x86/espfix/64: Stop assuming that pt_regs is on the entry stack

[sfrench/cifs-2.6.git] / arch / x86 / kernel / traps.c

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 5a6b8f809792bfb1de41063ad3247d53b2f54537..f0029d17b14b1e8c4cc9bf02140e7934d9231bb0 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -141,8 +141,7 @@ void ist_begin_non_atomic(struct pt_regs *regs)
         * will catch asm bugs and any attempt to use ist_preempt_enable
         * from double_fault.
         */
-       BUG_ON((unsigned long)(current_top_of_stack() -
-                              current_stack_pointer) >= THREAD_SIZE);
+       BUG_ON(!on_thread_stack());
 
        preempt_enable_no_resched();
 }
@@ -349,9 +348,15 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
 
        /*
         * If IRET takes a non-IST fault on the espfix64 stack, then we
-        * end up promoting it to a doublefault.  In that case, modify
-        * the stack to make it look like we just entered the #GP
-        * handler from user space, similar to bad_iret.
+        * end up promoting it to a doublefault.  In that case, take
+        * advantage of the fact that we're not using the normal (TSS.sp0)
+        * stack right now.  We can write a fake #GP(0) frame at TSS.sp0
+        * and then modify our own IRET frame so that, when we return,
+        * we land directly at the #GP(0) vector with the stack already
+        * set up according to its expectations.
+        *
+        * The net result is that our #GP handler will think that we
+        * entered from usermode with the bad user context.
         *
         * No need for ist_enter here because we don't use RCU.
         */
@@ -359,13 +364,26 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
                regs->cs == __KERNEL_CS &&
                regs->ip == (unsigned long)native_irq_return_iret)
        {
-               struct pt_regs *normal_regs = task_pt_regs(current);
+               struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1;
+
+               /*
+                * regs->sp points to the failing IRET frame on the
+                * ESPFIX64 stack.  Copy it to the entry stack.  This fills
+                * in gpregs->ss through gpregs->ip.
+                *
+                */
+               memmove(&gpregs->ip, (void *)regs->sp, 5*8);
+               gpregs->orig_ax = 0;  /* Missing (lost) #GP error code */
 
-               /* Fake a #GP(0) from userspace. */
-               memmove(&normal_regs->ip, (void *)regs->sp, 5*8);
-               normal_regs->orig_ax = 0;  /* Missing (lost) #GP error code */
+               /*
+                * Adjust our frame so that we return straight to the #GP
+                * vector with the expected RSP value.  This is safe because
+                * we won't enable interupts or schedule before we invoke
+                * general_protection, so nothing will clobber the stack
+                * frame we just set up.
+                */
                regs->ip = (unsigned long)general_protection;
-               regs->sp = (unsigned long)&normal_regs->orig_ax;
+               regs->sp = (unsigned long)&gpregs->orig_ax;
 
                return;
        }
@@ -390,7 +408,7 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
         *
         *   Processors update CR2 whenever a page fault is detected. If a
         *   second page fault occurs while an earlier page fault is being
-        *   deliv- ered, the faulting linear address of the second fault will
+        *   delivered, the faulting linear address of the second fault will
         *   overwrite the contents of CR2 (replacing the previous
         *   address). These updates to CR2 occur even if the page fault
         *   results in a double fault or occurs during the delivery of a
@@ -795,14 +813,13 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
        debug_stack_usage_dec();
 
 exit:
-#if defined(CONFIG_X86_32)
        /*
         * This is the most likely code path that involves non-trivial use
         * of the SYSENTER stack.  Check that we haven't overrun it.
         */
        WARN(this_cpu_read(cpu_tss.SYSENTER_stack_canary) != STACK_END_MAGIC,
             "Overran or corrupted SYSENTER stack\n");
-#endif
+
        ist_exit(regs);
 }
 NOKPROBE_SYMBOL(do_debug);