Re: [PATCH] proc: Track /proc/$pid/attr/ opener mm_struct

[Kees Cook <keescook@chromium.org>]

On Mon, Jun 14, 2021 at 08:32:35AM -0700, Kees Cook wrote:
> On Mon, Jun 14, 2021 at 06:02:34PM +0800, youling257 wrote:
> > I used mainline kernel on android, this patch cause "failed to retrieve pid context" problem.
> > 
> > 06-14 02:15:51.165  1685  1685 E ServiceManager: SELinux: getpidcon(pid=1682) failed to retrieve pid context.

I found getpidcon() in libselinux:
https://github.com/SELinuxProject/selinux/blob/master/libselinux/src/procattr.c#L159

> > 06-14 02:15:51.166  1685  1685 E ServiceManager: add_service('batteryproperties',1) uid=0 - PERMISSION DENIED
> > 06-14 02:15:51.166  1682  1682 I ServiceManager: addService() batteryproperties failed (err -1 - no service manager yet?).  Retrying...
> > 06-14 02:15:51.197  1685  1685 E ServiceManager: SELinux: getpidcon(pid=1695) failed to retrieve pid context.
> > 06-14 02:15:51.197  1685  1685 E ServiceManager: add_service('android.security.keystore',1) uid=1017 - PERMISSION DENIED
> > 06-14 02:15:51.198  1695  1695 I ServiceManager: addService() android.security.keystore failed (err -1 - no service manager yet?).  Retrying...
> > 06-14 02:15:51.207  1685  1685 E ServiceManager: SELinux: getpidcon(pid=1708) failed to retrieve pid context.
> > 06-14 02:15:51.207  1685  1685 E ServiceManager: add_service('android.service.gatekeeper.IGateKeeperService',1) uid=1000 - PERMISSION DENIED
> > 06-14 02:15:51.207  1708  1708 I ServiceManager: addService() android.service.gatekeeper.IGateKeeperService failed (err -1 - no service manager yet?).  Retrying...
> > 06-14 02:15:51.275  1685  1685 E ServiceManager: SELinux: getpidcon(pid=1693) failed to retrieve pid context.
> > 06-14 02:15:51.275  1692  1692 I cameraserver: ServiceManager: 0xf6d309e0
> > 06-14 02:15:51.275  1685  1685 E ServiceManager: add_service('drm.drmManager',1) uid=1019 - PERMISSION DENIED
> > 06-14 02:15:51.276  1693  1693 I ServiceManager: addService() drm.drmManager failed (err -1 - no service manager yet?).  Retrying...
> > 
> 
> Argh. Are you able to uncover what userspace is doing here?

It looks like this is a case of attempting to _read_ the attr file, and
the new opener check was requiring the opener/target relationship pass
the mm_access() checks, which is clearly too strict.

> So far, my test cases are:
> 
> 1) self: open, write, close: allowed
> 2) self: open, clone thread. thread: change privileges, write, close: allowed
> 3) self: open, give to privileged process. privileged process: write: reject

I've now added:

4) self: open privileged process's attr, read, close: allowed

Can folks please test this patch to double-check?


diff --git a/fs/proc/base.c b/fs/proc/base.c
index 7118ebe38fa6..7c55301674e0 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2676,7 +2676,14 @@ static int proc_pident_readdir(struct file *file, struct dir_context *ctx,
 #ifdef CONFIG_SECURITY
 static int proc_pid_attr_open(struct inode *inode, struct file *file)
 {
-	return __mem_open(inode, file, PTRACE_MODE_READ_FSCREDS);
+	struct mm_struct *mm = __mem_open(inode, file, PTRACE_MODE_READ_FSCREDS);
+
+	/* Reads do not require mm_struct access. */
+	if (IS_ERR(mm))
+		mm = NULL;
+
+	file->private_data = mm;
+	return 0;
 }
 
 static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,
@@ -2709,7 +2716,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	int rv;
 
 	/* A task may only write when it was the opener. */
-	if (file->private_data != current->mm)
+	if (!file->private_data || file->private_data != current->mm)
 		return -EPERM;
 
 	rcu_read_lock();


Wheee.

-- 
Kees Cook