[3.16.y-ckt stable] Patch "x86/fpu: Avoid math_state_restore() without used_math() in __restore_xstate_sig()" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    x86/fpu: Avoid math_state_restore() without used_math() in __restore_xstate_sig()

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 6c47e87be71fd9d9a7bc20e4771d6886cd356e03 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg at redhat.com>
Date: Fri, 13 Mar 2015 09:53:09 +0100
Subject: x86/fpu: Avoid math_state_restore() without used_math() in
 __restore_xstate_sig()

commit a7c80ebcac3068b1c3cb27d538d29558c30010c8 upstream.

math_state_restore() assumes it is called with irqs disabled,
but this is not true if the caller is __restore_xstate_sig().

This means that if ia32_fxstate == T and __copy_from_user()
fails, __restore_xstate_sig() returns with irqs disabled too.

This triggers:

  BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:41
   dump_stack
   ___might_sleep
   ? _raw_spin_unlock_irqrestore
   __might_sleep
   down_read
   ? _raw_spin_unlock_irqrestore
   print_vma_addr
   signal_fault
   sys32_rt_sigreturn

Change __restore_xstate_sig() to call set_used_math()
unconditionally. This avoids enabling and disabling interrupts
in math_state_restore(). If copy_from_user() fails, we can
simply do fpu_finit() by hand.

[ Note: this is only the first step. math_state_restore() should
        not check used_math(), it should set this flag. While
	init_fpu() should simply die. ]

Signed-off-by: Oleg Nesterov <oleg at redhat.com>
Signed-off-by: Borislav Petkov <bp at suse.de>
Cc: Andy Lutomirski <luto at amacapital.net>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Dave Hansen <dave.hansen at intel.com>
Cc: Fenghua Yu <fenghua.yu at intel.com>
Cc: H. Peter Anvin <hpa at zytor.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Pekka Riikonen <priikone at iki.fi>
Cc: Quentin Casasnovas <quentin.casasnovas at oracle.com>
Cc: Rik van Riel <riel at redhat.com>
Cc: Suresh Siddha <sbsiddha at gmail.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Link: http://lkml.kernel.org/r/20150307153844.GB25954@redhat.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/kernel/xsave.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
index dd50e26c58f6..7a09aca4b33a 100644
--- a/arch/x86/kernel/xsave.c
+++ b/arch/x86/kernel/xsave.c
@@ -375,7 +375,7 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 		 * thread's fpu state, reconstruct fxstate from the fsave
 		 * header. Sanitize the copied state etc.
 		 */
-		struct xsave_struct *xsave = &tsk->thread.fpu.state->xsave;
+		struct fpu *fpu = &tsk->thread.fpu;
 		struct user_i387_ia32_struct env;
 		int err = 0;

@@ -389,14 +389,15 @@ int __restore_xstate_sig(void __user *buf, void __user *buf_fx, int size)
 		 */
 		drop_fpu(tsk);

-		if (__copy_from_user(xsave, buf_fx, state_size) ||
+		if (__copy_from_user(&fpu->state->xsave, buf_fx, state_size) ||
 		    __copy_from_user(&env, buf, sizeof(env))) {
+			fpu_finit(fpu);
 			err = -1;
 		} else {
 			sanitize_restored_xstate(tsk, &env, xstate_bv, fx_only);
-			set_used_math();
 		}

+		set_used_math();
 		if (use_eager_fpu()) {
 			preempt_disable();
 			math_state_restore();