[uml-devel] [RFC PATCH 11/12] SKAS4 - Guest get_mm and switch_mm support

[uml-devel] [RFC PATCH 11/12] SKAS4 - Guest get_mm and switch_mm support

[Jeff Dike]

UML guest SKAS4 support.

diff --git a/arch/um/include/as-layout.h b/arch/um/include/as-layout.h
index cac542d..929d053 100644
--- a/arch/um/include/as-layout.h
+++ b/arch/um/include/as-layout.h
@@ -23,16 +23,15 @@
  */
 
 #ifdef __ASSEMBLY__
-#define _AC(X, Y)	(Y)
+#define _C(Y)	(Y)
 #else
-#define __AC(X, Y)	(X (Y))
-#define _AC(X, Y)	__AC(X, Y)
+#define _C(Y)	((unsigned long) (Y))
 #endif
 
-#define STUB_START _AC(, 0x100000)
-#define STUB_CODE _AC((unsigned long), STUB_START)
-#define STUB_DATA _AC((unsigned long), STUB_CODE + UM_KERN_PAGE_SIZE)
-#define STUB_END _AC((unsigned long), STUB_DATA + UM_KERN_PAGE_SIZE)
+#define STUB_START _C(0x100000)
+#define STUB_CODE STUB_START
+#define STUB_DATA (STUB_CODE + UM_KERN_PAGE_SIZE)
+#define STUB_END (STUB_DATA + UM_KERN_PAGE_SIZE)
 
 #ifndef __ASSEMBLY__
 
diff --git a/arch/um/include/skas/mm_id.h b/arch/um/include/skas/mm_id.h
index 48dd098..a2e7643 100644
--- a/arch/um/include/skas/mm_id.h
+++ b/arch/um/include/skas/mm_id.h
@@ -7,7 +7,7 @@
 #define __MM_ID_H
 
 struct mm_id {
-	union {
+	struct {
 		int mm_fd;
 		int pid;
 	} u;
diff --git a/arch/um/include/skas/skas.h b/arch/um/include/skas/skas.h
index 331f343..ea56428 100644
--- a/arch/um/include/skas/skas.h
+++ b/arch/um/include/skas/skas.h
@@ -6,9 +6,25 @@
 #ifndef __SKAS_H
 #define __SKAS_H
 
-#include "sysdep/ptrace.h"
+#ifndef __KERNEL__
+#include <unistd.h>
+#include <sys/syscall.h>
+#endif
+#include "uml-config.h"
 
-extern int have_siginfo_segv;
+#ifdef UML_CONFIG_X86_32
+#define __NR_new_mm             327
+#define __NR_switch_mm          328
+#else
+#define __NR_new_mm             288
+#define __NR_switch_mm          289
+#endif
+
+#define PTRACE_SWITCH_MM 34
+
+#ifndef __ASSEMBLY__
+
+#include "sysdep/ptrace.h"
 
 #define STUB_ADDR(x) (STUB_CODE + (unsigned long) (x) - \
 		      (unsigned long) &__syscall_stub_start)
@@ -17,6 +33,11 @@ extern int userspace_pid[];
 extern int proc_mm, ptrace_faultinfo, ptrace_ldt;
 extern int skas_needs_stub;
 
+extern int have_switch_mm;
+extern int have_ptrace_switch_mm;
+extern int have_siginfo_segv;
+extern int self_mm_fd;
+
 extern int user_thread(unsigned long stack, int flags);
 extern void new_thread_handler(void);
 extern void handle_syscall(struct uml_pt_regs *regs);
@@ -25,4 +46,45 @@ extern void get_skas_faultinfo(int pid, struct faultinfo * fi);
 extern long execute_syscall_skas(void *r);
 extern unsigned long current_stub_stack(void);
 
