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 92d140e21f1 ("x86: fix taking
DNA during 64bit sigreturn").

Post commit 92d140e21f1, 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>

diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index b45ef8d..ca316b5 100644 (file)
--- a/arch/x86/kernel/signal_64.c
+++ b/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;
 }
 
 /*

diff --git a/arch/x86/kernel/traps_64.c b/arch/x86/kernel/traps_64.c
index 3f18d73..513caac 100644 (file)
--- a/arch/x86/kernel/traps_64.c
+++ b/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++;
 }

diff --git a/include/asm-x86/i387.h b/include/asm-x86/i387.h
index 96fa844..0048fb7 100644 (file)
--- a/include/asm-x86/i387.h
+++ b/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;
 }