ARM: move signal handlers into a vdso-like page

Move the signal handlers into a VDSO page rather than keeping them in
the vectors page.  This allows us to place them randomly within this
page, and also map the page at a random location within userspace
further protecting these code fragments from ROP attacks.  The new
VDSO page is also poisoned in the same way as the vector page.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Author: Russell King

arch/arm/include/asm/elf.h

@@ -130,4 +130,8 @@ struct mm_struct;
 extern unsigned long arch_randomize_brk(struct mm_struct *mm);
 #define arch_randomize_brk arch_randomize_brk
 
+#define ARCH_HAS_SETUP_ADDITIONAL_PAGES 1
+struct linux_binprm;
+int arch_setup_additional_pages(struct linux_binprm *, int);
+
 #endif

arch/arm/include/asm/mmu.h

@@ -8,6 +8,7 @@ typedef struct {
 	atomic64_t	id;
 #endif
 	unsigned int	vmalloc_seq;
+	unsigned long	sigpage;
 } mm_context_t;
 
 #ifdef CONFIG_CPU_HAS_ASID

arch/arm/kernel/process.c

@@ -428,8 +428,8 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 #ifdef CONFIG_MMU
 /*
  * The vectors page is always readable from user space for the
- * atomic helpers and the signal restart code. Insert it into the
- * gate_vma so that it is visible through ptrace and /proc/<pid>/mem.
+ * atomic helpers. Insert it into the gate_vma so that it is visible
+ * through ptrace and /proc/<pid>/mem.
  */
 static struct vm_area_struct gate_vma = {
 	.vm_start	= 0xffff0000,
@@ -461,6 +461,40 @@ int in_gate_area_no_mm(unsigned long addr)
 
 const char *arch_vma_name(struct vm_area_struct *vma)
 {
-	return (vma == &gate_vma) ? "[vectors]" : NULL;
+	return (vma == &gate_vma) ? "[vectors]" :
+		(vma->vm_mm && vma->vm_start == vma->vm_mm->context.sigpage) ?
+		 "[sigpage]" : NULL;
+}
+
+extern struct page *get_signal_page(void);
+
+int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
+{
+	struct mm_struct *mm = current->mm;
+	struct page *page;
+	unsigned long addr;
+	int ret;
+
+	page = get_signal_page();
+	if (!page)
+		return -ENOMEM;
+
+	down_write(&mm->mmap_sem);
+	addr = get_unmapped_area(NULL, 0, PAGE_SIZE, 0, 0);
+	if (IS_ERR_VALUE(addr)) {
+		ret = addr;
+		goto up_fail;
+	}
+
+	ret = install_special_mapping(mm, addr, PAGE_SIZE,
+		VM_READ | VM_EXEC | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC,
+		&page);
+
+	if (ret == 0)
+		mm->context.sigpage = addr;
+
+ up_fail:
+	up_write(&mm->mmap_sem);
+	return ret;
 }
 #endif

arch/arm/kernel/signal.c

@@ -8,19 +8,19 @@
  * published by the Free Software Foundation.
  */
 #include <linux/errno.h>
+#include <linux/random.h>
 #include <linux/signal.h>
 #include <linux/personality.h>
 #include <linux/uaccess.h>
 #include <linux/tracehook.h>
 
 #include <asm/elf.h>
 #include <asm/cacheflush.h>
+#include <asm/traps.h>
 #include <asm/ucontext.h>
 #include <asm/unistd.h>
 #include <asm/vfp.h>
 
-#include "signal.h"
-
 /*
  * For ARM syscalls, we encode the syscall number into the instruction.
  */
@@ -40,11 +40,13 @@
 #define SWI_THUMB_SIGRETURN	(0xdf00 << 16 | 0x2700 | (__NR_sigreturn - __NR_SYSCALL_BASE))
 #define SWI_THUMB_RT_SIGRETURN	(0xdf00 << 16 | 0x2700 | (__NR_rt_sigreturn - __NR_SYSCALL_BASE))
 
-const unsigned long sigreturn_codes[7] = {
+static const unsigned long sigreturn_codes[7] = {
 	MOV_R7_NR_SIGRETURN,    SWI_SYS_SIGRETURN,    SWI_THUMB_SIGRETURN,
 	MOV_R7_NR_RT_SIGRETURN, SWI_SYS_RT_SIGRETURN, SWI_THUMB_RT_SIGRETURN,
 };
 
+static unsigned long signal_return_offset;
+
 #ifdef CONFIG_CRUNCH
 static int preserve_crunch_context(struct crunch_sigframe __user *frame)
 {
@@ -401,12 +403,15 @@ setup_return(struct pt_regs *regs, struct ksignal *ksig,
 			return 1;
 
 		if ((cpsr & MODE32_BIT) && !IS_ENABLED(CONFIG_ARM_MPU)) {
+			struct mm_struct *mm = current->mm;
+
 			/*
-			 * 32-bit code can use the new high-page
-			 * signal return code support except when the MPU has
-			 * protected the vectors page from PL0
+			 * 32-bit code can use the signal return page
+			 * except when the MPU has protected the vectors
+			 * page from PL0
 			 */
-			retcode = KERN_SIGRETURN_CODE + (idx << 2) + thumb;
+			retcode = mm->context.sigpage + signal_return_offset +
+				  (idx << 2) + thumb;
 		} else {
 			/*
 			 * Ensure that the instruction cache sees
@@ -608,3 +613,36 @@ do_work_pending(struct pt_regs *regs, unsigned int thread_flags, int syscall)
 	} while (thread_flags & _TIF_WORK_MASK);
 	return 0;
 }
+
+static struct page *signal_page;
+
+struct page *get_signal_page(void)
+{
+	if (!signal_page) {
+		unsigned long ptr;
+		unsigned offset;
+		void *addr;
+
+		signal_page = alloc_pages(GFP_KERNEL, 0);
+
+		if (!signal_page)
+			return NULL;
+
+		addr = page_address(signal_page);
+
+		/* Give the signal return code some randomness */
+		offset = 0x200 + (get_random_int() & 0x7fc);
+		signal_return_offset = offset;
+
+		/*
+		 * Copy signal return handlers into the vector page, and
+		 * set sigreturn to be a pointer to these.
+		 */
+		memcpy(addr + offset, sigreturn_codes, sizeof(sigreturn_codes));
+
+		ptr = (unsigned long)addr + offset;
+		flush_icache_range(ptr, ptr + sizeof(sigreturn_codes));
+	}
+
+	return signal_page;
+}

arch/arm/kernel/signal.h

@@ -35,8 +35,6 @@
 #include <asm/tls.h>
 #include <asm/system_misc.h>
 
-#include "signal.h"
-
 static const char *handler[]= { "prefetch abort", "data abort", "address exception", "interrupt" };
 
 void *vectors_page;
@@ -850,13 +848,6 @@ void __init early_trap_init(void *vectors_base)
 
 	kuser_init(vectors_base);
 
-	/*
-	 * Copy signal return handlers into the vector page, and
-	 * set sigreturn to be a pointer to these.
-	 */
-	memcpy((void *)(vectors + KERN_SIGRETURN_CODE - CONFIG_VECTORS_BASE),
-	       sigreturn_codes, sizeof(sigreturn_codes));
-
 	flush_icache_range(vectors, vectors + PAGE_SIZE * 2);
 	modify_domain(DOMAIN_USER, DOMAIN_CLIENT);
 #else /* ifndef CONFIG_CPU_V7M */