+#ifndef __KERNEL__
+#include <errno.h>
+#include <asm/user.h>
+
+struct user_regs {
+	unsigned long regs[MAX_REG_NR];
+#ifdef UML_CONFIG_X86_32
+	struct user_fxsr_struct *fp_state;
+	struct user_fxsr_struct fpregs;
+#else
+	struct user_i387_struct *fp_state;
+	struct user_i387_struct fpregs;
+#endif
+};
+
+static inline long new_mm(void)
+{
+	int ret = syscall(__NR_new_mm, 0, 0, 0, 0, 0, 0);
+
+	if (ret < 0)
+		return -errno;
+
+	return ret;
+}
+
+static inline long switch_mm(int mm_fd, struct user_regs *save_regs,
+			     struct user_regs *new_regs, unsigned long ip,
+			     unsigned long sp)
+{
+	int ret = syscall(__NR_switch_mm, mm_fd, save_regs, new_regs, ip, sp,
+			  0);
+
+	if (ret < 0)
+		return -errno;
+
+	return 0;
+}
+#endif
+
+#endif
+
 #endif
diff --git a/arch/um/kernel/reboot.c b/arch/um/kernel/reboot.c
index 00197d3..a597b5d 100644
--- a/arch/um/kernel/reboot.c
+++ b/arch/um/kernel/reboot.c
@@ -12,7 +12,7 @@ void (*pm_power_off)(void);
 
 static void kill_off_processes(void)
 {
-	if (proc_mm)
+	if (proc_mm || have_switch_mm)
 		/*
 		 * FIXME: need to loop over userspace_pids
 		 */
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index 412fadc..5f4c32e 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -46,6 +46,9 @@ static int init_stub_pte(struct mm_struct *mm, unsigned long proc,
 	return -ENOMEM;
 }
 
+extern int copy_context_skas4(struct mm_id *id);
+extern int get_new_mm(void);
+
 int init_new_context(struct task_struct *task, struct mm_struct *mm)
 {
  	struct mm_context *from_mm = NULL;
@@ -71,6 +74,19 @@ int init_new_context(struct task_struct *task, struct mm_struct *mm)
 			goto out_free;
 		}
 		to_mm->id.u.mm_fd = ret;
+	} else if (have_switch_mm) {
+		to_mm->id.u.mm_fd = get_new_mm();
+		if (to_mm->id.u.mm_fd < 0) {
+			ret = to_mm->id.u.mm_fd;
+			goto out_free;
+		}
+
+		ret = copy_context_skas4(&to_mm->id);
+		if (ret < 0) {
+			os_close_file(to_mm->id.u.mm_fd);
+			to_mm->id.u.mm_fd = -1;
+			goto out_free;
+		}
 	}
 	else {
 		if (from_mm)
@@ -167,7 +183,7 @@ void destroy_context(struct mm_struct *mm)
 {
 	struct mm_context *mmu = &mm->context;
 
-	if (proc_mm)
+	if (proc_mm || have_switch_mm)
 		os_close_file(mmu->id.u.mm_fd);
 	else {
 		/*
diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index 4f63928..32e84d7 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -55,7 +55,7 @@ int __init start_uml(void)
 {
 	stack_protections((unsigned long) &cpu0_irqstack);
 	set_sigstack(cpu0_irqstack, THREAD_SIZE);
-	if (proc_mm) {
+	if (proc_mm || have_switch_mm) {
 		userspace_pid[0] = start_userspace(0);
 		if (userspace_pid[0] < 0) {
 			printf("start_uml - start_userspace returned %d\n",
diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index a6c1dd1..d00ebbd 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -284,7 +284,9 @@ int __init linux_main(int argc, char **argv)
 
 	can_do_skas();
 
-	if (proc_mm && ptrace_faultinfo)
+	if (have_switch_mm)
+		mode = "SKAS4";
+	else if (proc_mm && ptrace_faultinfo)
 		mode = "SKAS3";
 	else
 		mode = "SKAS0";
diff --git a/arch/um/os-Linux/skas/mem.c b/arch/um/os-Linux/skas/mem.c
index 384e43b..42a3026 100644
--- a/arch/um/os-Linux/skas/mem.c
+++ b/arch/um/os-Linux/skas/mem.c
@@ -6,6 +6,7 @@
 #include <stddef.h>
 #include <unistd.h>
 #include <errno.h>
+#include <signal.h>
 #include <string.h>
 #include <sys/mman.h>
 #include "init.h"
@@ -22,7 +23,7 @@
 #include "sysdep/stub.h"
 #include "uml-config.h"
 
