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Biederman" , Linus Torvalds , =?UTF-8?q?Michal=20Koutn=C3=BD?= , Tejun Heo , Ovidiu Panait Subject: [PATCH 5.10 162/171] cgroup: Use open-time credentials for process migraton perm checks Date: Tue, 12 Apr 2022 08:30:53 +0200 Message-Id: <20220412062932.586417436@linuxfoundation.org> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220412062927.870347203@linuxfoundation.org> References: <20220412062927.870347203@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Tejun Heo commit 1756d7994ad85c2479af6ae5a9750b92324685af upstream. cgroup process migration permission checks are performed at write time as whether a given operation is allowed or not is dependent on the content of the write - the PID. This currently uses current's credentials which is a potential security weakness as it may allow scenarios where a less privileged process tricks a more privileged one into writing into a fd that it created. This patch makes both cgroup2 and cgroup1 process migration interfaces to use the credentials saved at the time of open (file->f_cred) instead of current's. Reported-by: "Eric W. Biederman" Suggested-by: Linus Torvalds Fixes: 187fe84067bd ("cgroup: require write perm on common ancestor when moving processes on the default hierarchy") Reviewed-by: Michal Koutný Signed-off-by: Tejun Heo [OP: apply original __cgroup_procs_write() changes to cgroup_threads_write() and cgroup_procs_write(), as the refactoring commit da70862efe006 ("cgroup: cgroup.{procs,threads} factor out common parts") is not present in 5.10-stable] Signed-off-by: Ovidiu Panait Signed-off-by: Greg Kroah-Hartman --- kernel/cgroup/cgroup-v1.c | 7 ++++--- kernel/cgroup/cgroup.c | 17 ++++++++++++++++- 2 files changed, 20 insertions(+), 4 deletions(-) --- a/kernel/cgroup/cgroup-v1.c +++ b/kernel/cgroup/cgroup-v1.c @@ -505,10 +505,11 @@ static ssize_t __cgroup1_procs_write(str goto out_unlock; /* - * Even if we're attaching all tasks in the thread group, we only - * need to check permissions on one of them. + * Even if we're attaching all tasks in the thread group, we only need + * to check permissions on one of them. Check permissions using the + * credentials from file open to protect against inherited fd attacks. */ - cred = current_cred(); + cred = of->file->f_cred; tcred = get_task_cred(task); if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) && !uid_eq(cred->euid, tcred->uid) && --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -4788,6 +4788,7 @@ static ssize_t cgroup_procs_write(struct struct cgroup_file_ctx *ctx = of->priv; struct cgroup *src_cgrp, *dst_cgrp; struct task_struct *task; + const struct cred *saved_cred; ssize_t ret; bool locked; @@ -4805,9 +4806,16 @@ static ssize_t cgroup_procs_write(struct src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root); spin_unlock_irq(&css_set_lock); + /* + * Process and thread migrations follow same delegation rule. Check + * permissions using the credentials from file open to protect against + * inherited fd attacks. + */ + saved_cred = override_creds(of->file->f_cred); ret = cgroup_attach_permissions(src_cgrp, dst_cgrp, of->file->f_path.dentry->d_sb, true, ctx->ns); + revert_creds(saved_cred); if (ret) goto out_finish; @@ -4832,6 +4840,7 @@ static ssize_t cgroup_threads_write(stru struct cgroup_file_ctx *ctx = of->priv; struct cgroup *src_cgrp, *dst_cgrp; struct task_struct *task; + const struct cred *saved_cred; ssize_t ret; bool locked; @@ -4851,10 +4860,16 @@ static ssize_t cgroup_threads_write(stru src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root); spin_unlock_irq(&css_set_lock); - /* thread migrations follow the cgroup.procs delegation rule */ + /* + * Process and thread migrations follow same delegation rule. Check + * permissions using the credentials from file open to protect against + * inherited fd attacks. + */ + saved_cred = override_creds(of->file->f_cred); ret = cgroup_attach_permissions(src_cgrp, dst_cgrp, of->file->f_path.dentry->d_sb, false, ctx->ns); + revert_creds(saved_cred); if (ret) goto out_finish;