[4.2.y-ckt stable] Patch "NFSv4: Don't perform cached access checks before we've OPENed the file" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    NFSv4: Don't perform cached access checks before we've OPENed the file

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 9ddd9c61f24f08c3ab0e5554d9564f9944852a72 Mon Sep 17 00:00:00 2001
From: Trond Myklebust <trond.myklebust at primarydata.com>
Date: Sat, 26 Dec 2015 21:54:58 -0500
Subject: NFSv4: Don't perform cached access checks before we've OPENed the
 file

commit 762674f86d0328d5dc923c966e209e1ee59663f2 upstream.

Donald Buczek reports that a nfs4 client incorrectly denies
execute access based on outdated file mode (missing 'x' bit).
After the mode on the server is 'fixed' (chmod +x) further execution
attempts continue to fail, because the nfs ACCESS call updates
the access parameter but not the mode parameter or the mode in
the inode.

The root cause is ultimately that the VFS is calling may_open()
before the NFS client has a chance to OPEN the file and hence revalidate
the access and attribute caches.

Al Viro suggests:
>>> Make nfs_permission() relax the checks when it sees MAY_OPEN, if you know
>>> that things will be caught by server anyway?
>>
>> That can work as long as we're guaranteed that everything that calls
>> inode_permission() with MAY_OPEN on a regular file will also follow up
>> with a vfs_open() or dentry_open() on success. Is this always the
>> case?
>
> 1) in do_tmpfile(), followed by do_dentry_open() (not reachable by NFS since
> it doesn't have ->tmpfile() instance anyway)
>
> 2) in atomic_open(), after the call of ->atomic_open() has succeeded.
>
> 3) in do_last(), followed on success by vfs_open()
>
> That's all.  All calls of inode_permission() that get MAY_OPEN come from
> may_open(), and there's no other callers of that puppy.

Reported-by: Donald Buczek <buczek at molgen.mpg.de>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=109771
Link: http://lkml.kernel.org/r/1451046656-26319-1-git-send-email-buczek@molgen.mpg.de
Cc: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Trond Myklebust <trond.myklebust at primarydata.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/nfs/dir.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 547308a..40d0dc6 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -2460,6 +2460,9 @@ int nfs_permission(struct inode *inode, int mask)
 		case S_IFLNK:
 			goto out;
 		case S_IFREG:
+			if ((mask & MAY_OPEN) &&
+			   nfs_server_capable(inode, NFS_CAP_ATOMIC_OPEN))
+				return 0;
 			break;
 		case S_IFDIR:
 			/*
--
1.9.1