Messages in this thread | | | From | Dmitry Vyukov <> | Date | Wed, 1 Feb 2023 10:53:44 +0100 | Subject | Re: [PATCH v2] perf: Allow restricted kernel breakpoints on user addresses |
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On Wed, 1 Feb 2023 at 10:34, Marco Elver <elver@google.com> wrote: > > 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"?
I also wonder if these special mappings are ever accessible in a user task context? If yes, can a racing process_vm_readv/writev mess with these special mappings?
We could use copy_from_user() to probe that the watchpoint address is legit. But I think the memory can be potentially PROT_NONE but still legit, so copy_from_user() won't work for these corner cases.
> 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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