netdev - Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request at 0000a7cf

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Date:   Sun, 12 Mar 2017 22:40:33 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>,
        ast@...com, the arch/x86 maintainers <x86@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [net/bpf] 3051bf36c2 BUG: unable to handle kernel paging request
 at 0000a7cf

On Thu, Mar 09, 2017 at 03:26:02PM -0800, Linus Torvalds wrote:
> Maybe it's the lguest games with PGE that need to be removed?

Btw, tglx suggested something else the other day: warn when we're
changing boot_cpu_data x86_capability bits *after* alternatives have
run. The reasoning behind it being that potentially some patching
static_cpu_has() has done won't be correct anymore.

And it is pretty cheap to do it, it fires nicely on the 32-bit config
with LGUEST=y.

---
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index d59c15c3defd..f06c3dc6db70 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -124,8 +124,18 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 
 #define boot_cpu_has(bit)	cpu_has(&boot_cpu_data, bit)
 
-#define set_cpu_cap(c, bit)	set_bit(bit, (unsigned long *)((c)->x86_capability))
-#define clear_cpu_cap(c, bit)	clear_bit(bit, (unsigned long *)((c)->x86_capability))
+#define set_cpu_cap(c, bit)					\
+({								\
+	WARN_ON(c == &boot_cpu_data && alternatives_patched);	\
+	set_bit(bit, (unsigned long *)((c)->x86_capability));	\
+})
+
+#define clear_cpu_cap(c, bit)					\
+({								\
+	WARN_ON(c == &boot_cpu_data && alternatives_patched);	\
+	clear_bit(bit, (unsigned long *)((c)->x86_capability));	\
+})
+
 #define setup_clear_cpu_cap(bit) do { \
 	clear_cpu_cap(&boot_cpu_data, bit);	\
 	set_bit(bit, (unsigned long *)cpu_caps_cleared); \

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--