[PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs

[Oleg Nesterov]

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 trgiggers

	BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:41
	[<ffffffff81381499>] dump_stack+0x59/0xa0
	[<ffffffff8106bd05>] ___might_sleep+0x105/0x110
	[<ffffffff8138786d>] ? _raw_spin_unlock_irqrestore+0x3d/0x70
	[<ffffffff8106bd8d>] __might_sleep+0x7d/0xb0
	[<ffffffff81385426>] down_read+0x26/0xa0
	[<ffffffff8138788a>] ? _raw_spin_unlock_irqrestore+0x5a/0x70
	[<ffffffff81136038>] print_vma_addr+0x58/0x130
	[<ffffffff8100239e>] signal_fault+0xbe/0xf0
	[<ffffffff810419aa>] sys32_rt_sigreturn+0xba/0xd0

Change math_state_restore() to check irqs_disabled().

Note: this is the minimal fix for -stable, it is horrible but simple.
We need to rewrite math_state_restore(), init_fpu(), and cleanup their
users.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/traps.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 51c4658..7310e0e 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -774,7 +774,10 @@ void math_state_restore(void)
 	struct task_struct *tsk = current;
 
 	if (!tsk_used_math(tsk)) {
-		local_irq_enable();
+		bool disabled = irqs_disabled();
+
+		if (disabled)
+			local_irq_enable();
 		/*
 		 * does a slab alloc which can sleep
 		 */
@@ -785,7 +788,9 @@ void math_state_restore(void)
 			do_group_exit(SIGKILL);
 			return;
 		}
-		local_irq_disable();
+
+		if (disabled)
+			local_irq_disable();
 	}
 
 	/* Avoid __kernel_fpu_begin() right after __thread_fpu_begin() */
-- 
1.5.5.1


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