[PATCH v2 4/5] x86, fpu: fix math_state_restore() race with kernel_fpu_begin()

Oleg Nesterov
2014-09-05T13:46:19Z

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.

Perhaps we should export in_kernel_fpu and make these helpers inline.
Probably they will have more users.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/include/asm/i387.h | 4 ++++
arch/x86/kernel/i387.c | 12 +++++++++++-
arch/x86/kernel/traps.c | 12 +++++-------
3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/i387.h b/arch/x86/include/asm/i387.h
index 5e275d3..605b1eb 100644
--- a/arch/x86/include/asm/i387.h
+++ b/arch/x86/include/asm/i387.h
@@ -51,6 +51,10 @@ static inline void kernel_fpu_end(void)
preempt_enable();
}

+/* Must be called woth 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
diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 7bc8236..ece91cf 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -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?
*
@@ -80,7 +91,6 @@ void __kernel_fpu_begin(void)

this_cpu_write(in_kernel_fpu, true);

- /* FIXME: race with math_state_restore()-like code */
if (__thread_has_fpu(me)) {
__save_init_fpu(me);
} else if (!use_eager_fpu()) {
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 0d0e922..c632843 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -667,18 +667,16 @@ 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);

--
1.5.5.1