIT_TRAP	BIT(SYSCALL_WORK_BIT_SYSCALL_EXIT_TRAP)
 #endif
 
 #include <asm/thread_info.h>

include/uapi/linux/prctl.h

@@ -251,5 +251,8 @@ struct prctl_mm_map {
 #define PR_SET_SYSCALL_USER_DISPATCH	59
 # define PR_SYS_DISPATCH_OFF		0
 # define PR_SYS_DISPATCH_ON		1
+/* The control values for the user space selector when dispatch is enabled */
+# define SYSCALL_DISPATCH_FILTER_ALLOW	0
+# define SYSCALL_DISPATCH_FILTER_BLOCK	1
 
 #endif /* _LINUX_PRCTL_H */

kernel/entry/common.c

@@ -209,15 +209,9 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
 	lockdep_sys_exit();
 }
 
-#ifndef _TIF_SINGLESTEP
-static inline bool report_single_step(unsigned long work)
-{
-	return false;
-}
-#else
 /*
  * If SYSCALL_EMU is set, then the only reason to report is when
- * TIF_SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP).  This syscall
+ * SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP).  This syscall
  * instruction has been already reported in syscall_enter_from_user_mode().
  */
 static inline bool report_single_step(unsigned long work)
@@ -225,10 +219,8 @@ static inline bool report_single_step(unsigned long work)
 	if (work & SYSCALL_WORK_SYSCALL_EMU)
 		return false;
 
-	return !!(current_thread_info()->flags & _TIF_SINGLESTEP);
+	return work & SYSCALL_WORK_SYSCALL_EXIT_TRAP;
 }
-#endif
-
 
 static void syscall_exit_work(struct pt_regs *regs, unsigned long work)
 {

kernel/entry/syscall_user_dispatch.c

@@ -50,10 +50,10 @@ bool syscall_user_dispatch(struct pt_regs *regs)
 		if (unlikely(__get_user(state, sd->selector)))
 			do_exit(SIGSEGV);
 
-		if (likely(state == PR_SYS_DISPATCH_OFF))
+		if (likely(state == SYSCALL_DISPATCH_FILTER_ALLOW))
 			return false;
 
-		if (state != PR_SYS_DISPATCH_ON)
+		if (state != SYSCALL_DISPATCH_FILTER_BLOCK)
 			do_exit(SIGSYS);
 	}
 

tools/testing/selftests/syscall_user_dispatch/sud_benchmark.c

@@ -22,6 +22,8 @@
 # define PR_SET_SYSCALL_USER_DISPATCH	59
 # define PR_SYS_DISPATCH_OFF	0
 # define PR_SYS_DISPATCH_ON	1
+# define SYSCALL_DISPATCH_FILTER_ALLOW	0
+# define SYSCALL_DISPATCH_FILTER_BLOCK	1
 #endif
 
 #ifdef __NR_syscalls
@@ -55,8 +57,8 @@ unsigned long trapped_call_count = 0;
 unsigned long native_call_count = 0;
 
 char selector;
-#define SYSCALL_BLOCK   (selector = PR_SYS_DISPATCH_ON)
-#define SYSCALL_UNBLOCK (selector = PR_SYS_DISPATCH_OFF)
+#define SYSCALL_BLOCK   (selector = SYSCALL_DISPATCH_FILTER_BLOCK)
+#define SYSCALL_UNBLOCK (selector = SYSCALL_DISPATCH_FILTER_ALLOW)
 
 #define CALIBRATION_STEP 100000
 #define CALIBRATE_TO_SECS 5
@@ -170,7 +172,7 @@ int main(void)
 	syscall(MAGIC_SYSCALL_1);
 
 #ifdef TEST_BLOCKED_RETURN
-	if (selector == PR_SYS_DISPATCH_OFF) {
+	if (selector == SYSCALL_DISPATCH_FILTER_ALLOW) {
 		fprintf(stderr, "Failed to return with selector blocked.\n");
 		exit(-1);
 	}

tools/testing/selftests/syscall_user_dispatch/sud_test.c

@@ -18,6 +18,8 @@
 # define PR_SET_SYSCALL_USER_DISPATCH	59
 # define PR_SYS_DISPATCH_OFF	0
 # define PR_SYS_DISPATCH_ON	1
+# define SYSCALL_DISPATCH_FILTER_ALLOW	0
+# define SYSCALL_DISPATCH_FILTER_BLOCK	1
 #endif
 
 #ifndef SYS_USER_DISPATCH
@@ -30,8 +32,8 @@
 # define MAGIC_SYSCALL_1 (0xff00)  /* Bad Linux syscall number */
 #endif
 
-#define SYSCALL_DISPATCH_ON(x) ((x) = 1)
-#define SYSCALL_DISPATCH_OFF(x) ((x) = 0)
+#define SYSCALL_DISPATCH_ON(x) ((x) = SYSCALL_DISPATCH_FILTER_BLOCK)
+#define SYSCALL_DISPATCH_OFF(x) ((x) = SYSCALL_DISPATCH_FILTER_ALLOW)
 
 /* Test Summary:
  *
@@ -56,7 +58,7 @@
 
 TEST_SIGNAL(dispatch_trigger_sigsys, SIGSYS)
 {
-	char sel = 0;
+	char sel = SYSCALL_DISPATCH_FILTER_ALLOW;
 	struct sysinfo info;
 	int ret;
 
@@ -79,7 +81,7 @@ TEST_SIGNAL(dispatch_trigger_sigsys, SIGSYS)
 
 TEST(bad_prctl_param)
 {
-	char sel = 0;
+	char sel = SYSCALL_DISPATCH_FILTER_ALLOW;
 	int op;
 
 	/* Invalid op */
@@ -220,7 +222,7 @@ TEST_SIGNAL(bad_selector, SIGSYS)
 	sigset_t mask;
 	struct sysinfo info;
 
-	glob_sel = 0;
+	glob_sel = SYSCALL_DISPATCH_FILTER_ALLOW;
 	nr_syscalls_emulated = 0;
 	si_code = 0;
 	si_errno = 0;
@@ -288,7 +290,7 @@ TEST(direct_dispatch_range)
 {
 	int ret = 0;
 	struct sysinfo info;
-	char sel = 0;
+	char sel = SYSCALL_DISPATCH_FILTER_ALLOW;
 
 	/*
 	 * Instead of calculating libc addresses; allow the entire