linux-kernel - Re: [tip:x86/efi] x86/efi: Save and restore FPU context around efi_calls (x86

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Date:	Fri, 25 Apr 2014 13:40:10 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	ricardo.neri-calderon@...ux.intel.com, tglx@...utronix.de,
	matt.fleming@...el.com, linux-tip-commits@...r.kernel.org,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: Re: [tip:x86/efi] x86/efi: Save and restore FPU context around
 efi_calls (x86_64)

On Fri, Apr 25, 2014 at 12:09:36PM +0100, Matt Fleming wrote:
> Hmm... note that we may call EFI runtime services from interrupt context
> in efi_pstore_write(), so it seems like it would be possible to trigger
> that WARN_ON_ONCE() there.
> 
> Seiji (Cc'd) might have some opinions on this.
> 
> Either way, if someone sends me a patch ontop of this one that swaps the
> __kernel_fpu_begin() for kernel_fpu_begin() I can try them out in my
> lab.

Well, the more I think about it, the more I'm persuaded that
you actually do *really* need that WARN_ON_ONCE check there to
make sure you're not fiddling with the FPU while in an interrupt
context and in an unsafe way (see interrupted_kernel_fpu_idle() and
interrupted_user_mode()). And so you do need the variants without the
"__" which include the check.

Anyway, here it is, do give it a good run:

---
From: Borislav Petkov <bp@...e.de>
Date: Fri, 25 Apr 2014 13:30:21 +0200
Subject: [PATCH] efi: Check for unsafe dealing with FPU state in irq ctxt

efi_call can happen in an irq context (pstore) and there we really need
to make sure we're not scribbling over FPU state while we've interrupted
a thread or kernel mode with a live FPU state. Therefore, use the
kernel_fpu_begin/end() variants which do that check.

Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/include/asm/efi.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index 1eb5f6433ad8..f969ce8bea24 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -67,11 +67,9 @@ extern u64 asmlinkage efi_call(void *fp, ...);
 	efi_status_t __s;						\
 									\
 	efi_sync_low_kernel_mappings();					\
-	preempt_disable();						\
-	__kernel_fpu_begin();						\
+	kernel_fpu_begin();						\
 	__s = efi_call((void *)efi.systab->runtime->f, __VA_ARGS__);	\
-	__kernel_fpu_end();						\
-	preempt_enable();						\
+	kernel_fpu_end();						\
 	__s;								\
 })
 
-- 
1.9.0.258.g00eda23

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