Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [RFC PATCH 1/4] x86/entry/nmi: Switch to the entry stack before switching to the thread stack | Date | Sat, 26 Jun 2021 09:03:23 +0200 |
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On Fri, Jun 25 2021 at 13:00, Peter Zijlstra wrote: > On Fri, Jun 25, 2021 at 12:40:53PM +0200, Peter Zijlstra wrote: >> On Sat, Jun 19, 2021 at 08:13:15PM -0700, Andy Lutomirski wrote: >> > >> > >> > On Sat, Jun 19, 2021, at 3:51 PM, Thomas Gleixner wrote: >> > > On Tue, Jun 01 2021 at 14:52, Lai Jiangshan wrote: >> > > > From: Lai Jiangshan <laijs@linux.alibaba.com> >> > > > >> > > > Current kernel has no code to enforce data breakpoint not on the thread >> > > > stack. If there is any data breakpoint on the top area of the thread >> > > > stack, there might be problem. >> > > >> > > And because the kernel does not prevent data breakpoints on the thread >> > > stack we need to do more complicated things in the already horrible >> > > entry code instead of just doing the obvious and preventing data >> > > breakpoints on the thread stack? >> > >> > Preventing breakpoints on the thread stack is a bit messy: it’s >> > possible for a breakpoint to be set before the address in question is >> > allocated for the thread stack. >> >> How about we call into C from the entry stack and have the from-user >> stack swizzle there. The from-kernel entries land on the ISTs and those >> are already excluded. >> >> > None of this is NMI-specific. #DB itself has the same problem. We >> > could plausibly solve it differently by disarming breakpoints in the >> > entry asm before switching stacks. I’m not sure how much I like that >> > approach. >> >> I'm not sure I see how, from-user #DB already doesn't clear DR7, and if >> we recurse, we'll get a from-kernel trap, which will land on the IST, >> whcih is excluded, and then we clear DR7 there. >> >> IST and entry stack are excluded, the only problem we have is thread >> stack, and that can be solved by calling into C from the entry stack. >> >> I should put teaching objtool about .data references from .noinstr.text >> and .entry.text higher on the todo list I suppose ... > > Also, I think we can run the from-user exceptions on the entry stack, > without ever switching to the kernel stack, except for #PF, which is > magical and schedules.
No. Pretty much any exception coming from user space can schedule and even if it does not voluntary it can be preempted.
Thanks,
tglx
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