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    Subject[PATCH mm RESEND] c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal
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    [ fix for "c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-v2" from mm tree ]

    After removing mm->num_exe_file_vmas kernel keeps mm->exe_file until final mmput(),
    it never becomes NULL while task is alive.

    We can check for other mapped files in mm instead of checking mm->num_exe_file_vmas,
    and mark mm with flag MMF_EXE_FILE_CHANGED in order to forbid second changing of mm->exe_file.

    Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
    Cc: Cyrill Gorcunov <gorcunov@openvz.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Matt Helsley <matthltc@us.ibm.com>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Pavel Emelyanov <xemul@parallels.com>
    ---
    include/linux/sched.h | 1 +
    kernel/sys.c | 31 +++++++++++++++++++------------
    2 files changed, 20 insertions(+), 12 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 81a173c..ac61e51 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -437,6 +437,7 @@ extern int get_dumpable(struct mm_struct *mm);
    /* leave room for more dump flags */
    #define MMF_VM_MERGEABLE 16 /* KSM may merge identical pages */
    #define MMF_VM_HUGEPAGE 17 /* set when VM_HUGEPAGE is set on vma */
    +#define MMF_EXE_FILE_CHANGED 18 /* see prctl_set_mm_exe_file() */

    #define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)

    diff --git a/kernel/sys.c b/kernel/sys.c
    index 089cb11..c6cdef5 100644
    --- a/kernel/sys.c
    +++ b/kernel/sys.c
    @@ -1714,17 +1714,11 @@ static bool vma_flags_mismatch(struct vm_area_struct *vma,

    static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
    {
    + struct vm_area_struct *vma;
    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;
    @@ -1745,17 +1739,30 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
    if (err)
    goto exit;

    + down_write(&mm->mmap_sem);
    +
    + /*
    + * Forbid mm->exe_file change if there are mapped other files.
    + */
    + err = -EBUSY;
    + for (vma = mm->mmap; vma; vma = vma->vm_next) {
    + if (vma->vm_file && !path_equal(&vma->vm_file->f_path,
    + &exe_file->f_path))
    + goto exit_unlock;
    + }
    +
    /*
    * 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;
    + err = -EPERM;
    + if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags))
    + goto exit_unlock;
    +
    + set_mm_exe_file(mm, exe_file);
    +exit_unlock:
    up_write(&mm->mmap_sem);

    exit:


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