mm: Hold a file reference in madvise_remove

Otherwise the code races with munmap (causing a use-after-free
of the vma) or with close (causing a use-after-free of the struct
file).

The bug was introduced by commit 90ed52ebe481 ("[PATCH] holepunch: fix
mmap_sem i_mutex deadlock")

Cc: Hugh Dickins <hugh@veritas.com>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/madvise.c b/mm/madvise.c
index deff1b64a08c36ef4857590e9913717488953014..14d260fa0d17939a2279c244df91789cd30720e4 100644 (file)
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -15,6 +15,7 @@
 #include <linux/sched.h>
 #include <linux/ksm.h>
 #include <linux/fs.h>
+#include <linux/file.h>
 
 /*
  * Any behaviour which results in changes to the vma->vm_flags needs to
@@ -204,14 +205,16 @@ static long madvise_remove(struct vm_area_struct *vma,
 {
        loff_t offset;
        int error;
+       struct file *f;
 
        *prev = NULL;   /* tell sys_madvise we drop mmap_sem */
 
        if (vma->vm_flags & (VM_LOCKED|VM_NONLINEAR|VM_HUGETLB))
                return -EINVAL;
 
-       if (!vma->vm_file || !vma->vm_file->f_mapping
-               || !vma->vm_file->f_mapping->host) {
+       f = vma->vm_file;
+
+       if (!f || !f->f_mapping || !f->f_mapping->host) {
                        return -EINVAL;
        }
 
@@ -221,11 +224,18 @@ static long madvise_remove(struct vm_area_struct *vma,
        offset = (loff_t)(start - vma->vm_start)
                        + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
 
-       /* filesystem's fallocate may need to take i_mutex */
+       /*
+        * Filesystem's fallocate may need to take i_mutex.  We need to
+        * explicitly grab a reference because the vma (and hence the
+        * vma's reference to the file) can go away as soon as we drop
+        * mmap_sem.
+        */
+       get_file(f);
        up_read(&current->mm->mmap_sem);
-       error = do_fallocate(vma->vm_file,
+       error = do_fallocate(f,
                                FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                                offset, end - start);
+       fput(f);
        down_read(&current->mm->mmap_sem);
        return error;
 }