[3.16.y-ckt extended stable] Patch "fs: allow open(dir, O_TMPFILE|..., 0) with mode 0" has been added to staging queue

[Luis Henriques]

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

    fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt1.

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

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 84584307387dd9bf9a08eb345e96196f29a4994b Mon Sep 17 00:00:00 2001
From: Eric Rannaud <e at nanocritical.com>
Date: Thu, 30 Oct 2014 01:51:01 -0700
Subject: fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

commit 69a91c237ab0ebe4e9fdeaf6d0090c85275594ec upstream.

The man page for open(2) indicates that when O_CREAT is specified, the
'mode' argument applies only to future accesses to the file:

	Note that this mode applies only to future accesses of the newly
	created file; the open() call that creates a read-only file
	may well return a read/write file descriptor.

The man page for open(2) implies that 'mode' is treated identically by
O_CREAT and O_TMPFILE.

O_TMPFILE, however, behaves differently:

	int fd = open("/tmp", O_TMPFILE | O_RDWR, 0);
	assert(fd == -1);
	assert(errno == EACCES);

	int fd = open("/tmp", O_TMPFILE | O_RDWR, 0600);
	assert(fd > 0);

For O_CREAT, do_last() sets acc_mode to MAY_OPEN only:

	if (*opened & FILE_CREATED) {
		/* Don't check for write permission, don't truncate */
		open_flag &= ~O_TRUNC;
		will_truncate = false;
		acc_mode = MAY_OPEN;
		path_to_nameidata(path, nd);
		goto finish_open_created;
	}

But for O_TMPFILE, do_tmpfile() passes the full op->acc_mode to
may_open().

This patch lines up the behavior of O_TMPFILE with O_CREAT. After the
inode is created, may_open() is called with acc_mode = MAY_OPEN, in
do_tmpfile().

A different, but related glibc bug revealed the discrepancy:
https://sourceware.org/bugzilla/show_bug.cgi?id=17523

The glibc lazily loads the 'mode' argument of open() and openat() using
va_arg() only if O_CREAT is present in 'flags' (to support both the 2
argument and the 3 argument forms of open; same idea for openat()).
However, the glibc ignores the 'mode' argument if O_TMPFILE is in
'flags'.

On x86_64, for open(), it magically works anyway, as 'mode' is in
RDX when entering open(), and is still in RDX on SYSCALL, which is where
the kernel looks for the 3rd argument of a syscall.

But openat() is not quite so lucky: 'mode' is in RCX when entering the
glibc wrapper for openat(), while the kernel looks for the 4th argument
of a syscall in R10. Indeed, the syscall calling convention differs from
the regular calling convention in this respect on x86_64. So the kernel
sees mode = 0 when trying to use glibc openat() with O_TMPFILE, and
fails with EACCES.

Signed-off-by: Eric Rannaud <e at nanocritical.com>
Acked-by: Andy Lutomirski <luto at amacapital.net>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/namei.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 8a3bdeae730c..0b79fe47502a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3151,7 +3151,8 @@ static int do_tmpfile(int dfd, struct filename *pathname,
 	if (error)
 		goto out2;
 	audit_inode(pathname, nd->path.dentry, 0);
-	error = may_open(&nd->path, op->acc_mode, op->open_flag);
+	/* Don't check for other permissions, the inode was just created */
+	error = may_open(&nd->path, MAY_OPEN, op->open_flag);
 	if (error)
 		goto out2;
 	file->f_path.mnt = nd->path.mnt;
--
2.1.0