From ba46dc96adbb2d79bf91bbd7a30cde6279acea48 Mon Sep 17 00:00:00 2001 From: Eric Rannaud Date: Thu, 30 Oct 2014 01:51:01 -0700 Subject: [PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0 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. Change-Id: If41388897576291d3cfb0515fd71efb70bf2b3df Signed-off-by: Eric Rannaud Acked-by: Andy Lutomirski Cc: stable@vger.kernel.org Signed-off-by: Linus Torvalds --- fs/namei.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/fs/namei.c b/fs/namei.c index ac3e12c60b8..d60833ea60b 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -3036,7 +3036,8 @@ 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; -- 2.20.1