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>

1 files changed, 45 insertions, 7 deletions

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 1c16c35c271..0f17e06d51e 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -8,6 +8,7 @@
  * 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>
@@ -15,12 +16,11 @@
 
 #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;
+}