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    SubjectRe: [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@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.

    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@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>
    CC: richard -rw- weinberger <richard.weinberger@gmail.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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