Re: [tip:x86/efi] x86/efi: Save and restore FPU context around efi_calls (x86_64)
From: Borislav Petkov
Date: Fri Apr 25 2014 - 07:40:21 EST
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@xxxxxxx>
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@xxxxxxx>
---
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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