From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754219AbaKOPij (ORCPT ); Sat, 15 Nov 2014 10:38:39 -0500 Received: from out03.mta.xmission.com ([166.70.13.233]:46796 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754049AbaKOPig (ORCPT ); Sat, 15 Nov 2014 10:38:36 -0500 From: ebiederm@xmission.com (Eric W. Biederman) To: Josh Triplett Cc: Andrew Morton , Kees Cook , mtk.manpages@gmail.com, linux-api@vger.kernel.org, linux-man@vger.kernel.org, linux-kernel@vger.kernel.org References: <3ccec8a13019b5e8ce7b1d7889677b778b070dc8.1416041823.git.josh@joshtriplett.org> <0895c1f268bc0b01cc6c8ed4607d7c3953f49728.1416041823.git.josh@joshtriplett.org> Date: Sat, 15 Nov 2014 09:37:27 -0600 In-Reply-To: <0895c1f268bc0b01cc6c8ed4607d7c3953f49728.1416041823.git.josh@joshtriplett.org> (Josh Triplett's message of "Sat, 15 Nov 2014 01:01:02 -0800") Message-ID: <87d28osceg.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-AID: U2FsdGVkX19Voq+N6L8xmV2k+EhoD9Mjl0l61+xdUSk= X-SA-Exim-Connect-IP: 97.121.92.161 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 0.7 XMSubLong Long Subject * 1.5 XMNoVowels Alpha-numberic number with no vowels * 0.0 TVD_RCVD_IP Message was received from an IP address * 0.0 T_TM2_M_HEADER_IN_MSG BODY: No description available. * 0.8 BAYES_50 BODY: Bayes spam probability is 40 to 60% * [score: 0.4999] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa06 1397; Body=1 Fuz1=1 Fuz2=1] * 1.0 T_XMDrugObfuBody_08 obfuscated drug references X-Spam-DCC: XMission; sa06 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: ***;Josh Triplett X-Spam-Relay-Country: X-Spam-Timing: total 971 ms - load_scoreonly_sql: 0.11 (0.0%), signal_user_changed: 6 (0.6%), b_tie_ro: 3.5 (0.4%), parse: 1.22 (0.1%), extract_message_metadata: 20 (2.1%), get_uri_detail_list: 3.8 (0.4%), tests_pri_-1000: 6 (0.6%), tests_pri_-950: 1.52 (0.2%), tests_pri_-900: 1.30 (0.1%), tests_pri_-400: 30 (3.1%), check_bayes: 28 (2.9%), b_tokenize: 9 (0.9%), b_tok_get_all: 9 (1.0%), b_comp_prob: 4.0 (0.4%), b_tok_touch_all: 3.0 (0.3%), b_finish: 1.01 (0.1%), tests_pri_0: 894 (92.1%), tests_pri_500: 5 (0.6%), rewrite_mail: 0.00 (0.0%) Subject: Re: [PATCH 2/2] groups: Allow unprivileged processes to use setgroups to drop groups X-Spam-Flag: No X-SA-Exim-Version: 4.2.1 (built Wed, 24 Sep 2014 11:00:52 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@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;