x86-64: Improve vsyscall emulation CS and RIP handling

Three fixes here:
 - Send SIGSEGV if called from compat code or with a funny CS.
 - Don't BUG on impossible addresses.
 - Add a missing local_irq_disable.

This patch also removes an unused variable.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/6fb2b13ab39b743d1e4f466eef13425854912f7f.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

2 files changed, 41 insertions, 32 deletions

diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
index bb710cb0cdc1..d55597351f6a 100644
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -31,18 +31,6 @@ extern struct timezone sys_tz;
 
 extern void map_vsyscall(void);
 
-/* Emulation */
-
-static inline bool is_vsyscall_entry(unsigned long addr)
-{
-        return (addr & ~0xC00UL) == VSYSCALL_START;
-}
-
-static inline int vsyscall_entry_nr(unsigned long addr)
-{
-        return (addr & 0xC00UL) >> 10;
-}
-
 #endif /* __KERNEL__ */
 
 #endif /* _ASM_X86_VSYSCALL_H */
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 10cd8ac3395a..a262400c3479 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -38,6 +38,7 @@
 
 #include <asm/vsyscall.h>
 #include <asm/pgtable.h>
+#include <asm/compat.h>
 #include <asm/page.h>
 #include <asm/unistd.h>
 #include <asm/fixmap.h>
@@ -97,33 +98,63 @@ static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
 
         tsk = current;
 
-        printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
+        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->sp, regs->ax, regs->si, regs->di);
+               message, regs->ip - 2, regs->cs,
+               regs->sp, regs->ax, regs->si, regs->di);
+}
+
+static int addr_to_vsyscall_nr(unsigned long addr)
+{
+        int nr;
+
+        if ((addr & ~0xC00UL) != VSYSCALL_START)
+                return -EINVAL;
+
+        nr = (addr & 0xC00UL) >> 10;
+        if (nr >= 3)
+                return -EINVAL;
+
+        return nr;
 }
 
 void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
 {
-        const char *vsyscall_name;
         struct task_struct *tsk;
         unsigned long caller;
         int vsyscall_nr;
         long ret;
 
-        /* Kernel code must never get here. */
-        BUG_ON(!user_mode(regs));
-
         local_irq_enable();
 
         /*
+         * Real 64-bit user mode code has cs == __USER_CS.  Anything else
+         * is bogus.
+         */
+        if (regs->cs != __USER_CS) {
+                /*
+                 * 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));
+
+                /* 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;
+        }
+
+        /*
          * x86-ism here: regs->ip points to the instruction after the int 0xcc,
          * and int 0xcc is two bytes long.
          */
-        if (!is_vsyscall_entry(regs->ip - 2)) {
-                warn_bad_vsyscall(KERN_WARNING, regs, "illegal int 0xcc (exploit attempt?)");
+        vsyscall_nr = addr_to_vsyscall_nr(regs->ip - 2);
+        if (vsyscall_nr < 0) {
+                warn_bad_vsyscall(KERN_WARNING, regs,
+                                  "illegal int 0xcc (exploit attempt?)");
                 goto sigsegv;
         }
-        vsyscall_nr = vsyscall_entry_nr(regs->ip - 2);
 
         if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
                 warn_bad_vsyscall(KERN_WARNING, regs, "int 0xcc with bad stack (exploit attempt?)");
@@ -136,31 +167,20 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
 
         switch (vsyscall_nr) {
         case 0:
-                vsyscall_name = "gettimeofday";
                 ret = sys_gettimeofday(
                         (struct timeval __user *)regs->di,
                         (struct timezone __user *)regs->si);
                 break;
 
         case 1:
-                vsyscall_name = "time";
                 ret = sys_time((time_t __user *)regs->di);
                 break;
 
         case 2:
-                vsyscall_name = "getcpu";
                 ret = sys_getcpu((unsigned __user *)regs->di,
                                  (unsigned __user *)regs->si,
                                  0);
                 break;
-
-        default:
-                /*
-                 * If we get here, then vsyscall_nr indicates that int 0xcc
-                 * happened at an address in the vsyscall page that doesn't
-                 * contain int 0xcc.  That can't happen.
-                 */
-                BUG();
         }
 
         if (ret == -EFAULT) {
@@ -188,6 +208,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
 sigsegv:
         regs->ip -= 2;  /* The faulting instruction should be the int 0xcc. */
         force_sig(SIGSEGV, current);
+        local_irq_disable();
 }
 
 /*