From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) Subject: Re: [PATCH 2/2] groups: Allow unprivileged processes to use setgroups to drop groups Date: Sat, 15 Nov 2014 09:37:27 -0600 Message-ID: <87d28osceg.fsf@x220.int.ebiederm.org> References: <3ccec8a13019b5e8ce7b1d7889677b778b070dc8.1416041823.git.josh@joshtriplett.org> <0895c1f268bc0b01cc6c8ed4607d7c3953f49728.1416041823.git.josh@joshtriplett.org> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <0895c1f268bc0b01cc6c8ed4607d7c3953f49728.1416041823.git.josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org> (Josh Triplett's message of "Sat, 15 Nov 2014 01:01:02 -0800") Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Josh Triplett Cc: Andrew Morton , Kees Cook , mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org Josh Triplett writes: > Currently, unprivileged processes (without CAP_SETGID) cannot call > setgroups at all. In particular, processes with a set of supplementary > groups cannot further drop permissions without obtaining elevated > permissions first. > > Allow unprivileged processes to call setgroups with a subset of their > current groups; only require CAP_SETGID to add a group the process does > not currently have. A couple of questions. - Is there precedence in other unix flavors for this? - What motiviates this change? - Have you looked to see if anything might for bug compatibilty require applications not to be able to drop supplementary groups? > The kernel already maintains the list of supplementary group IDs in > sorted order, and setgroups already needs to sort the new list, so this > just requires a linear comparison of the two sorted lists. > > This moves the CAP_SETGID test from setgroups into set_current_groups. > > Tested via the following test program: > > #include > #include > #include > #include > #include > > void run_id(void) > { > pid_t p = fork(); > switch (p) { > case -1: > err(1, "fork"); > case 0: > execl("/usr/bin/id", "id", NULL); > err(1, "exec"); > default: > if (waitpid(p, NULL, 0) < 0) > err(1, "waitpid"); > } > } > > int main(void) > { > gid_t list1[] = { 1, 2, 3, 4, 5 }; > gid_t list2[] = { 2, 3, 4 }; > run_id(); > if (setgroups(5, list1) < 0) > err(1, "setgroups 1"); > run_id(); > if (setresgid(1, 1, 1) < 0) > err(1, "setresgid"); > if (setresuid(1, 1, 1) < 0) > err(1, "setresuid"); > run_id(); > if (setgroups(3, list2) < 0) > err(1, "setgroups 2"); > run_id(); > if (setgroups(5, list1) < 0) > err(1, "setgroups 3"); > run_id(); > > return 0; > } > > Without this patch, the test program gets EPERM from the second > setgroups call, after dropping root privileges. With this patch, the > test program successfully drops groups 1 and 5, but then gets EPERM from > the third setgroups call, since that call attempts to add groups the > process does not currently have. > > Signed-off-by: Josh Triplett > --- > kernel/groups.c | 33 ++++++++++++++++++++++++++++++--- > kernel/uid16.c | 2 -- > 2 files changed, 30 insertions(+), 5 deletions(-) > > diff --git a/kernel/groups.c b/kernel/groups.c > index f0667e7..fe7367d 100644 > --- a/kernel/groups.c > +++ b/kernel/groups.c > @@ -153,6 +153,29 @@ int groups_search(const struct group_info *group_info, kgid_t grp) > return 0; > } > > +/* Compare two sorted group lists; return true if the first is a subset of the > + * second. > + */ > +static bool is_subset(const struct group_info *g1, const struct group_info *g2) > +{ > + unsigned int i, j; > + > + for (i = 0, j = 0; i < g1->ngroups; i++) { > + kgid_t gid1 = GROUP_AT(g1, i); > + kgid_t gid2; > + for (; j < g2->ngroups; j++) { > + gid2 = GROUP_AT(g2, j); > + if (gid_lte(gid1, gid2)) > + break; > + } > + if (j >= g2->ngroups || !gid_eq(gid1, gid2)) > + return false; > + j++; > + } > + > + return true; > +} > + > /** > * set_groups_sorted - Change a group subscription in a set of credentials > * @new: The newly prepared set of credentials to alter > @@ -189,11 +212,17 @@ int set_current_groups(struct group_info *group_info) > { > struct cred *new; > > + groups_sort(group_info); > new = prepare_creds(); > if (!new) > return -ENOMEM; > + if (!ns_capable(current_user_ns(), CAP_SETGID) > + && !is_subset(group_info, new->group_info)) { > + abort_creds(new); > + return -EPERM; > + } > > - set_groups(new, group_info); > + set_groups_sorted(new, group_info); > return commit_creds(new); > } > > @@ -233,8 +262,6 @@ SYSCALL_DEFINE2(setgroups, int, gidsetsize, gid_t __user *, grouplist) > struct group_info *group_info; > int retval; > > - if (!ns_capable(current_user_ns(), CAP_SETGID)) > - return -EPERM; > if ((unsigned)gidsetsize > NGROUPS_MAX) > return -EINVAL; > > diff --git a/kernel/uid16.c b/kernel/uid16.c > index 602e5bb..b27e167 100644 > --- a/kernel/uid16.c > +++ b/kernel/uid16.c > @@ -176,8 +176,6 @@ SYSCALL_DEFINE2(setgroups16, int, gidsetsize, old_gid_t __user *, grouplist) > struct group_info *group_info; > int retval; > > - if (!ns_capable(current_user_ns(), CAP_SETGID)) > - return -EPERM; > if ((unsigned)gidsetsize > NGROUPS_MAX) > return -EINVAL;