Re: [PATCH mm-unstable fix] mm: userfaultfd: check for start + len overflow in validate_range: fix

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Aug 10, 2023 at 9:49 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 10.08.23 17:53, Ryan Roberts wrote:
> > On 14/07/2023 19:29, Axel Rasmussen wrote:
> >> This commit removed an extra check for zero-length ranges, and folded it
> >> into the common validate_range() helper used by all UFFD ioctls.
> >>
> >> It failed to notice though that UFFDIO_COPY *only* called validate_range
> >> on the dst range, not the src range. So removing this check actually let
> >> us proceed with zero-length source ranges, eventually hitting a BUG
> >> further down in the call stack.
> >>
> >> The correct fix seems clear: call validate_range() on the src range too.
> >>
> >> Other ioctls are not affected by this, as they only have one range, not
> >> two (src + dst).
> >>
> >> Reported-by: syzbot+42309678e0bc7b32f8e9@syzkaller.appspotmail.com
> >> Closes: https://syzkaller.appspot.com/bug?extid=42309678e0bc7b32f8e9
> >> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> >> ---
> >>   fs/userfaultfd.c | 3 +++
> >>   1 file changed, 3 insertions(+)
> >>
> >> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> >> index 53a7220c4679..36d233759233 100644
> >> --- a/fs/userfaultfd.c
> >> +++ b/fs/userfaultfd.c
> >> @@ -1759,6 +1759,9 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
> >>                         sizeof(uffdio_copy)-sizeof(__s64)))
> >>              goto out;
> >>
> >> +    ret = validate_range(ctx->mm, uffdio_copy.src, uffdio_copy.len);
> >> +    if (ret)
> >> +            goto out;
> >>      ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
> >>      if (ret)
> >>              goto out;
> >
> >
> > Hi Axel,
> >
> > I've just noticed that this patch, now in mm-unstable, regresses the mkdirty mm
> > selftest:
> >
> > # [INFO] detected THP size: 2048 KiB
> > TAP version 13
> > 1..6
> > # [INFO] PTRACE write access
> > ok 1 SIGSEGV generated, page not modified
> > # [INFO] PTRACE write access to THP
> > ok 2 SIGSEGV generated, page not modified
> > # [INFO] Page migration
> > ok 3 SIGSEGV generated, page not modified
> > # [INFO] Page migration of THP
> > ok 4 SIGSEGV generated, page not modified
> > # [INFO] PTE-mapping a THP
> > ok 5 SIGSEGV generated, page not modified
> > # [INFO] UFFDIO_COPY
> > not ok 6 UFFDIO_COPY failed
> > Bail out! 1 out of 6 tests failed
> > # Totals: pass:5 fail:1 xfail:0 xpass:0 skip:0 error:0
> >
> > Whereas all 6 tests pass against v6.5-rc4.
> >
> > I'm afraid I don't know the test well and haven't looked at what the issue might
> > be, but noticed and thought I should point it out.
>
> That test (written by me ;) ) essentially does
>
> src = malloc(pagesize);
> dst = mmap(NULL, pagesize, PROT_READ, MAP_PRIVATE|MAP_ANON, -1, 0)
> ...
>
> uffdio_copy.dst = (unsigned long) dst;
> uffdio_copy.src = (unsigned long) src;
> uffdio_copy.len = pagesize;
> uffdio_copy.mode = 0;
> if (ioctl(uffd, UFFDIO_COPY, &uffdio_copy)) {
> ...
>
>
> So src might not be aligned to a full page.
>
> According to the man page:
>
> "EINVAL Either dst or len was not a multiple of the system page size, or
> the range specified by src and len or dst and len was invalid."
>
> So, AFAIKT, there is no requirement for src to be page-aligned.
>
> Using validate_range() on the src is wrong.

Thanks for the report and the suggestions! I sent a fixup patch which
should resolve this [1]. At least, I ran the test in question a bunch
of times and it passed reliably with this fix.

[1]: https://patchwork.kernel.org/project/linux-mm/patch/20230810192128.1855570-1-axelrasmussen@google.com/

>
> --
> Cheers,
>
> David / dhildenb
>