Re: [parch 4/4] vfs: utimensat(): fix write access check for futimens()

[Miklos Szeredi]

> The POSIX.1 draft spec for futimens()/utimensat() says:
> 
>         Only a process with the effective user ID equal to the
>         user ID of the file, *or with write access to the file*,
>         or with appropriate privileges may use futimens() or
>         utimensat() with a null pointer as the times argument
>         or with both tv_nsec fields set to the special value
>         UTIME_NOW.
> 
> The important piece here is "with write access to the file", and
> this matters for futimens(), which deals with an argument that
> is a file descriptor referring to the file whose timestamps are
> being updated,  The standard is saying that the "writability"
> check is based on the file permissions, not the access mode with
> which the file is opened.  (This behavior is consistent with the
> semantics of FreeBSD's futimes().)  However, Linux is currently
> doing the latter -- futimens(fd, times) is a library
> function implemented as
> 
>        utimensat(fd, NULL, times, 0)
> 
> and within the utimensat() implementation we have the code:
> 
>                 f = fget(dfd);  // dfd is 'fd'
>                 ...
>                 if (f) {
>                         if (!(f->f_mode & FMODE_WRITE))
>                                 goto mnt_drop_write_and_out;
> 
> The check should instead be based on the file permissions.
> 
> Thanks to Miklos for pointing out how to do this check.
> 
> CC: Miklos Szeredi <miklos@xxxxxxxxxx>
> CC: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> CC: Ulrich Drepper <drepper@xxxxxxxxxx>
> Signed-off-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> 
> --- linux-2.6.26-rc4/fs/utimes.c	2008-06-03 23:13:31.000000000 +0200
> +++ linux-2.6.26-rc4-utimensat-fix-v4/fs/utimes.c	2008-06-03 23:15:12.000000000 +0200
> @@ -137,7 +137,8 @@
> 
>  		if (!is_owner_or_cap(inode)) {
>  			if (f) {
> -				if (!(f->f_mode & FMODE_WRITE))
> +				error = permission(inode, MAY_WRITE, NULL);
> +				if (error)
>  					goto mnt_drop_write_and_out;
>  			} else {
>  				error = vfs_permission(&nd, MAY_WRITE);

At which point the "if (f)" and the "else" branches become equivalent
(the nameidata isn't interesting in the other case either).  So that
could be written as:

		if (!is_owner_or_cap(inode)) {
			error = permission(inode, MAY_WRITE, NULL);
			if (error)
				goto mnt_drop_write_and_out;
		}

Miklos
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