-extern unsigned long batch_syscall_stub, __syscall_stub_start;
+extern unsigned long batch_syscall_stub, switch_mm_stub, __syscall_stub_start;
 
 extern void wait_stub_done(int pid);
 
@@ -40,13 +41,9 @@ static unsigned long syscall_regs[MAX_REG_NR];
 
 static int __init init_syscall_regs(void)
 {
-	unsigned long *stub_entry;
-
 	get_safe_registers(syscall_regs);
-	stub_entry = &batch_syscall_stub;
-
-	syscall_regs[REGS_IP_INDEX] = STUB_ADDR(stub_entry);
 
+	syscall_regs[REGS_IP_INDEX] = STUB_ADDR(&batch_syscall_stub);
 	return 0;
 }
 
@@ -125,6 +122,32 @@ static long do_syscall_stub(struct mm_id *mm_idp, void **addr)
 	return ret;
 }
 
+long do_syscall_stub_skas4(struct mm_id *mm_idp, void **addr, unsigned long ip,
+			   unsigned long sp)
+{
+	long ret;
+	struct user_regs return_regs;
+	unsigned long *ptr;
+	int err;
+	sigset_t sigs, old;
+
+	ptr = (unsigned long *) (mm_idp->stack + UM_KERN_PAGE_SIZE -
+				 sizeof(long));
+	*ptr = (unsigned long) &return_regs;
+	*(ptr - 1) = self_mm_fd;
+
+	sigfillset(&sigs);
+	sigprocmask(SIG_SETMASK, &sigs, &old);
+	err = switch_mm(mm_idp->u.mm_fd, &return_regs, NULL, ip, sp);
+	sigprocmask(SIG_SETMASK, &old, NULL);
+
+	ret = syscall_stub_done(mm_idp->stack);
+
+	*addr = check_init_stack(mm_idp, NULL);
+
+	return ret;
+}
+
 static int flush_syscalls(struct mm_id *mm_idp, void **addr, int extra)
 {
 	unsigned long *stack = check_init_stack(mm_idp, *addr);
@@ -133,10 +156,17 @@ static int flush_syscalls(struct mm_id *mm_idp, void **addr, int extra)
 	current = ((unsigned long) stack) & ~UM_KERN_PAGE_MASK;
 	end = UM_KERN_PAGE_SIZE;
 
+	if (have_switch_mm)
+		end -= 2 * sizeof(long);
+
 	if (current + (10 + extra) * sizeof(long) < end)
 		return 0;
 
-	return do_syscall_stub(mm_idp, addr);
+	if (have_switch_mm)
+		return do_syscall_stub_skas4(mm_idp, addr,
+					     STUB_ADDR(&switch_mm_stub), 0);
+	else
+		return do_syscall_stub(mm_idp, addr);
 }
 
 long run_syscall_stub(struct mm_id *mm_idp, int syscall,
@@ -165,11 +195,18 @@ long run_syscall_stub(struct mm_id *mm_idp, int syscall,
 	*stack++ = expected;
 	*stack = 0;
 
-	if (done)
+	if (!done) {
+		*addr = stack;
+		return 0;
+	}
+
+	if (have_switch_mm)
+		return do_syscall_stub_skas4(mm_idp, addr,
+					     STUB_ADDR(&switch_mm_stub), 0);
+	else
 		return do_syscall_stub(mm_idp, addr);
 
 	*addr = stack;
-
 	return 0;
 }
 
diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 08297ae..23a9b42 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -262,7 +262,7 @@ static int userspace_tramp(void *stack)
 			}
 		}
 	}
-	if (!ptrace_faultinfo && (stack != NULL)) {
+	if (!ptrace_faultinfo) {
 		struct sigaction sa;
 		unsigned long v = STUB_ADDR(stub_segv_handler);
 
