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Commit 7575637a authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Thomas Gleixner
Browse files

x86, fpu: Fix math_state_restore() race with kernel_fpu_begin() 



math_state_restore() can race with kernel_fpu_begin() if irq comes
right after __thread_fpu_begin(), __save_init_fpu() will overwrite
fpu->state we are going to restore.

Add 2 simple helpers, kernel_fpu_disable() and kernel_fpu_enable()
which simply set/clear in_kernel_fpu, and change math_state_restore()
to exclude kernel_fpu_begin() in between.

Alternatively we could use local_irq_save/restore, but probably these
new helpers can have more users.

Perhaps they should disable/enable preemption themselves, in this case
we can remove preempt_disable() in __restore_xstate_sig().

Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Cc: matt.fleming@intel.com
Cc: bp@suse.de
Cc: pbonzini@redhat.com
Cc: luto@amacapital.net
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150115192028.GD27332@redhat.com


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 33a3ebdc

arch/x86/include/asm/i387.h

+4 −0
Original line number Diff line number Diff line
@@ -51,6 +51,10 @@ static inline void kernel_fpu_end(void) 
	preempt_enable();

}



/* Must be called with preempt disabled */

extern void kernel_fpu_disable(void);

extern void kernel_fpu_enable(void);



/*

 * Some instructions like VIA's padlock instructions generate a spurious

 * DNA fault but don't modify SSE registers. And these instructions

arch/x86/kernel/i387.c

+11 −0
Original line number Diff line number Diff line
@@ -21,6 +21,17 @@ 


static DEFINE_PER_CPU(bool, in_kernel_fpu);



void kernel_fpu_disable(void)

{

	WARN_ON(this_cpu_read(in_kernel_fpu));

	this_cpu_write(in_kernel_fpu, true);

}



void kernel_fpu_enable(void)

{

	this_cpu_write(in_kernel_fpu, false);

}



/*

 * Were we in an interrupt that interrupted kernel mode?

 *

arch/x86/kernel/traps.c

+5 −7
Original line number Diff line number Diff line
@@ -788,19 +788,17 @@ void math_state_restore(void) 
		local_irq_disable();

	}



	/* Avoid __kernel_fpu_begin() right after __thread_fpu_begin() */

	kernel_fpu_disable();

	__thread_fpu_begin(tsk);



	/*

	 * Paranoid restore. send a SIGSEGV if we fail to restore the state.

	 */

	if (unlikely(restore_fpu_checking(tsk))) {

		drop_init_fpu(tsk);

		force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);

		return;

	}



	} else {

		tsk->thread.fpu_counter++;

	}

	kernel_fpu_enable();

}

EXPORT_SYMBOL_GPL(math_state_restore);



dotraplinkage void