Messages in this thread | | | Subject | Re: [PATCH] x86/split_lock: Sanitize userspace and guest error output | From | Xiaoyao Li <> | Date | Sat, 6 Jun 2020 11:02:17 +0800 |
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On 6/6/2020 12:42 AM, Prarit Bhargava wrote: > > > On 6/5/20 11:29 AM, Xiaoyao Li wrote: >> On 6/5/2020 7:44 PM, Prarit Bhargava wrote: >>> There are two problems with kernel messages in fatal mode that >>> were found during testing of guests and userspace programs. >>> >>> The first is that no kernel message is output when the split lock detector >>> is triggered with a userspace program. As a result the userspace process >>> dies from receiving SIGBUS with no indication to the user of what caused >>> the process to die. >>> >>> The second problem is that only the first triggering guest causes a kernel >>> message to be output because the message is output with pr_warn_once(). >>> This also results in a loss of information to the user. >>> >>> While fixing these I noticed that the same message was being output >>> three times so I'm cleaning that up too. >>> >>> Fix fatal mode output, and use consistent messages for fatal and >>> warn modes for both userspace and guests. >>> >>> Signed-off-by: Prarit Bhargava <prarit@redhat.com> >>> Cc: Thomas Gleixner <tglx@linutronix.de> >>> Cc: Ingo Molnar <mingo@redhat.com> >>> Cc: Borislav Petkov <bp@alien8.de> >>> Cc: x86@kernel.org >>> Cc: "H. Peter Anvin" <hpa@zytor.com> >>> Cc: Tony Luck <tony.luck@intel.com> >>> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org> >>> Cc: Sean Christopherson <sean.j.christopherson@intel.com> >>> Cc: Rahul Tanwar <rahul.tanwar@linux.intel.com> >>> Cc: Xiaoyao Li <xiaoyao.li@intel.com> >>> Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com> >>> Cc: Dave Hansen <dave.hansen@linux.intel.com> >>> --- >>> arch/x86/kernel/cpu/intel.c | 24 ++++++++++-------------- >>> 1 file changed, 10 insertions(+), 14 deletions(-) >>> >>> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c >>> index 166d7c355896..463022aa9b7a 100644 >>> --- a/arch/x86/kernel/cpu/intel.c >>> +++ b/arch/x86/kernel/cpu/intel.c >>> @@ -1074,10 +1074,14 @@ static void split_lock_init(void) >>> split_lock_verify_msr(sld_state != sld_off); >>> } >>> -static void split_lock_warn(unsigned long ip) >>> +static bool split_lock_warn(unsigned long ip, int fatal) >>> { >>> - pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n", >>> - current->comm, current->pid, ip); >>> + pr_warn_ratelimited("#AC: %s/%d %ssplit_lock trap at address: 0x%lx\n", >>> + current->comm, current->pid, >>> + sld_state == sld_fatal ? "fatal " : "", ip); >>> + >>> + if (sld_state == sld_fatal || fatal) >>> + return false; >>> /* >>> * Disable the split lock detection for this task so it can make >>> @@ -1086,18 +1090,13 @@ static void split_lock_warn(unsigned long ip) >>> */ >>> sld_update_msr(false); >>> set_tsk_thread_flag(current, TIF_SLD); >>> + return true; >>> } >>> bool handle_guest_split_lock(unsigned long ip) >>> { >>> - if (sld_state == sld_warn) { >>> - split_lock_warn(ip); >>> + if (split_lock_warn(ip, 0)) >>> return true; >>> - } >>> - >>> - pr_warn_once("#AC: %s/%d %s split_lock trap at address: 0x%lx\n", >>> - current->comm, current->pid, >>> - sld_state == sld_fatal ? "fatal" : "bogus", ip); >>> current->thread.error_code = 0; >>> current->thread.trap_nr = X86_TRAP_AC; >>> @@ -1108,10 +1107,7 @@ EXPORT_SYMBOL_GPL(handle_guest_split_lock); >>> bool handle_user_split_lock(struct pt_regs *regs, long error_code) >>> { >>> - if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal) >>> - return false; >>> - split_lock_warn(regs->ip); >>> - return true; >>> + return split_lock_warn(regs->ip, regs->flags & X86_EFLAGS_AC); >> >> It's incorrect. You change the behavior that it will print the split lock >> warning even when CPL 3 Alignment Check is turned on. > > Do you want the message to be displayed in the fatal case of CPL 3 Alignment check? >
No. It should never be displayed if #AC happens in CPL 3 and X86_EFLAGS_AC is set. In this case, an unaligned access triggers #AC regardless of #LOCK prefix. What's more, even there is a #LOCK prefix, we still cannot tell the cause because we don't know the priority of legacy alignment check #AC and split lock #AC.
If you do want a message, we can only say "unaligned access at address xxx".
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