Re: [PATCH v10 5/6] x86/split_lock: Handle #AC exception for split lock

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 20, 2019, at 5:45 PM, Fenghua Yu <fenghua.yu@intel.com> wrote:
> 
> Currently Linux does not expect to see an alignment check exception 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.
> 
> Add a new handler for #AC exceptions that will force a panic on split
> lock for kernel mode.
> 
> 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(-)
> 
> 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    =        1 << 4,
>    X86_PF_PK    =        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
> 
> +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
> 
> +dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code)
> +{
> +    unsigned int trapnr = X86_TRAP_AC;
> +    char str[] = "alignment check";
> +    int signr = SIGBUS;
> +
> +    RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +
> +    if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) == NOTIFY_STOP)
> +        return;
> +
> +    if (!user_mode(regs) && split_lock_detect_enabled)
> +        panic("Split lock detected\n");

NAK.

1. Don’t say “split lock detected” if you don’t know that you detected a split lock.  Or is this genuinely the only way to get #AC from kernel mode?

2. Don’t panic. Use die() just like every other error where nothing is corrupted.

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