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SubjectRe: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3
On Tue, Mar 13, 2012 at 10:26:25AM +0400, Cyrill Gorcunov wrote:
> > I am quite aware of the c/r use for this prctl :). However I also
> > wonder if there aren't serious malicious uses of it. I'm not saying the
> > symlink has to be perfectly accurate at all times, but it's easy and
> > reasonable to make it much harder to abuse this particular prctl for
> > malicious purposes by making it one-shot.
> >
>
> ok, convinced, I'll update the patch ;)

Matt, Oleg, there is a final version I hope,
which should fit everyone.

Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.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.

Also this action is one-shot only. For secutiry reason
we don't allow to chanage the symlink several times.

This feature is available iif CONFIG_CHECKPOINT_RESTORE is set.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Kees Cook <keescook@chromium.org>
CC: Tejun Heo <tj@kernel.org>
CC: Matt Helsley <matthltc@us.ibm.com>
---
include/linux/prctl.h | 1
kernel/sys.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 60 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 +1703,60 @@ 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 left. This is
+ * a special C/R case when a restored program
+ * need to change own /proc/$pid/exe symlink.
+ * After this call mm::num_exe_file_vmas become
+ * meaningless.
+ */
+ 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
+ * this one is executable as well to not
+ * break 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;
+
+ /*
+ * For security reason changing mm->exe_file
+ * is one-shot action.
+ */
+ 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 +1771,9 @@ 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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