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Date: Tue, 20 Mar 2012 10:55:43 +0400
From: Cyrill Gorcunov <gorcunov@...nvz.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: richard -rw- weinberger <richard.weinberger@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Oleg Nesterov <oleg@...hat.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Pavel Emelyanov <xemul@...allels.com>,
Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
Matt Helsley <matthltc@...ibm.com>
Subject: Re: [patch 1/2] c/r: prctl: Add ability to set new
mm_struct::exe_file
On Mon, Mar 19, 2012 at 03:46:49PM -0700, Andrew Morton wrote:
> > >>
> > >> What is this mysterious "security reason"?
> > >>
> > >
> > > Oh, sorry I should have included Matt's comment here
>
> Please send a patch with the updated changelog and improved comment?
>
Andrew, take a look please, will the changelog and comments look better?
Cyrill
---
From: Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: c/r: prctl: add ability to set new mm_struct::exe_file
When we do restore we would like to have a way to setup
a former mm_struct::exe_file so that /proc/pid/exe would
point to the original executable file a process had at
checkpoint time.
For this the PR_SET_MM_EXE_FILE code is introduced.
This option takes a file descriptor which will be
set as a source for new /proc/$pid/exe symlink.
Note it allows to change /proc/$pid/exe iif there
are no VM_EXECUTABLE vmas present for current process,
simply because this feature is a special to C/R
and mm::num_exe_file_vmas become meaningless after
that.
To minimize the amount of transition the /proc/pid/exe
symlink might have, this feature is implemented in one-shot
manner. Thus once changed the symlink can't be changed
again. This should help sysadmins to monitor the symlinks
over all process running in a system.
In particular one could make a snapshot of processes and
ring alarm if there unexpected changes of /proc/pid/exe's
in a system.
Note -- this feature is available iif CONFIG_CHECKPOINT_RESTORE
is set and the caller must have CAP_SYS_RESOURCE capability
granted, otherwise the request to change symlink will be
rejected.
Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
Reviewed-by: Oleg Nesterov <oleg@...hat.com>
CC: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC: Pavel Emelyanov <xemul@...allels.com>
CC: Kees Cook <keescook@...omium.org>
CC: Tejun Heo <tj@...nel.org>
CC: Matt Helsley <matthltc@...ibm.com>
CC: richard -rw- weinberger <richard.weinberger@...il.com>
---
include/linux/prctl.h | 1
kernel/sys.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 57 insertions(+)
Index: linux-2.6.git/include/linux/prctl.h
===================================================================
--- linux-2.6.git.orig/include/linux/prctl.h
+++ linux-2.6.git/include/linux/prctl.h
@@ -118,5 +118,6 @@
# define PR_SET_MM_ENV_START 10
# define PR_SET_MM_ENV_END 11
# define PR_SET_MM_AUXV 12
+# define PR_SET_MM_EXE_FILE 13
#endif /* _LINUX_PRCTL_H */
Index: linux-2.6.git/kernel/sys.c
===================================================================
--- linux-2.6.git.orig/kernel/sys.c
+++ linux-2.6.git/kernel/sys.c
@@ -36,6 +36,8 @@
#include <linux/personality.h>
#include <linux/ptrace.h>
#include <linux/fs_struct.h>
+#include <linux/file.h>
+#include <linux/mount.h>
#include <linux/gfp.h>
#include <linux/syscore_ops.h>
#include <linux/version.h>
@@ -1701,6 +1703,57 @@ static bool vma_flags_mismatch(struct vm
(vma->vm_flags & banned);
}
+static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
+{
+ struct file *exe_file;
+ struct dentry *dentry;
+ int err;
+
+ /*
+ * Setting new mm::exe_file is only allowed when no VM_EXECUTABLE vma's
+ * remain. So perform a quick test first.
+ */
+ if (mm->num_exe_file_vmas)
+ return -EBUSY;
+
+ exe_file = fget(fd);
+ if (!exe_file)
+ return -EBADF;
+
+ dentry = exe_file->f_path.dentry;
+
+ /*
+ * Because the original mm->exe_file points to executable file, make
+ * sure that this one is executable as well, to avoid breaking an
+ * overall picture.
+ */
+ err = -EACCES;
+ if (!S_ISREG(dentry->d_inode->i_mode) ||
+ exe_file->f_path.mnt->mnt_flags & MNT_NOEXEC)
+ goto exit;
+
+ err = inode_permission(dentry->d_inode, MAY_EXEC);
+ if (err)
+ goto exit;
+
+ /*
+ * The symlink can be changed only once, just to disallow arbitrary
+ * transitions malicious software might bring in. This means one
+ * could make a snapshot over all processes running and monitor
+ * /proc/pid/exe changes to notice unusual activity if needed.
+ */
+ down_write(&mm->mmap_sem);
+ if (likely(!mm->exe_file))
+ set_mm_exe_file(mm, exe_file);
+ else
+ err = -EBUSY;
+ up_write(&mm->mmap_sem);
+
+exit:
+ fput(exe_file);
+ return err;
+}
+
static int prctl_set_mm(int opt, unsigned long addr,
unsigned long arg4, unsigned long arg5)
{
@@ -1715,6 +1768,9 @@ static int prctl_set_mm(int opt, unsigne
if (!capable(CAP_SYS_RESOURCE))
return -EPERM;
+ if (opt == PR_SET_MM_EXE_FILE)
+ return prctl_set_mm_exe_file(mm, (unsigned int)addr);
+
if (addr >= TASK_SIZE)
return -EINVAL;
--
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