[ 046/104] sysfs: fix use after free in case of concurrent read/write and readdir

Greg Kroah-Hartman
2013-05-06T23:31:20Z

From: Ming Lei <ming.lei@canonical.com>

commit f7db5e7660b122142410dcf36ba903c73d473250 upstream.

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.

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>

---
fs/sysfs/dir.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -1012,6 +1012,7 @@ static int sysfs_readdir(struct file * f
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];
@@ -1034,6 +1035,7 @@ static int sysfs_readdir(struct file * f
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)) {
@@ -1045,19 +1047,24 @@ static int sysfs_readdir(struct file * f
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;
}