@@ -306,7 +306,7 @@ int start_userspace(unsigned long stub_stack)
 	sp = (unsigned long) stack + UM_KERN_PAGE_SIZE - sizeof(void *);
 
 	flags = CLONE_FILES;
-	if (proc_mm)
+	if (proc_mm || have_switch_mm)
 		flags |= CLONE_VM;
 	else
 		flags |= SIGCHLD;
@@ -569,6 +569,56 @@ int copy_context_skas0(unsigned long new_stack, int pid)
 	return err;
 }
 
+extern unsigned long switch_mm_stub;
+extern long task_size;
+
+static void unmap_new_as(void)
+{
+	void (*p)(void);
+	void *addr;
+	unsigned long stack = (unsigned long) &stack & ~(UM_KERN_PAGE_SIZE - 1);
+	unsigned long long data_offset, code_offset;
+	int fd = phys_mapping(to_phys((void *) stack), &data_offset);
+
+	addr = mmap((void *) STUB_DATA, UM_KERN_PAGE_SIZE,
+		    PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd,
+		    data_offset);
+	if (addr == MAP_FAILED)
+		panic("Failed to remap stack");
+
+	fd = phys_mapping(to_phys(&__syscall_stub_start), &code_offset);
+	addr = mmap((void *) STUB_CODE, UM_KERN_PAGE_SIZE,
+		    PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd,
+		    code_offset);
+	if (addr == MAP_FAILED)
+		panic("Failed to remap code");
+
+	p = (void (*)(void)) (STUB_ADDR(&switch_mm_stub));
+	(*p)();
+}
+
+extern long do_syscall_stub_skas4(struct mm_id *mm_idp, void **addr,
+				  unsigned long ip, unsigned long sp);
+
+int copy_context_skas4(struct mm_id *id)
+{
+	void *data = NULL;
+	int err;
+
+	err = unmap(id, 0, STUB_START, 0, &data);
+	if (err)
+		return err;
+
+	if (STUB_END < task_size) {
+		err = unmap(id, STUB_END, task_size - STUB_END, 0, &data);
+		if (err)
+			return err;
+	}
+
+	return do_syscall_stub_skas4(id, &data, (unsigned long) unmap_new_as,
+				     id->stack + UM_KERN_PAGE_SIZE / 2);
+}
+
 /*
  * This is used only, if stub pages are needed, while proc_mm is
  * available. Opening /proc/mm creates a new mm_context, which lacks
@@ -738,6 +788,11 @@ void __switch_mm(struct mm_id *mm_idp)
 			       "failed, errno = %d\n", errno);
 			fatal_sigsegv();
 		}
-	}
-	else userspace_pid[0] = mm_idp->u.pid;
+	} else if (have_ptrace_switch_mm) {
+		err = ptrace(PTRACE_SWITCH_MM, userspace_pid[0], 0,
+			     mm_idp->u.mm_fd);
+		if (err)
+			panic("__switch_mm - PTRACE_SWITCH_MM "
+			      "failed, errno = %d\n", errno);
+	} else userspace_pid[0] = mm_idp->u.pid;
 }
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 81e1333..021d41c 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -162,6 +162,9 @@ static int disable_switch_mm;
 int have_siginfo_segv;
 static int disable_siginfo_segv;
 
+int have_ptrace_switch_mm;
+static int disable_ptrace_switch_mm;
+
 int skas_needs_stub;
 
 static int __init skas0_cmd_param(char *str, int* add)
@@ -169,8 +172,10 @@ static int __init skas0_cmd_param(char *str, int* add)
 	disable_ptrace_faultinfo = 1;
 	disable_ptrace_ldt = 1;
 	disable_proc_mm = 1;
+
 	disable_switch_mm = 1;
 	disable_siginfo_segv = 1;
+	disable_ptrace_switch_mm = 1;
 
 	return 0;
 }
@@ -415,7 +420,7 @@ __uml_setup("noptraceldt", noptraceldt_cmd_param,
 "    To support PTRACE_LDT, the host needs to be patched using\n"
 "    the current skas3 patch.\n\n");
 
