Re: [PATCH v6 11/11] arm64: annotate user pointers casts detected by sparse

Andrey Konovalov
2018-09-24T21:07:07Z

On Mon, Sep 17, 2018 at 7:01 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> I took another look at the changes this patchset does to the kernel
> and here are my thoughts:
>
> I see two ways how a (potentially tagged) user pointer gets into the kernel:
>
> 1. A pointer is passed to a syscall (directly as an argument or
> indirectly as a struct field).
> 2. A pointer is extracted from user context (registers, etc.) by some
> kind of a trap/fault handler.
> (Is there something else?)
>
> In case 1 we also have a special case of a pointer passed to one of
> the memory syscalls (mmap, mprotect, etc.). These syscalls "are not
> doing memory accesses but rather dealing with the memory range, hence
> an untagged pointer is better suited" as pointed out by Catalin (these
> syscalls do not always use "unsigned long" instead of "void __user *"
> though, for example shmat uses "void __user *").
>
> Looking at patch #8 ("usb, arm64: untag user addresses in devio") in
> this series, it seems that that devio ioctl actually accepts a pointer
> into a vma, so we shouldn't actually be untagging its argument and the
> patch needs to be dropped. Otherwise there's quite a few more cases
> that needs to be changed (like tcp_zerocopy_receive() for example,
> more can be found by grepping find_vma() in generic code).
>
> Regarding case 2, it seems that analyzing casts of __user pointers
> won't really help, since the code (arch/arm64/mm/fault.c) doesn't
> really use them. However all of this code is arch specific, so it
> shouldn't really change over time (right?). It looks like dealing with
> tags passed to the kernel through these fault handlers is already
> resolved with these patches (and therefore patch #6 ("arm64: untag
> user address in __do_user_fault") in this series is not actually
> needed and can be dropped (need to test that)):
>
> 276e9327 ("arm64: entry: improve data abort handling of tagged pointers"),
> 81cddd65 ("arm64: traps: fix userspace cache maintenance emulation on
> a tagged pointer")
> 7dcd9dd8 ("arm64: hw_breakpoint: fix watchpoint matching for tagged pointers")
>
> Now, I also see two cases when kernel behavior changes depending on
> whether a pointer is tagged:
>
> 1. Kernel code checks that a pointer belongs to userspace by comparing
> it with TASK_SIZE/addr_limit/user_addr_max()/USER_DS/... .
> 2. A pointer gets passed to find_vma() or similar functions.
> (Is there something else?)
>
> The initial thought that I had here is that the pointers that reach
> find_vma() must be passed through memory syscalls and therefore
> shouldn't be untagged and don't require any fixes. There are at least
> two exceptions to this: 1. get_user_pages() (see patch #4 ("mm, arm64:
> untag user addresses in mm/gup.c") in this patch series) and 2.
> __do_page_fault() in arch/arm64/mm/fault.c. Are there any other
> obvious exceptions? I've tried adding BUG_ON(has_tag(addr)) to
> find_vma() and running a modified syzkaller version that passes tagged
> pointers to the kernel and failed to find anything else.
>
> As for case 1, the places where pointers are compared with TASK_SIZE
> and others can be found with grep. Maybe it makes sense to introduce
> some kind of routine like is_user_pointer() that handles tagged
> pointers and refactor the existing code to use it? And maybe add a
> rule to checkpatch.pl that forbids the direct usage of TASK_SIZE and
> others.
>
> So I think detecting direct comparisons with TASK_SIZE and others
> would more useful than finding __user pointer casts (it seems that the
> latter requires a lot of annotations to be fixed/added), and I should
> just drop this patch with annotations.
>
> WDYT?

ping