x86-64: Rework vsyscall emulation and add vsyscall= parameter

There are three choices:

vsyscall=native: Vsyscalls are native code that issues the
corresponding syscalls.

vsyscall=emulate (default): Vsyscalls are emulated by instruction
fault traps, tested in the bad_area path.  The actual contents of
the vsyscall page is the same as the vsyscall=native case except
that it's marked NX.  This way programs that make assumptions about
what the code in the page does will not be confused when they read
that code.

vsyscall=none: Trying to execute a vsyscall will segfault.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/8449fb3abf89851fd6b2260972666a6f82542284.1312988155.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 49 insertions, 30 deletions

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index bf8e9ffee6e9..18ae83dd1cd7 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -56,6 +56,27 @@ DEFINE_VVAR(struct vsyscall_gtod_data, vsyscall_gtod_data) =
 	.lock = __SEQLOCK_UNLOCKED(__vsyscall_gtod_data.lock),
 };
 
+static enum { EMULATE, NATIVE, NONE } vsyscall_mode = EMULATE;
+
+static int __init vsyscall_setup(char *str)
+{
+	if (str) {
+		if (!strcmp("emulate", str))
+			vsyscall_mode = EMULATE;
+		else if (!strcmp("native", str))
+			vsyscall_mode = NATIVE;
+		else if (!strcmp("none", str))
+			vsyscall_mode = NONE;
+		else
+			return -EINVAL;
+
+		return 0;
+	}
+
+	return -EINVAL;
+}
+early_param("vsyscall", vsyscall_setup);
+
 void update_vsyscall_tz(void)
 {
 	unsigned long flags;
@@ -100,7 +121,7 @@ static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
 
 	printk("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
 	       level, tsk->comm, task_pid_nr(tsk),
-	       message, regs->ip - 2, regs->cs,
+	       message, regs->ip, regs->cs,
 	       regs->sp, regs->ax, regs->si, regs->di);
 }
 
@@ -118,45 +139,39 @@ static int addr_to_vsyscall_nr(unsigned long addr)
 	return nr;
 }
 
-void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
+bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
 {
 	struct task_struct *tsk;
 	unsigned long caller;
 	int vsyscall_nr;
 	long ret;
 
-	local_irq_enable();
+	/*
+	 * No point in checking CS -- the only way to get here is a user mode
+	 * trap to a high address, which means that we're in 64-bit user code.
+	 */
 
-	if (!user_64bit_mode(regs)) {
-		/*
-		 * If we trapped from kernel mode, we might as well OOPS now
-		 * instead of returning to some random address and OOPSing
-		 * then.
-		 */
-		BUG_ON(!user_mode(regs));
+	WARN_ON_ONCE(address != regs->ip);
 
-		/* Compat mode and non-compat 32-bit CS should both segfault. */
-		warn_bad_vsyscall(KERN_WARNING, regs,
-				  "illegal int 0xcc from 32-bit mode");
-		goto sigsegv;
+	if (vsyscall_mode == NONE) {
+		warn_bad_vsyscall(KERN_INFO, regs,
+				  "vsyscall attempted with vsyscall=none");
+		return false;
 	}
 
-	/*
-	 * x86-ism here: regs->ip points to the instruction after the int 0xcc,
-	 * and int 0xcc is two bytes long.
-	 */
-	vsyscall_nr = addr_to_vsyscall_nr(regs->ip - 2);
+	vsyscall_nr = addr_to_vsyscall_nr(address);
 
 	trace_emulate_vsyscall(vsyscall_nr);
 
 	if (vsyscall_nr < 0) {
 		warn_bad_vsyscall(KERN_WARNING, regs,
-				  "illegal int 0xcc (exploit attempt?)");
+				  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
 		goto sigsegv;
 	}
 
 	if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
-		warn_bad_vsyscall(KERN_WARNING, regs, "int 0xcc with bad stack (exploit attempt?)");
+		warn_bad_vsyscall(KERN_WARNING, regs,
+				  "vsyscall with bad stack (exploit attempt?)");
 		goto sigsegv;
 	}
 
@@ -201,13 +216,11 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
 	regs->ip = caller;
 	regs->sp += 8;
 
-	local_irq_disable();
-	return;
+	return true;
 
 sigsegv:
-	regs->ip -= 2;  /* The faulting instruction should be the int 0xcc. */
 	force_sig(SIGSEGV, current);
-	local_irq_disable();
+	return true;
 }
 
 /*
@@ -255,15 +268,21 @@ cpu_vsyscall_notifier(struct notifier_block *n, unsigned long action, void *arg)
 
 void __init map_vsyscall(void)
 {
-	extern char __vsyscall_0;
-	unsigned long physaddr_page0 = __pa_symbol(&__vsyscall_0);
+	extern char __vsyscall_page;
+	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 	extern char __vvar_page;
 	unsigned long physaddr_vvar_page = __pa_symbol(&__vvar_page);
 
-	/* Note that VSYSCALL_MAPPED_PAGES must agree with the code below. */
-	__set_fixmap(VSYSCALL_FIRST_PAGE, physaddr_page0, PAGE_KERNEL_VSYSCALL);
+	__set_fixmap(VSYSCALL_FIRST_PAGE, physaddr_vsyscall,
+		     vsyscall_mode == NATIVE
+		     ? PAGE_KERNEL_VSYSCALL
+		     : PAGE_KERNEL_VVAR);
+	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_FIRST_PAGE) !=
+		     (unsigned long)VSYSCALL_START);
+
 	__set_fixmap(VVAR_PAGE, physaddr_vvar_page, PAGE_KERNEL_VVAR);
-	BUILD_BUG_ON((unsigned long)__fix_to_virt(VVAR_PAGE) != (unsigned long)VVAR_ADDRESS);
+	BUILD_BUG_ON((unsigned long)__fix_to_virt(VVAR_PAGE) !=
+		     (unsigned long)VVAR_ADDRESS);
 }
 
 static int __init vsyscall_init(void)