-static inline void check_skas3_ptrace_faultinfo(void)
+static inline void __init check_skas3_ptrace_faultinfo(void)
 {
 	struct ptrace_faultinfo fi;
 	int pid, n;
@@ -439,7 +444,7 @@ static inline void check_skas3_ptrace_faultinfo(void)
 	stop_ptraced_child(pid, 1, 1);
 }
 
-static inline void check_skas3_ptrace_ldt(void)
+static inline void __init check_skas3_ptrace_ldt(void)
 {
 #ifdef PTRACE_LDT
 	int pid, n;
@@ -470,7 +475,7 @@ static inline void check_skas3_ptrace_ldt(void)
 #endif
 }
 
-static inline void check_skas3_proc_mm(void)
+static inline void __init check_skas3_proc_mm(void)
 {
 	non_fatal("  - /proc/mm...");
 	if (access("/proc/mm", W_OK) < 0)
@@ -483,6 +488,19 @@ static inline void check_skas3_proc_mm(void)
 	}
 }
 
+static void __init can_do_skas3(void)
+{
+	non_fatal("Checking for the skas3 patch in the host:\n");
+
+	check_skas3_proc_mm();
+	check_skas3_ptrace_faultinfo();
+	check_skas3_ptrace_ldt();
+
+	if (!proc_mm || (!ptrace_faultinfo && !have_siginfo_segv) ||
+	    !ptrace_ldt)
+		skas_needs_stub = 1;
+}
+
 static void *fault_address;
 
 static int check_fault_info(struct faultinfo *fi)
@@ -493,7 +511,7 @@ static int check_fault_info(struct faultinfo *fi)
 
 static jmp_buf siginfo_buf;
 
-static void segv_handler(int sig, siginfo_t *si, void *foo)
+static void __init segv_handler(int sig, siginfo_t *si, void *foo)
 {
 	struct faultinfo fi;
 	int n;
@@ -503,7 +521,7 @@ static void segv_handler(int sig, siginfo_t *si, void *foo)
 	longjmp(siginfo_buf, n);
 }
 
-static int fault(void)
+static int __init fault(void)
 {
 	struct sigaction sa, old;
 	int err, n;
@@ -557,7 +575,7 @@ __uml_setup("nogetsiginfo", nogetsiginfo_cmd_param,
 #define PTRACE_GETSIGINFO	0x4202
 #endif
 
-static int check_siginfo(void)
+static int __init check_siginfo(void)
 {
 	siginfo_t si;
 	struct faultinfo fi;
@@ -607,25 +625,213 @@ static int check_siginfo(void)
 
 	if (disable_siginfo_segv)
 		non_fatal("Extended PTRACE_GETSIGINFO disabled on command "
-			  "line");
+			  "line\n");
 	else
 		have_siginfo_segv = 1;
 
 	return ok;
 }
 
-void can_do_skas(void)
+static struct user_regs return_regs;
+int self_mm_fd;
+
+static int switch_mm_works;
+
+static void after_switch(void)
 {
-	non_fatal("Checking for the skas3 patch in the host:\n");
+	/*
+	 * If we are really in a new address space, setting this to
+	 * zero won't affect the value of 1 already set in the old
+	 * address space.
+	 */
+	switch_mm_works = 0;
 
-	check_skas3_proc_mm();
-	check_skas3_ptrace_faultinfo();
-	check_skas3_ptrace_ldt();
-	check_siginfo();
+	switch_mm(self_mm_fd, NULL, &return_regs, 0, 0);
+}
 
