arch/tile: don't allow user code to set the PL via ptrace or signal return

The kernel was allowing any component of the pt_regs to be updated either
by signal handlers writing to the stack, or by processes writing via
PTRACE_POKEUSR or PTRACE_SETREGS, which meant they could set their PL
up from 0 to 1 and get access to kernel code and data (or, in practice,
cause a kernel panic).  We now always reset the ex1 field, allowing the
user to set their ICS bit only.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

2 files changed, 24 insertions, 18 deletions

diff --git a/arch/tile/kernel/ptrace.c b/arch/tile/kernel/ptrace.c
index 9cd29884c09..e92e40527d6 100644
--- a/arch/tile/kernel/ptrace.c
+++ b/arch/tile/kernel/ptrace.c
@@ -50,10 +50,10 @@ long arch_ptrace(struct task_struct *child, long request,
 {
         unsigned long __user *datap = (long __user __force *)data;
         unsigned long tmp;
-        int i;
         long ret = -EIO;
-        unsigned long *childregs;
         char *childreg;
+        struct pt_regs copyregs;
+        int ex1_offset;
 
         switch (request) {
 
@@ -80,6 +80,16 @@ long arch_ptrace(struct task_struct *child, long request,
                 if (addr >= PTREGS_SIZE)
                         break;
                 childreg = (char *)task_pt_regs(child) + addr;
+
+                /* Guard against overwrites of the privilege level. */
+                ex1_offset = PTREGS_OFFSET_EX1;
+#if defined(CONFIG_COMPAT) && defined(__BIG_ENDIAN)
+                if (is_compat_task())   /* point at low word */
+                        ex1_offset += sizeof(compat_long_t);
+#endif
+                if (addr == ex1_offset)
+                        data = PL_ICS_EX1(USER_PL, EX1_ICS(data));
+
 #ifdef CONFIG_COMPAT
                 if (is_compat_task()) {
                         if (addr & (sizeof(compat_long_t)-1))
@@ -96,26 +106,19 @@ long arch_ptrace(struct task_struct *child, long request,
                 break;
 
         case PTRACE_GETREGS:  /* Get all registers from the child. */
-                if (!access_ok(VERIFY_WRITE, datap, PTREGS_SIZE))
-                        break;
-                childregs = (long *)task_pt_regs(child);
-                for (i = 0; i < sizeof(struct pt_regs)/sizeof(unsigned long);
-                                ++i) {
-                        ret = __put_user(childregs[i], &datap[i]);
-                        if (ret != 0)
-                                break;
+                if (copy_to_user(datap, task_pt_regs(child),
+                                 sizeof(struct pt_regs)) == 0) {
+                        ret = 0;
                 }
                 break;
 
         case PTRACE_SETREGS:  /* Set all registers in the child. */
-                if (!access_ok(VERIFY_READ, datap, PTREGS_SIZE))
-                        break;
-                childregs = (long *)task_pt_regs(child);
-                for (i = 0; i < sizeof(struct pt_regs)/sizeof(unsigned long);
-                                ++i) {
-                        ret = __get_user(childregs[i], &datap[i]);
-                        if (ret != 0)
-                                break;
+                if (copy_from_user(&copyregs, datap,
+                                   sizeof(struct pt_regs)) == 0) {
+                        copyregs.ex1 =
+                                PL_ICS_EX1(USER_PL, EX1_ICS(copyregs.ex1));
+                        *task_pt_regs(child) = copyregs;
+                        ret = 0;
                 }
                 break;
 
diff --git a/arch/tile/kernel/signal.c b/arch/tile/kernel/signal.c
index 704ce0bce83..687719d4abd 100644
--- a/arch/tile/kernel/signal.c
+++ b/arch/tile/kernel/signal.c
@@ -71,6 +71,9 @@ int restore_sigcontext(struct pt_regs *regs,
         for (i = 0; i < sizeof(struct pt_regs)/sizeof(long); ++i)
                 err |= __get_user(regs->regs[i], &sc->gregs[i]);
 
+        /* Ensure that the PL is always set to USER_PL. */
+        regs->ex1 = PL_ICS_EX1(USER_PL, EX1_ICS(regs->ex1));
+
         regs->faultnum = INT_SWINT_1_SIGRETURN;
 
         err |= __get_user(*pr0, &sc->gregs[0]);