sysfs: fix use after free in case of concurrent read/write and readdir

The inode->i_mutex isn't hold when updating filp->f_pos
in read()/write(), so the filp->f_pos might be read as
0 or 1 in readdir() when there is concurrent read()/write()
on this same file, then may cause use after free in readdir().

The bug can be reproduced with Li Zefan's test code on the
link:

	https://patchwork.kernel.org/patch/2160771/

This patch fixes the use after free under this situation.

Cc: stable <stable@vger.kernel.org>
Reported-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index c6f54abe9852..1bf016b5e88f 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -999,6 +999,7 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
         enum kobj_ns_type type;
         const void *ns;
         ino_t ino;
+        loff_t off;
 
         type = sysfs_ns_type(parent_sd);
         ns = sysfs_info(dentry->d_sb)->ns[type];
@@ -1021,6 +1022,7 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
                         return 0;
         }
         mutex_lock(&sysfs_mutex);
+        off = filp->f_pos;
         for (pos = sysfs_dir_pos(ns, parent_sd, filp->f_pos, pos);
              pos;
              pos = sysfs_dir_next_pos(ns, parent_sd, filp->f_pos, pos)) {
@@ -1032,19 +1034,24 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
                 len = strlen(name);
                 ino = pos->s_ino;
                 type = dt_type(pos);
-                filp->f_pos = pos->s_hash;
+                off = filp->f_pos = pos->s_hash;
                 filp->private_data = sysfs_get(pos);
 
                 mutex_unlock(&sysfs_mutex);
-                ret = filldir(dirent, name, len, filp->f_pos, ino, type);
+                ret = filldir(dirent, name, len, off, ino, type);
                 mutex_lock(&sysfs_mutex);
                 if (ret < 0)
                         break;
         }
         mutex_unlock(&sysfs_mutex);
-        if ((filp->f_pos > 1) && !pos) { /* EOF */
-                filp->f_pos = INT_MAX;
+
+        /* don't reference last entry if its refcount is dropped */
+        if (!pos) {
                 filp->private_data = NULL;
+
+                /* EOF and not changed as 0 or 1 in read/write path */
+                if (off == filp->f_pos && off > 1)
+                        filp->f_pos = INT_MAX;
         }
         return 0;
 }