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Message-Id: <20220211021324.4116773-3-ebiederm@xmission.com>
Date: Thu, 10 Feb 2022 20:13:19 -0600
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: linux-kernel@...r.kernel.org
Cc: Alexey Gladkov <legion@...nel.org>,
Kees Cook <keescook@...omium.org>,
Shuah Khan <shuah@...nel.org>,
Christian Brauner <brauner@...nel.org>,
Solar Designer <solar@...nwall.com>,
Ran Xiaokai <ran.xiaokai@....com.cn>,
containers@...ts.linux-foundation.org,
Michal Koutný <mkoutny@...e.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>, stable@...r.kernel.org
Subject: [PATCH 3/8] ucounts: Fix and simplify RLIMIT_NPROC handling during setuid()+execve
Michal Koutný <mkoutny@...e.com> wrote:
> The check is currently against the current->cred but since those are
> going to change and we want to check RLIMIT_NPROC condition after the
> switch, supply the capability check with the new cred.
> But since we're checking new_user being INIT_USER any new cred's
> capability-based allowance may be redundant when the check fails and the
> alternative solution would be revert of the commit 2863643fb8b9
> ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds")
As of commit 2863643fb8b9 ("set_user: add capability check when
rlimit(RLIMIT_NPROC) exceeds") setting the flag to see if execve
should check RLIMIT_NPROC is buggy, as it tests the capabilites from
before the credential change and not aftwards.
As of commit 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of
ucounts") examining the rlimit is buggy as cred->ucounts has not yet
been properly set in the new credential.
Make the code correct and more robust moving the test to see if
execve() needs to test RLIMIT_NPROC into commit_creds, and defer all
of the rest of the logic into execve() itself.
As the flag only indicateds that RLIMIT_NPROC should be checked
in execve rename it from PF_NPROC_EXCEEDED to PF_NPROC_CHECK.
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/20220207121800.5079-2-mkoutny@suse.com
Link: https://lkml.kernel.org/r/20220207121800.5079-3-mkoutny@suse.com
Reported-by: Michal Koutný <mkoutny@...e.com>
Fixes: 2863643fb8b9 ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds")
Fixes: 21d1c5e386bc ("Reimplement RLIMIT_NPROC on top of ucounts")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
fs/exec.c | 15 ++++++++-------
include/linux/sched.h | 2 +-
kernel/cred.c | 13 +++++++++----
kernel/fork.c | 2 +-
kernel/sys.c | 14 --------------
5 files changed, 19 insertions(+), 27 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..1e7f757cbc2c 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1875,20 +1875,21 @@ static int do_execveat_common(int fd, struct filename *filename,
return PTR_ERR(filename);
/*
- * We move the actual failure in case of RLIMIT_NPROC excess from
- * set*uid() to execve() because too many poorly written programs
- * don't check setuid() return code. Here we additionally recheck
- * whether NPROC limit is still exceeded.
+ * After calling set*uid() is RLIMT_NPROC exceeded?
+ * This can not be checked in set*uid() because too many programs don't
+ * check the setuid() return code.
*/
- if ((current->flags & PF_NPROC_EXCEEDED) &&
- is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
+ if ((current->flags & PF_NPROC_CHECK) &&
+ is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
+ (current_user() != INIT_USER) &&
+ !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
retval = -EAGAIN;
goto out_ret;
}
/* We're below the limit (still or again), so we don't want to make
* further execve() calls fail. */
- current->flags &= ~PF_NPROC_EXCEEDED;
+ current->flags &= ~PF_NPROC_CHECK;
bprm = alloc_bprm(fd, filename);
if (IS_ERR(bprm)) {
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 75ba8aa60248..6605a262a6be 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1678,7 +1678,7 @@ extern struct pid *cad_pid;
#define PF_DUMPCORE 0x00000200 /* Dumped core */
#define PF_SIGNALED 0x00000400 /* Killed by a signal */
#define PF_MEMALLOC 0x00000800 /* Allocating memory */
-#define PF_NPROC_EXCEEDED 0x00001000 /* set_user() noticed that RLIMIT_NPROC was exceeded */
+#define PF_NPROC_CHECK 0x00001000 /* Check in execve if RLIMIT_NPROC was exceeded */
#define PF_USED_MATH 0x00002000 /* If unset the fpu must be initialized before use */
#define PF_NOFREEZE 0x00008000 /* This thread should not be frozen */
#define PF_FROZEN 0x00010000 /* Frozen for system suspend */
diff --git a/kernel/cred.c b/kernel/cred.c
index 933155c96922..229cff081167 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -490,13 +490,18 @@ int commit_creds(struct cred *new)
if (!gid_eq(new->fsgid, old->fsgid))
key_fsgid_changed(new);
- /* do it
- * RLIMIT_NPROC limits on user->processes have already been checked
- * in set_user().
+ /*
+ * Remember if the NPROC limit may be exceeded. The set*uid() functions
+ * can not fail if the NPROC limit is exceeded as too many programs
+ * don't check the return code. Instead enforce the NPROC limit for
+ * programs doing set*uid()+execve by harmlessly defering the failure
+ * to the execve() stage.
*/
alter_cred_subscribers(new, 2);
- if (new->user != old->user || new->user_ns != old->user_ns)
+ if (new->user != old->user || new->user_ns != old->user_ns) {
inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+ task->flags |= PF_NPROC_CHECK;
+ }
rcu_assign_pointer(task->real_cred, new);
rcu_assign_pointer(task->cred, new);
if (new->user != old->user || new->user_ns != old->user_ns)
diff --git a/kernel/fork.c b/kernel/fork.c
index 17d8a8c85e3b..2b6a28a86325 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2031,7 +2031,7 @@ static __latent_entropy struct task_struct *copy_process(
!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
goto bad_fork_cleanup_count;
}
- current->flags &= ~PF_NPROC_EXCEEDED;
+ current->flags &= ~PF_NPROC_CHECK;
/*
* If multiple threads are within copy_process(), then this check
diff --git a/kernel/sys.c b/kernel/sys.c
index ecc4cf019242..b1ed21d79f3b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -472,20 +472,6 @@ static int set_user(struct cred *new)
if (!new_user)
return -EAGAIN;
- /*
- * We don't fail in case of NPROC limit excess here because too many
- * poorly written programs don't check set*uid() return code, assuming
- * it never fails if called by root. We may still enforce NPROC limit
- * for programs doing set*uid()+execve() by harmlessly deferring the
- * failure to the execve() stage.
- */
- if (is_ucounts_overlimit(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) &&
- new_user != INIT_USER &&
- !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
- current->flags |= PF_NPROC_EXCEEDED;
- else
- current->flags &= ~PF_NPROC_EXCEEDED;
-
free_uid(new->user);
new->user = new_user;
return 0;
--
2.29.2
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