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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v10 5/6] x86/split_lock: Handle #AC exception for split lock
Date:   Thu, 21 Nov 2019 14:10:38 -0800
Message-Id: <5BDDAE0C-2D31-4779-B3A0-5BF206FF3E50@amacapital.net>
References: <1574297603-198156-6-git-send-email-fenghua.yu@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tony Luck <tony.luck@intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
In-Reply-To: <1574297603-198156-6-git-send-email-fenghua.yu@intel.com>
To:     Fenghua Yu <fenghua.yu@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Nov 20, 2019, at 5:45 PM, Fenghua Yu <fenghua.yu@intel.com> wrote:
>=20
> =EF=BB=BFCurrently Linux does not expect to see an alignment check excepti=
on in
> kernel mode (since it does not set CR4.AC). The existing #AC handlers
> will just return from exception to the faulting instruction which will
> trigger another exception.
>=20
> Add a new handler for #AC exceptions that will force a panic on split
> lock for kernel mode.
>=20
> Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> ---
> arch/x86/include/asm/traps.h |  3 +++
> arch/x86/kernel/cpu/intel.c  |  2 ++
> arch/x86/kernel/traps.c      | 22 +++++++++++++++++++++-
> 3 files changed, 26 insertions(+), 1 deletion(-)
>=20
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index b25e633033c3..0fa4eef83057 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -172,4 +172,7 @@ enum x86_pf_error_code {
>    X86_PF_INSTR    =3D        1 << 4,
>    X86_PF_PK    =3D        1 << 5,
> };
> +
> +extern bool split_lock_detect_enabled;
> +
> #endif /* _ASM_X86_TRAPS_H */
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 2614616fb6d3..bc0c2f288509 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -32,6 +32,8 @@
> #include <asm/apic.h>
> #endif
>=20
> +bool split_lock_detect_enabled;
> +
> /*
>  * Just in case our CPU detection goes bad, or you have a weird system,
>  * allow a way to override the automatic disabling of MPX.
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 4bb0f8447112..044033ff4326 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -293,9 +293,29 @@ DO_ERROR(X86_TRAP_OLD_MF, SIGFPE,           0, NULL, "=
coprocessor segment overru
> DO_ERROR(X86_TRAP_TS,     SIGSEGV,          0, NULL, "invalid TSS",       =
  invalid_TSS)
> DO_ERROR(X86_TRAP_NP,     SIGBUS,           0, NULL, "segment not present"=
, segment_not_present)
> DO_ERROR(X86_TRAP_SS,     SIGBUS,           0, NULL, "stack segment",     =
  stack_segment)
> -DO_ERROR(X86_TRAP_AC,     SIGBUS,  BUS_ADRALN, NULL, "alignment check",  =
   alignment_check)
> #undef IP
>=20
> +dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_co=
de)
> +{
> +    unsigned int trapnr =3D X86_TRAP_AC;
> +    char str[] =3D "alignment check";
> +    int signr =3D SIGBUS;
> +
> +    RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +
> +    if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) =3D=3D=
 NOTIFY_STOP)
> +        return;
> +
> +    if (!user_mode(regs) && split_lock_detect_enabled)
> +        panic("Split lock detected\n");

NAK.

1. Don=E2=80=99t say =E2=80=9Csplit lock detected=E2=80=9D if you don=E2=80=99=
t know that you detected a split lock.  Or is this genuinely the only way to=
 get #AC from kernel mode?

2. Don=E2=80=99t panic. Use die() just like every other error where nothing i=
s corrupted.

And maybe instead turn off split lock detection and print a stack trace inst=
ead.  Then the kernel is even more likely to survive to log something useful=
.