-	if (!proc_mm || (!ptrace_faultinfo && !have_siginfo_segv) ||
-	    !ptrace_ldt)
+static int __init check_switch_mm(void)
+{
+	char *mm_stack;
+	int err, there = -1;
+
+	non_fatal("\t/proc/self/mm ... ");
+	self_mm_fd = open("/proc/self/mm", O_RDONLY);
+	if (self_mm_fd < 0)
+		goto bad;
+	non_fatal("OK\n");
+
+	mm_stack = mmap(NULL, UM_KERN_PAGE_SIZE, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (mm_stack == MAP_FAILED)
+		goto bad;
+
+	non_fatal("\tnew_mm ... ");
+	there = new_mm();
+	if (there < 0)
+		goto bad_unmap;
+	non_fatal("OK\n");
+
+	switch_mm_works = 1;
+
+	non_fatal("\tswitching over ... ");
+	err = switch_mm(there, &return_regs, NULL, (unsigned long) after_switch,
+			((unsigned long) &mm_stack[UM_KERN_PAGE_SIZE]) -
+			sizeof(void *));
+	if (err < 0)
+		goto bad_close;
+	non_fatal("switched back ... ");
+	if (!switch_mm_works)
+		goto bad_close;
+	else
+		non_fatal("OK\n");
+
+	munmap(mm_stack, UM_KERN_PAGE_SIZE);
+	close(there);
+
+	if (disable_switch_mm)
+		non_fatal("switch_mm support disabled on command line\n");
+	else
+		have_switch_mm = 1;
+
+	return 1;
+ bad_close:
+	if (there > 0)
+		close(there);
+ bad_unmap:
+	munmap(mm_stack, UM_KERN_PAGE_SIZE);
+ bad:
+	non_fatal("Failed - \n");
+	perror("");
+	return 0;
+}
+
+static int ptrace_switch_mm_works;
+
+static int __init after_ptrace_switch(void)
+{
+	ptrace_switch_mm_works = 1;
+	exit(0);
+}
+
+static int __init check_ptrace_switch_mm(void)
+{
+	void *stack;
+	unsigned long regs[MAX_REG_NR];
+	int pid, here, err, status;
+
+	non_fatal("\tPTRACE_SWITCH_MM ... ");
+	pid = fork();
+	if (pid == 0){
+		ptrace(PTRACE_TRACEME, 0, 0, 0);
+		kill(getpid(), SIGSTOP);
+
+		exit(0);
+	}
+	else if (pid < 0)
+		goto bad;
+
+	stack = mmap(NULL, UM_KERN_PAGE_SIZE, PROT_READ | PROT_WRITE,
+		     MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+	if (stack == MAP_FAILED)
+		goto bad;
+
+	here = open("/proc/self/mm", O_RDONLY);
+	if (here < 0)
+		goto bad_unmap;
+
+	err = waitpid(pid, &status, WUNTRACED);
+	if (err < 0)
+		goto bad_close;
+	else if (err != pid) {
+		non_fatal("waitpid returned %d, expected %d\n", err, pid);
+		goto bad_close;
+	} else if (!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGSTOP)) {
+		non_fatal("waitpid returned status 0x%d\n", status);
+		goto bad_close;
+	}
+
+	err = ptrace(PTRACE_GETREGS, pid, 0, regs);
+	if (err < 0)
+		goto bad_close;
+
+	regs[REGS_IP_INDEX] = (unsigned long) after_ptrace_switch;
+	regs[REGS_SP_INDEX] = (unsigned long) stack + UM_KERN_PAGE_SIZE -
+		sizeof(void *);
+
+	if (ptrace(PTRACE_SETREGS, pid, 0, regs) < 0)
+		goto bad_close;
+
+	if (ptrace(PTRACE_SWITCH_MM, pid, NULL, here) < 0)
+		goto bad_close;
+
+	if (ptrace(PTRACE_CONT, pid, NULL, 0) < 0)
+		goto bad_close;
+
+	err = waitpid(pid, &status, WUNTRACED);
+	if (err < 0)
+		goto bad_close;
+	else if(err != pid) {
+		non_fatal("waitpid returned %d, expected %d\n", err, pid);
+		goto bad_close;
+	} else if (!WIFEXITED(status) || (WEXITSTATUS(status) != 0)) {
+		non_fatal("waitpid returned status 0x%d\n", status);
+		goto bad_close;
+	}
+
+	if (!ptrace_switch_mm_works)
+		goto bad_close;
+	else
+		non_fatal("OK\n");
+
+	if (disable_ptrace_switch_mm)
+		non_fatal("PTRACE_SWITCH_MM support disabled on command "
+			  "line\n");
+	else
+		have_ptrace_switch_mm = 1;
+
+	close(here);
+	munmap(stack, UM_KERN_PAGE_SIZE);
+
+	return 1;
+
+ bad_close:
+	close(here);
+ bad_unmap:
+	munmap(stack, UM_KERN_PAGE_SIZE);
+ bad:
+	non_fatal("Failed - \n");
+	perror("");
+	return 0;
+}
+
+static int __init can_do_skas4(void)
+{
+	int ret;
+
+	non_fatal("Checking for SKAS4 support in the host:\n");
+
+	ret = check_switch_mm() && check_ptrace_switch_mm() && check_siginfo();
+	if (ret)
 		skas_needs_stub = 1;
+
+	return ret;
+}
+
+void __init can_do_skas(void)
+{
+	if (!can_do_skas4())
+		can_do_skas3();
+}
+
+int get_new_mm(void)
+{
+	int err;
+
+	err = new_mm();
+	if (err < 0)
+		err = -errno;
+
+	return err;
 }
 
