Re: [PATCH] Fix unsigned integer underflow in fd-trans.c

[Shu-Chun Weng <scw@google.com>]

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(Re-sending to the list because I forgot to turn off HTML before and
it was bounced.)

That does prevent the integer underflow, but it also changes the
behavior and I don't think the new behavior is desirable.

If the extra payload has a smaller alignment than the header, it makes
sense for the user program to generate a nlmsg_len that is not a
multiple of the alignment. When it's the last entry, the new condition
will it because NLMSG_ALIGN pushes the aligned length over `len`, yet
the single entry processing function won't actually read beyond the
buffer as long as it's bounded by nlmsg_len.

Shu-Chun


On Fri, Oct 18, 2019 at 12:26 AM Laurent Vivier <laurent@vivier.eu> wrote:
>
> Le 18/10/2019 à 02:19, Shu-Chun Weng a écrit :
> > In any of these `*_for_each_*` functions, the last entry in the buffer (so the
> > "remaining length in the buffer" `len` is equal to the length of the
> > entry `nlmsg_len`/`nla_len`/etc) has size that is not a multiple of the
> > alignment, the aligned lengths `*_ALIGN(*_len)` will be greater than `len`.
> > Since `len` is unsigned (`size_t`), it underflows and the loop will read
> > pass the buffer.
> >
> > This may manifest as random EINVAL or EOPNOTSUPP error on IO or network
> > system calls.
> >
> > Signed-off-by: Shu-Chun Weng <scw@google.com>
> > ---
> >  linux-user/fd-trans.c | 51 +++++++++++++++++++++++++++++++++----------
> >  1 file changed, 40 insertions(+), 11 deletions(-)
> >
> > diff --git a/linux-user/fd-trans.c b/linux-user/fd-trans.c
> > index 60077ce531..9b92386abf 100644
> > --- a/linux-user/fd-trans.c
> > +++ b/linux-user/fd-trans.c
> > @@ -279,6 +279,7 @@ static abi_long host_to_target_for_each_nlmsg(struct nlmsghdr *nlh,
> >                                                         (struct nlmsghdr *))
> >  {
> >      uint32_t nlmsg_len;
> > +    uint32_t aligned_nlmsg_len;
> >      abi_long ret;
> >
> >      while (len > sizeof(struct nlmsghdr)) {
> > @@ -312,8 +313,13 @@ static abi_long host_to_target_for_each_nlmsg(struct nlmsghdr *nlh,
> >              break;
> >          }
> >          tswap_nlmsghdr(nlh);
> > -        len -= NLMSG_ALIGN(nlmsg_len);
> > -        nlh = (struct nlmsghdr *)(((char*)nlh) + NLMSG_ALIGN(nlmsg_len));
> > +
> > +        aligned_nlmsg_len = NLMSG_ALIGN(nlmsg_len);
> > +        if (aligned_nlmsg_len >= len) {
> > +            break;
> > +        }
> > +        len -= aligned_nlmsg_len;
> > +        nlh = (struct nlmsghdr *)(((char*)nlh) + aligned_nlmsg_len);
> >      }
> >      return 0;
> >  }
>
> Nice catch.
>
> But the first "if" in the loop is already here for that, we only need to
> fix it with something like that in all the for_each functions:
>
> @@ -285,7 +285,7 @@ static abi_long host_to_target_for_each_nlmsg(struct
> nlmsghdr *nlh,
>
>          nlmsg_len = nlh->nlmsg_len;
>          if (nlmsg_len < sizeof(struct nlmsghdr) ||
> -            nlmsg_len > len) {
> +            NLMSG_ALIGN(nlmsg_len) > len) {
>              break;
>          }
>
> Thanks,
> Laurent
>

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