x64, fpu: fix possible FPU leakage in error conditions

On Thu, Jul 24, 2008 at 03:43:44PM -0700, Linus Torvalds wrote:
> So how about this patch as a starting point? This is the RightThing(tm) to
> do regardless, and if it then makes it easier to do some other cleanups,
> we should do it first. What do you think?

restore_fpu_checking() calls init_fpu() in error conditions.

While this is wrong(as our main intention is to clear the fpu state of
the thread), this was benign before commit 92d140e2 ("x86: fix taking
DNA during 64bit sigreturn").

Post commit 92d140e2, live FPU registers may not belong to this
process at this error scenario.

In the error condition for restore_fpu_checking() (especially during the
64bit signal return), we are doing init_fpu(), which saves the live FPU
register state (possibly belonging to some other process context) into
the thread struct (through unlazy_fpu() in init_fpu()). This is wrong
and can leak the FPU data.

For the signal handler restore error condition in restore_i387(), clear
the fpu state present in the thread struct(before ultimately sending a
SIGSEGV for badframe).

For the paranoid error condition check in math_state_restore(), send a
SIGSEGV, if we fail to restore the state.
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: <stable@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/signal_64.c

@@ -104,7 +104,16 @@ static inline int restore_i387(struct _fpstate __user *buf)
 		clts();
 		task_thread_info(current)->status |= TS_USEDFPU;
 	}
-	return restore_fpu_checking((__force struct i387_fxsave_struct *)buf);
+	err = restore_fpu_checking((__force struct i387_fxsave_struct *)buf);
+	if (unlikely(err)) {
+		/*
+		 * Encountered an error while doing the restore from the
+		 * user buffer, clear the fpu state.
+		 */
+		clear_fpu(tsk);
+		clear_used_math();
+	}
+	return err;
 }
 
 /*

arch/x86/kernel/traps_64.c

@@ -1131,7 +1131,14 @@ asmlinkage void math_state_restore(void)
 	}
 
 	clts();				/* Allow maths ops (or we recurse) */
-	restore_fpu_checking(&me->thread.xstate->fxsave);
+	/*
+	 * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+	 */
+	if (unlikely(restore_fpu_checking(&me->thread.xstate->fxsave))) {
+		stts();
+		force_sig(SIGSEGV, me);
+		return;
+	}
 	task_thread_info(me)->status |= TS_USEDFPU;
 	me->fpu_counter++;
 }

include/asm-x86/i387.h

@@ -62,8 +62,6 @@ static inline int restore_fpu_checking(struct i387_fxsave_struct *fx)
 #else
 		     : [fx] "cdaSDb" (fx), "m" (*fx), "0" (0));
 #endif
-	if (unlikely(err))
-		init_fpu(current);
 	return err;
 }