Messages in this thread | | | From | Arvind Sankar <> | Date | Wed, 22 Jan 2020 17:42:51 -0500 | Subject | Re: [PATCH v12] x86/split_lock: Enable split lock detection by kernel |
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On Wed, Jan 22, 2020 at 10:55:14AM -0800, Luck, Tony wrote: > + > +static enum split_lock_detect_state sld_state = sld_warn; > +
This sets sld_state to sld_warn even on CPUs that don't support split-lock detection. split_lock_init will then try to read/write the MSR to turn it on. Would it be better to initialize it to sld_off and set it to sld_warn in split_lock_setup instead, which is only called if the CPU supports the feature?
> > +dotraplinkage void do_alignment_check(struct pt_regs *regs, long error_code) > +{ > + const char str[] = "alignment check"; > + > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU"); > + > + if (notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_AC, SIGBUS) == NOTIFY_STOP) > + return; > + > + if (!split_lock_detect_enabled()) > + return;
This misses one comment from Sean [1] that this check should be dropped, otherwise user-space alignment check via EFLAGS.AC will get ignored when split lock detection is disabled.
[1] https://lore.kernel.org/lkml/20191122184457.GA31235@linux.intel.com/
> + > + if (!user_mode(regs)) > + die("Split lock detected\n", regs, error_code); > + > + cond_local_irq_enable(regs); > + > + if (handle_user_split_lock(regs, error_code)) > + return; > + > + do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs, > + error_code, BUS_ADRALN, NULL); > +} > +
Peter [2] called this a possible DOS vector. If userspace is malicious rather than buggy, couldn't it simply ignore SIGBUS?
[2] https://lore.kernel.org/lkml/20191121131522.GX5671@hirez.programming.kicks-ass.net/
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