On Thu, 9 Oct 2008, Matthew Wilcox wrote: > On Thu, Oct 09, 2008 at 02:23:19PM +0200, Pavel Machek wrote: > > On Tue 2008-10-07 20:52:09, Matthew Wilcox wrote: > > > And it's worth saying that letter-of-the-standard arguments aren't > > > necessarily enough. Linux does not honour the POSIX guarantee that > > > writes are atomic (if they cross page boundaries, it's not certain). > > > This seems like even more of a corner case to me. > > > > We have append-only files, and normal users should not be able to work > > around that restriction. > > Is it possible to work around this restriction by exploiting this? > > IS_APPEND() forces the user to have O_APPEND in their flags. > O_APPEND is only checked in generic_write_checks() where it sets '*pos' > to i_size. > > For the majority of filesystems, generic_write_checks() is called from > __generic_file_aio_write_nolock. __generic_file_aio_write_nolock is > only called from generic_file_aio_write_nolock (which passes the address > of a kiocb->ki_pos) and generic_file_aio_write (same). > > The filesystems that call generic_write_checks() directly are: > XFS (xfs_write): Passes the address of a local variable > OCFS2 (ocfs2_file_aio_write): Passes the address of a ki_pos > CIFS (cifs_user_write): Not sure. > NFS (nfs_file_direct_write): "Note that O_APPEND is not supported". > NTFS (ntfs_file_aio_write_nolock): Address of a local variable > FUSE (fuse_file_aio_write): Address of a local variable > FUSE (fuse_direct_write): Not sure. > > So the only two that might be affected are CIFS and FUSE (O_DIRECT?!) as > far as I can tell. I'm having a hard time believing this is a security > problem. And even in those cases it's actually a local variable, since sys_write() does: loff_t pos = file_pos_read(file); ret = vfs_write(file, buf, count, &pos); file_pos_write(file, pos); So there's no way to corrupt the starting position for an append mode write, as that always comes from the file size. Two append writes to the same file could corrupt f_pos, but that would only matter for subsequent reads or non-append mode writes. Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