 int __init parse_iomem(char *str, int *add)
diff --git a/arch/um/sys-i386/stub.S b/arch/um/sys-i386/stub.S
index a63397f..117462e 100644
--- a/arch/um/sys-i386/stub.S
+++ b/arch/um/sys-i386/stub.S
@@ -1,5 +1,6 @@
 #include "uml-config.h"
 #include "as-layout.h"
+#include "skas/skas.h"
 
 #define PROCESS_LIST \
 	/* load pointer to first operation */ \
@@ -43,3 +44,17 @@ batch_syscall_stub:
 	PROCESS_LIST
 	/* stop */
 	int3
+
+ 	.globl switch_mm_stub
+switch_mm_stub:
+	PROCESS_LIST
+
+	mov	$__NR_switch_mm, %eax
+	mov	STUB_DATA + UM_KERN_PAGE_SIZE - 8, %ebx
+	xor	%ecx, %ecx
+	mov	STUB_DATA + UM_KERN_PAGE_SIZE - 4, %edx
+	xor	%esi, %esi
+	xor	%edi, %edi
+	int	$0x80
+
+	int3
diff --git a/arch/um/sys-x86_64/stub.S b/arch/um/sys-x86_64/stub.S
index 69460e6..b4043b0 100644
--- a/arch/um/sys-x86_64/stub.S
+++ b/arch/um/sys-x86_64/stub.S
@@ -1,5 +1,6 @@
 #include "uml-config.h"
 #include "as-layout.h"
+#include "skas/skas.h"
 
 #define PROCESS_LIST \
 	mov	$(STUB_DATA >> 32), %rbx; \
@@ -46,3 +47,22 @@ batch_syscall_stub:
 	PROCESS_LIST
 	/* stop */
 	int3
+
+ 	.globl switch_mm_stub
+switch_mm_stub:
+	PROCESS_LIST
+
+	mov	$__NR_switch_mm, %rax
+	mov	$(STUB_DATA >> 32), %rdi
+	sal	$32, %rdi
+	mov	$(STUB_DATA & 0xffffffff + 4096 - 8), %rsi
+	add	%rdi, %rsi
+	mov	(%rsi), %rdx
+	sub	$8, %rsi
+	mov	(%rsi), %rdi
+	xor	%rsi, %rsi
+	xor	%r10, %r10
+	xor	%r8, %r8
+	syscall
+
+	int3
diff --git a/arch/um/sys-x86_64/syscalls.c b/arch/um/sys-x86_64/syscalls.c
index f1199fd..e861ad6 100644
--- a/arch/um/sys-x86_64/syscalls.c
+++ b/arch/um/sys-x86_64/syscalls.c
@@ -30,7 +30,7 @@ long arch_prctl(struct task_struct *task, int code, unsigned long __user *addr)
 {
 	unsigned long *ptr = addr, tmp;
 	long ret;
-	int pid = task->mm->context.id.u.pid;
+	int pid = userspace_pid[0];
 
 	/*
 	 * With ARCH_SET_FS (and ARCH_SET_GS is treated similarly to

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