Re: [tip:core/locking] futex: Do not leak robust list to unprivileged process

[Wanlong Gao <gaowanlong@cn.fujitsu.com>]

On 03/29/2012 05:55 PM, tip-bot for Kees Cook wrote:
> Commit-ID:  bdbb776f882f5ad431aa1e694c69c1c3d6a4a5b8
> Gitweb:     http://git.kernel.org/tip/bdbb776f882f5ad431aa1e694c69c1c3d6a4a5b8
> Author:     Kees Cook <keescook@chromium.org>
> AuthorDate: Mon, 19 Mar 2012 16:12:53 -0700
> Committer:  Thomas Gleixner <tglx@linutronix.de>
> CommitDate: Thu, 29 Mar 2012 11:37:17 +0200
> 
> futex: Do not leak robust list to unprivileged process
> 
> It was possible to extract the robust list head address from a setuid
> process if it had used set_robust_list(), allowing an ASLR info leak. This
> changes the permission checks to be the same as those used for similar
> info that comes out of /proc.
> 
> Running a setuid program that uses robust futexes would have had:
>   cred->euid != pcred->euid
>   cred->euid == pcred->uid
> so the old permissions check would allow it. I'm not aware of any setuid
> programs that use robust futexes, so this is just a preventative measure.
> 

I'm not sure this change prevents the unprivileged process.
Please refer to LTP test, recently I saw that this change broke
the following test.

https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/get_robust_list/get_robust_list01.c#L155
		if (seteuid(1) == -1)
			tst_brkm(TBROK|TERRNO, cleanup, "seteuid(1) failed");

		TEST(retval = syscall(__NR_get_robust_list, 1,
				      (struct robust_list_head *)&head,
				      &len_ptr));

We set the euid to an unprivileged user, and expect to FAIL with EPERM,
without this patch, it FAIL as we expected, but with it, this call succeed.

Seems that we leaked the check of (cred->euid == pcred->euid && cred->euid == pcred->uid),
I'm not sure which one is right, can you please give an explanation?


Thanks in advance,
Wanlong Gao

> (This patch is based on changes from grsecurity.)
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Cc: Darren Hart <dvhart@linux.intel.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Jiri Kosina <jkosina@suse.cz>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: David Howells <dhowells@redhat.com>
> Cc: Serge E. Hallyn <serge.hallyn@canonical.com>
> Cc: kernel-hardening@lists.openwall.com
> Cc: spender@grsecurity.net
> Link: http://lkml.kernel.org/r/20120319231253.GA20893@www.outflux.net
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  kernel/futex.c        |   36 +++++++++++++-----------------------
>  kernel/futex_compat.c |   36 +++++++++++++-----------------------
>  2 files changed, 26 insertions(+), 46 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 72efa1e..d701be5 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -59,6 +59,7 @@
>  #include <linux/magic.h>
>  #include <linux/pid.h>
>  #include <linux/nsproxy.h>
> +#include <linux/ptrace.h>
>  
>  #include <asm/futex.h>
>  
> @@ -2443,40 +2444,29 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
>  {
>  	struct robust_list_head __user *head;
>  	unsigned long ret;
> -	const struct cred *cred = current_cred(), *pcred;
> +	struct task_struct *p;
>  
>  	if (!futex_cmpxchg_enabled)
>  		return -ENOSYS;
>  
> +	rcu_read_lock();
> +
> +	ret = -ESRCH;
>  	if (!pid)
> -		head = current->robust_list;
> +		p = current;
>  	else {
> -		struct task_struct *p;
> -
> -		ret = -ESRCH;
> -		rcu_read_lock();
>  		p = find_task_by_vpid(pid);
>  		if (!p)
>  			goto err_unlock;
> -		ret = -EPERM;
> -		pcred = __task_cred(p);
> -		/* If victim is in different user_ns, then uids are not
> -		   comparable, so we must have CAP_SYS_PTRACE */
> -		if (cred->user->user_ns != pcred->user->user_ns) {
> -			if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
> -				goto err_unlock;
> -			goto ok;
> -		}
> -		/* If victim is in same user_ns, then uids are comparable */
> -		if (cred->euid != pcred->euid &&
> -		    cred->euid != pcred->uid &&
> -		    !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
> -			goto err_unlock;
> -ok:
> -		head = p->robust_list;
> -		rcu_read_unlock();
>  	}
>  
> +	ret = -EPERM;
> +	if (!ptrace_may_access(p, PTRACE_MODE_READ))
> +		goto err_unlock;
> +
> +	head = p->robust_list;
> +	rcu_read_unlock();
> +
>  	if (put_user(sizeof(*head), len_ptr))
>  		return -EFAULT;
>  	return put_user(head, head_ptr);
> diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
> index 5f9e689..a9642d5 100644
> --- a/kernel/futex_compat.c
> +++ b/kernel/futex_compat.c
> @@ -10,6 +10,7 @@
>  #include <linux/compat.h>
>  #include <linux/nsproxy.h>
>  #include <linux/futex.h>
> +#include <linux/ptrace.h>
>  
>  #include <asm/uaccess.h>
>  
> @@ -136,40 +137,29 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr,
>  {
>  	struct compat_robust_list_head __user *head;
>  	unsigned long ret;
> -	const struct cred *cred = current_cred(), *pcred;
> +	struct task_struct *p;
>  
>  	if (!futex_cmpxchg_enabled)
>  		return -ENOSYS;
>  
> +	rcu_read_lock();
> +
> +	ret = -ESRCH;
>  	if (!pid)
> -		head = current->compat_robust_list;
> +		p = current;
>  	else {
> -		struct task_struct *p;
> -
> -		ret = -ESRCH;
> -		rcu_read_lock();
>  		p = find_task_by_vpid(pid);
>  		if (!p)
>  			goto err_unlock;
> -		ret = -EPERM;
> -		pcred = __task_cred(p);
> -		/* If victim is in different user_ns, then uids are not
> -		   comparable, so we must have CAP_SYS_PTRACE */
> -		if (cred->user->user_ns != pcred->user->user_ns) {
> -			if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
> -				goto err_unlock;
> -			goto ok;
> -		}
> -		/* If victim is in same user_ns, then uids are comparable */
> -		if (cred->euid != pcred->euid &&
> -		    cred->euid != pcred->uid &&
> -		    !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE))
> -			goto err_unlock;
> -ok:
> -		head = p->compat_robust_list;
> -		rcu_read_unlock();
>  	}
>  
> +	ret = -EPERM;
> +	if (!ptrace_may_access(p, PTRACE_MODE_READ))
> +		goto err_unlock;
> +
> +	head = p->compat_robust_list;
> +	rcu_read_unlock();
> +
>  	if (put_user(sizeof(*head), len_ptr))
>  		return -EFAULT;
>  	return put_user(ptr_to_compat(head), head_ptr);
> --
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