Re: [RESEND PATCH] vfs: add RWF_NOAPPEND flag for pwritev2

On Sun, Aug 30, 2020 at 05:05:45PM +0200, Jann Horn wrote:

On Sat, Aug 29, 2020 at 4:00 AM Rich Felker [off-list ref] wrote:

quoted

The pwrite function, originally defined by POSIX (thus the "p"), is
defined to ignore O_APPEND and write at the offset passed as its
argument. However, historically Linux honored O_APPEND if set and
ignored the offset. This cannot be changed due to stability policy,
but is documented in the man page as a bug.

Now that there's a pwritev2 syscall providing a superset of the pwrite
functionality that has a flags argument, the conforming behavior can
be offered to userspace via a new flag.

[...]

quoted

diff --git a/include/linux/fs.h b/include/linux/fs.h

[...]

quoted

@@ -3411,6 +3413,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
                ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
        if (flags & RWF_APPEND)
                ki->ki_flags |= IOCB_APPEND;
+       if (flags & RWF_NOAPPEND)
+               ki->ki_flags &= ~IOCB_APPEND;
        return 0;
 }

Linux enforces the S_APPEND flag (set by "chattr +a") only at open()
time, not at write() time:

# touch testfile
# exec 100>testfile
# echo foo > testfile
# cat testfile
foo
# chattr +a testfile
# echo bar > testfile
bash: testfile: Operation not permitted
# echo bar >&100
# cat testfile
bar
#

At open() time, the kernel enforces that you can't use O_WRONLY/O_RDWR
without also setting O_APPEND if the file is marked as append-only:

static int may_open(const struct path *path, int acc_mode, int flag)
{
[...]
  /*
   * An append-only file must be opened in append mode for writing.
   */
  if (IS_APPEND(inode)) {
    if  ((flag & O_ACCMODE) != O_RDONLY && !(flag & O_APPEND))
      return -EPERM;
    if (flag & O_TRUNC)
      return -EPERM;
  }
[...]
}

It seems to me like your patch will permit bypassing S_APPEND by
opening an append-only file with O_WRONLY|O_APPEND, then calling
pwritev2() with RWF_NOAPPEND? I think you'll have to add an extra
check for IS_APPEND() somewhere.


One could also argue that if an O_APPEND file descriptor is handed
across privilege boundaries, a programmer might reasonably expect that
the recipient will not be able to use the file descriptor for
non-append writes; if that is not actually true, that should probably
be noted in the open.2 manpage, at the end of the description of
O_APPEND.

fcntl F_SETFL can remove O_APPEND, so it is not a security boundary.
I'm not sure how this interacts with S_APPEND; presumably fcntl
rechecks it. So just checking IS_APPEND in the code paths used by
pwritev2 (and erroring out rather than silently writing output at the
wrong place) should suffice to preserve all existing security
invariants.

Rich