On 03/20/2014 09:38 AM, tytso@xxxxxxx wrote: > On Thu, Mar 20, 2014 at 04:48:30PM +0100, David Herrmann wrote: >> On Thu, Mar 20, 2014 at 4:32 PM, <tytso@xxxxxxx> wrote: >>> Why not make sealing an attribute of the "struct file", and enforce it >>> at the VFS layer? That way all file system objects would have access >>> to sealing interface, and for memfd_shmem, you can't get another >>> struct file pointing at the object, the security properties would be >>> identical. >> >> Sealing as introduced here is an inode-attribute, not "struct file". >> This is intentional. For instance, a gfx-client can get a read-only FD >> via /proc/self/fd/ and pass it to the compositor so it can never >> overwrite the contents (unless the compositor has write-access to the >> inode itself, in which case it can just re-open it read-write). > > Hmm, good point. I had forgotten about the /proc/self/fd hole. > Hmm... what if we have a SEAL_PROC which forces the permissions of > /proc/self/fd to be 000? This is the second time in a week that someone has asked for a way to have a struct file (or struct inode or whatever) that can't be reopened through /proc/pid/fd. This should be quite easy to implement as a separate feature. Actually, that feature would solve a major pet peeve of mine, I think: I want something like memfd that allows me to keep the thing read-write but that whomever I pass the fd to can't change. With this feature, I could do: fd_rw = memfd_create (or O_TMPFILE or whatever) fd_ro = open(/proc/self/fd/fd_ro, O_RDONLY); fcntl(fd_ro, F_RESTRICT, F_RESTRICT_REOPEN); send fd_ro via SCM_RIGHTS. To really make this work well, I also want to SEAL_SHRINK the inode so that the receiver can verify that I'm not going to truncate the file out from under it. Bingo, fast and secure one-way IPC. --Andy -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>