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Date: Tue, 9 Aug 2011 12:16:32 +1000
From: NeilBrown <neilb@...e.de>
To: Vasiliy Kulikov <segoon@...nwall.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
"David S. Miller" <davem@...emloft.net>,
Jiri Slaby <jslaby@...e.cz>, James Morris <jmorris@...ei.org>,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v3 -resend] move RLIMIT_NPROC check from set_user() to
do_execve_common()
On Mon, 8 Aug 2011 19:02:04 +0400 Vasiliy Kulikov <segoon@...nwall.com> wrote:
> The patch http://lkml.org/lkml/2003/7/13/226 introduced an RLIMIT_NPROC
> check in set_user() to check for NPROC exceeding via setuid() and
> similar functions. Before the check there was a possibility to greatly
> exceed the allowed number of processes by an unprivileged user if the
> program relied on rlimit only. But the check created new security
> threat: many poorly written programs simply don't check setuid() return
> code and believe it cannot fail if executed with root privileges. So,
> the check is removed in this patch because of too often privilege
> escalations related to buggy programs.
>
> The NPROC can still be enforced in the common code flow of daemons
> spawning user processes. Most of daemons do fork()+setuid()+execve().
> The check introduced in execve() (1) enforces the same limit as in
> setuid() and (2) doesn't create similar security issues.
>
> Neil Brown suggested to track what specific process has exceeded the
> limit by setting PF_NPROC_EXCEEDED process flag. With the change only
> this process would fail on execve(), and other processes' execve()
> behaviour is not changed.
>
> Solar Designer suggested to re-check whether NPROC limit is still
> exceeded at the moment of execve(). If the process was sleeping for
> days between set*uid() and execve(), and the NPROC counter step down
> under the limit, the defered execve() failure because NPROC limit was
> exceeded days ago would be unexpected. If the limit is not exceeded
> anymore, we clear the flag on successful calls to execve() and fork().
>
> The flag is also cleared on successful calls to set_user() as the limit
> was exceeded for the previous user, not the current one.
>
> Similar check was introduced in -ow patches (without the process flag).
>
> v3 - clear PF_NPROC_EXCEEDED on successful calls to set_user().
>
> Reviewed-by: James Morris <jmorris@...ei.org>
> Signed-off-by: Vasiliy Kulikov <segoon@...nwall.com>
Acked-by: NeilBrown <neilb@...e.de>
I'm not 100% happy with this for reasons that have been mentioned, but there
is a real problem, and this is a real fix, and I think it is as good as we
are likely to be able to achieve.
Thanks for persisting.
NeilBrown
> ---
> fs/exec.c | 17 +++++++++++++++++
> include/linux/sched.h | 1 +
> kernel/cred.c | 6 ++----
> kernel/fork.c | 1 +
> kernel/sys.c | 15 +++++++++++----
> 5 files changed, 32 insertions(+), 8 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index da80612..25dcbe5 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1459,6 +1459,23 @@ static int do_execve_common(const char *filename,
> struct files_struct *displaced;
> bool clear_in_exec;
> int retval;
> + const struct cred *cred = current_cred();
> +
> + /*
> + * 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.
> + */
> + if ((current->flags & PF_NPROC_EXCEEDED) &&
> + atomic_read(&cred->user->processes) > rlimit(RLIMIT_NPROC)) {
> + 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;
>
> retval = unshare_files(&displaced);
> if (retval)
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 20b03bf..4ac2c05 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1767,6 +1767,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
> #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_USED_MATH 0x00002000 /* if unset the fpu must be initialized before use */
> #define PF_FREEZING 0x00004000 /* freeze in progress. do not account to load */
> #define PF_NOFREEZE 0x00008000 /* this thread should not be frozen */
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 174fa84..8ef31f5 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -508,10 +508,8 @@ int commit_creds(struct cred *new)
> key_fsgid_changed(task);
>
> /* do it
> - * - What if a process setreuid()'s and this brings the
> - * new uid over his NPROC rlimit? We can check this now
> - * cheaply with the new uid cache, so if it matters
> - * we should be checking for it. -DaveM
> + * RLIMIT_NPROC limits on user->processes have already been checked
> + * in set_user().
> */
> alter_cred_subscribers(new, 2);
> if (new->user != old->user)
> diff --git a/kernel/fork.c b/kernel/fork.c
> index e7ceaca..8e6b6f4 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1111,6 +1111,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> p->real_cred->user != INIT_USER)
> goto bad_fork_free;
> }
> + current->flags &= ~PF_NPROC_EXCEEDED;
>
> retval = copy_creds(p, clone_flags);
> if (retval < 0)
> diff --git a/kernel/sys.c b/kernel/sys.c
> index a101ba3..dd948a1 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -621,11 +621,18 @@ 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 (atomic_read(&new_user->processes) >= rlimit(RLIMIT_NPROC) &&
> - new_user != INIT_USER) {
> - free_uid(new_user);
> - return -EAGAIN;
> - }
> + new_user != INIT_USER)
> + current->flags |= PF_NPROC_EXCEEDED;
> + else
> + current->flags &= ~PF_NPROC_EXCEEDED;
>
> free_uid(new->user);
> new->user = new_user;
--
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