Re: [PATCH] libsemanage: genhomedircon - improve handling large groups

[Nicolas Iooss <nicolas.iooss@m4x.org>]

On Wed, Feb 6, 2019 at 8:45 PM Petr Lautrbach <plautrba@redhat.com> wrote:
>
> getgrnam_r() uses a preallocated buffer to store a structure containing
> the broken-out fields of the record in the group database. The size of
> this buffer is usually sysconf(_SC_GETGR_R_SIZE_MAX) == 1024 and it is
> not enough for groups with a large number of users.  In these cases,
> getgrnam_r() returns -1 and sets errno to ERANGE and the caller can
> retry with a larger buffer.
>
> Fixes:
> $ semanage login -a -s user_u -r s0-s0:c1.c2 '%largegroup'
> libsemanage.semanage_direct_commit: semanage_genhomedircon returned error code -1. (Numerical result out of range).
> OSError: Numerical result out of range
>
> Signed-off-by: Petr Lautrbach <plautrba@redhat.com>
> ---
>  libsemanage/src/genhomedircon.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/libsemanage/src/genhomedircon.c b/libsemanage/src/genhomedircon.c
> index 591941fb..ac376671 100644
> --- a/libsemanage/src/genhomedircon.c
> +++ b/libsemanage/src/genhomedircon.c
> @@ -1077,10 +1077,20 @@ static int get_group_users(genhomedircon_settings_t * s,
>
>         const char *grname = selogin + 1;
>
> -       if (getgrnam_r(grname, &grstorage, grbuf,
> -                       (size_t) grbuflen, &group) != 0) {
> -               goto cleanup;
> +       errno = 0;
> +       while (
> +               (retval = getgrnam_r(grname, &grstorage, grbuf, (size_t) grbuflen, &group)) != 0 &&
> +               errno == ERANGE
> +       ) {
> +               char *new_grbuf;
> +               grbuflen *= 2;
> +               new_grbuf = realloc(grbuf, grbuflen);
> +               if (new_grbuf == NULL)
> +                       goto cleanup;
> +               grbuf = new_grbuf;
>         }

Hello,
When reading this for loop, I am concerned about the case where the
member list exceeds 2Go on a system with a 32-bit CPU (where
sizeof(long) = 4). Even if it seems very unlikely, if this ever
happens, the loop will become infinite. Would you agree with adding
"&& grbuflen > 0" to the condition of the while statement?

Anyway, if you do not agree, this patch looks good to me.
Acked-by: Nicolas Iooss <nicolas.iooss@m4x.org>

Thanks,
Nicolas

> +       if (retval == -1)
> +               goto cleanup;
>
>         if (group == NULL) {
>                 ERR(s->h_semanage, "Can't find group named %s\n", grname);
> --
> 2.20.1
>