c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal

A fix for commit b32dfe377102 ("c/r: prctl: add ability to set new
mm_struct::exe_file").

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>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 20 insertions, 12 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6029d8c54476..c688d4cc2e40 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -439,6 +439,7 @@ 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 9ff89cb9657a..54f20fdee93c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1796,17 +1796,11 @@ 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;
@@ -1827,17 +1821,30 @@ 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: