One last try on this path before I switch to the other options. On Wed, Oct 27, 2021 at 02:14:48PM -0700, Linus Torvalds wrote: > On Wed, Oct 27, 2021 at 12:13 PM Catalin Marinas > <catalin.marinas@xxxxxxx> wrote: > > As an alternative, you mentioned earlier that a per-thread fault status > > was not feasible on x86 due to races. Was this only for the hw poison > > case? I think the uaccess is slightly different. > > It's not x86-specific, it's very generic. > > If we set some flag in the per-thread status, we'll need to be careful > about not overwriting it if we then have a subsequent NMI that _also_ > takes a (completely unrelated) page fault - before we then read the > per-thread flag. > > Think 'perf' and fetching backtraces etc. > > Note that the NMI page fault can easily also be a pointer coloring > fault on arm64, for exactly the same reason that whatever original > copy_from_user() code was. So this is not a "oh, pointer coloring > faults are different". They have the same re-entrancy issue. > > And both the "pagefault_disable" and "fault happens in interrupt > context" cases are also the exact same 'faulthandler_disabled()' > thing. So even at fault time they look very similar. They do look fairly similar but we should have the information in the fault handler to distinguish: not a page fault (pte permission or p*d translation), in_task(), user address, fixup handler. But I agree the logic looks fragile. I think for nested contexts we can save the uaccess fault state on exception entry, restore it on return. Or (needs some thinking on atomicity) save it in a local variable. The high-level API would look something like: unsigned long uaccess_flags; /* we could use TIF_ flags */ uaccess_flags = begin_retriable_uaccess(); copied = copy_page_from_iter_atomic(...); retry = end_retriable_uaccess(uaccess_flags); ... if (!retry) break; I think we'd need a TIF flag to mark the retriable region and another to track whether a non-recoverable fault occurred. It needs prototyping. Anyway, if you don't like this approach, I'll look at error codes being returned but rather than changing all copy_from_user() etc., introduce a new API that returns different error codes depending on the fault (e.g -EFAULT vs -EACCES). We already have copy_from_user_nofault(), we'd need something for the iov_iter stuff to use in the fs code. -- Catalin
