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

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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: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Rik 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: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Oleg Nesterov
2015-01-15 20:20:28 +01:00
committed by Thomas Gleixner
parent 33a3ebdc07
commit 7575637ab2
3 changed files with 20 additions and 7 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
arch/x86/kernel/traps.c
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@@ -788,18 +788,16 @@ 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++;
}
tsk->thread.fpu_counter++;
kernel_fpu_enable();
}
EXPORT_SYMBOL_GPL(math_state_restore);