Messages in this thread | | | From | Marco Elver <> | Date | Wed, 1 Feb 2023 10:33:40 +0100 | Subject | Re: [PATCH v2] perf: Allow restricted kernel breakpoints on user addresses |
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On Mon, 30 Jan 2023 at 11:46, Mark Rutland <mark.rutland@arm.com> wrote: [...] > > This again feels like a deficiency with access_ok(). Is there a better > > primitive than access_ok(), or can we have something that gives us the > > guarantee that whatever it says is "ok" is a userspace address? > > I don't think so, since this is contextual and temporal -- a helper can't give > a single correct answert in all cases because it could change.
That's fair, but unfortunate. Just curious: would copy_from_user_nofault() reliably fail if it tries to access one of those mappings but where access_ok() said "ok"?
Though that would probably restrict us to only creating watchpoints for addresses that are actually mapped in the task.
> In the cases we switch to another mapping, we could try to ensure that we > enable/disable potentially unsafe watchpoints/breakpoints.
That seems it'd be too hard to reason that it's 100% safe, everywhere, on every arch. I'm still convinced we can prohibit creation of such watchpoints in the first place, but need something other than access_ok().
> Taking a look at arm64, our idmap code might actually be ok, since we usually > mask all the DAIF bits (and the 'D' or 'Debug' bit masks HW > breakpoints/watchpoints). For EFI we largely switch to another thread (but not > always), so that would need some auditing. > > So if this only needs to work in per-task mode rather than system-wide mode, I > reckon we can have some save/restore logic around those special cases where we > transiently install a mapping, which would protect us.
It should only work in per-task mode.
> For the threads that run with special mappings in the low half, I'm not sure > what to do. If we've ruled out system-wide monitoring I believe those would be > protected from unprivileged users.
Can the task actually access those special mappings, or is it only accessible by the kernel?
Thanks, -- Marco
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