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[D/E,2/2] vfs: fix do_last() regression

Message ID 20200304124918.16389-5-cascardo@canonical.com
State New
Headers show
Series [X/B,1/2] do_last(): fetch directory ->i_mode and ->i_uid before it's too late | expand

Commit Message

Thadeu Lima de Souza Cascardo March 4, 2020, 12:49 p.m. UTC 
From: Al Viro <viro@zeniv.linux.org.uk>

CVE-2020-8428

Brown paperbag time: fetching ->i_uid/->i_mode really should've been
done from nd->inode.  I even suggested that, but the reason for that has
slipped through the cracks and I went for dir->d_inode instead - made
for more "obvious" patch.

Analysis:

 - at the entry into do_last() and all the way to step_into(): dir (aka
   nd->path.dentry) is known not to have been freed; so's nd->inode and
   it's equal to dir->d_inode unless we are already doomed to -ECHILD.
   inode of the file to get opened is not known.

 - after step_into(): inode of the file to get opened is known; dir
   might be pointing to freed memory/be negative/etc.

 - at the call of may_create_in_sticky(): guaranteed to be out of RCU
   mode; inode of the file to get opened is known and pinned; dir might
   be garbage.

The last was the reason for the original patch.  Except that at the
do_last() entry we can be in RCU mode and it is possible that
nd->path.dentry->d_inode has already changed under us.

In that case we are going to fail with -ECHILD, but we need to be
careful; nd->inode is pointing to valid struct inode and it's the same
as nd->path.dentry->d_inode in "won't fail with -ECHILD" case, so we
should use that.

Reported-by: "Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@nokia.com>
Reported-by: syzbot+190005201ced78a74ad6@syzkaller.appspotmail.com
Wearing-brown-paperbag: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org
Fixes: d0cb50185ae9 ("do_last(): fetch directory ->i_mode and ->i_uid before it's too late")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 6404674acd596de41fd3ad5f267b4525494a891a)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
 fs/namei.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
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Patch

diff --git a/fs/namei.c b/fs/namei.c
index 0bd1f2de6aa0..6a9b8aae2401 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3257,8 +3257,8 @@  static int do_last(struct nameidata *nd,
 		   struct file *file, const struct open_flags *op)
 {
 	struct dentry *dir = nd->path.dentry;
-	kuid_t dir_uid = dir->d_inode->i_uid;
-	umode_t dir_mode = dir->d_inode->i_mode;
+	kuid_t dir_uid = nd->inode->i_uid;
+	umode_t dir_mode = nd->inode->i_mode;
 	int open_flag = op->open_flag;
 	bool will_truncate = (open_flag & O_TRUNC) != 0;
 	bool got_write = false;