From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f200.google.com (mail-pg1-f200.google.com [209.85.215.200]) by kanga.kvack.org (Postfix) with ESMTP id BFE1A6B0728 for ; Fri, 9 Nov 2018 16:41:00 -0500 (EST) Received: by mail-pg1-f200.google.com with SMTP id a18so2048891pga.16 for ; Fri, 09 Nov 2018 13:41:00 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id cb11-v6sor10740166plb.57.2018.11.09.13.40.58 for (Google Transport Security); Fri, 09 Nov 2018 13:40:59 -0800 (PST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (1.0) Subject: Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd From: Andy Lutomirski In-Reply-To: Date: Fri, 9 Nov 2018 13:40:56 -0800 Content-Transfer-Encoding: quoted-printable Message-Id: References: <20181108041537.39694-1-joel@joelfernandes.org> Sender: owner-linux-mm@kvack.org List-ID: To: Jann Horn Cc: joel@joelfernandes.org, kernel list , jreck@google.com, John Stultz , Todd Kjos , Greg Kroah-Hartman , Christoph Hellwig , Al Viro , Andrew Morton , Daniel Colascione , Bruce Fields , jlayton@kernel.org, Khalid Aziz , Lei.Yang@windriver.com, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, Linux-MM , marcandre.lureau@redhat.com, Mike Kravetz , minchan@kernel.org, shuah@kernel.org, valdis.kletnieks@vt.edu, Hugh Dickins , Linux API > On Nov 9, 2018, at 1:06 PM, Jann Horn wrote: >=20 > +linux-api for API addition > +hughd as FYI since this is somewhat related to mm/shmem >=20 > On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google) > wrote: >> Android uses ashmem for sharing memory regions. We are looking forward >> to migrating all usecases of ashmem to memfd so that we can possibly >> remove the ashmem driver in the future from staging while also >> benefiting from using memfd and contributing to it. Note staging drivers >> are also not ABI and generally can be removed at anytime. >>=20 >> One of the main usecases Android has is the ability to create a region >> and mmap it as writeable, then add protection against making any >> "future" writes while keeping the existing already mmap'ed >> writeable-region active. This allows us to implement a usecase where >> receivers of the shared memory buffer can get a read-only view, while >> the sender continues to write to the buffer. >> See CursorWindow documentation in Android for more details: >> https://developer.android.com/reference/android/database/CursorWindow >>=20 >> This usecase cannot be implemented with the existing F_SEAL_WRITE seal. >> To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal >> which prevents any future mmap and write syscalls from succeeding while >> keeping the existing mmap active. >=20 > Please CC linux-api@ on patches like this. If you had done that, I > might have criticized your v1 patch instead of your v3 patch... >=20 >> The following program shows the seal >> working in action: > [...] >> Cc: jreck@google.com >> Cc: john.stultz@linaro.org >> Cc: tkjos@google.com >> Cc: gregkh@linuxfoundation.org >> Cc: hch@infradead.org >> Reviewed-by: John Stultz >> Signed-off-by: Joel Fernandes (Google) >> --- > [...] >> diff --git a/mm/memfd.c b/mm/memfd.c >> index 2bb5e257080e..5ba9804e9515 100644 >> --- a/mm/memfd.c >> +++ b/mm/memfd.c > [...] >> @@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsign= ed int seals) >> } >> } >>=20 >> + if ((seals & F_SEAL_FUTURE_WRITE) && >> + !(*file_seals & F_SEAL_FUTURE_WRITE)) { >> + /* >> + * The FUTURE_WRITE seal also prevents growing and shrink= ing >> + * so we need them to be already set, or requested now. >> + */ >> + int test_seals =3D (seals | *file_seals) & >> + (F_SEAL_GROW | F_SEAL_SHRINK); >> + >> + if (test_seals !=3D (F_SEAL_GROW | F_SEAL_SHRINK)) { >> + error =3D -EINVAL; >> + goto unlock; >> + } >> + >> + spin_lock(&file->f_lock); >> + file->f_mode &=3D ~(FMODE_WRITE | FMODE_PWRITE); >> + spin_unlock(&file->f_lock); >> + } >=20 > So you're fiddling around with the file, but not the inode? How are > you preventing code like the following from re-opening the file as > writable? >=20 > $ cat memfd.c > #define _GNU_SOURCE > #include > #include > #include > #include > #include > #include >=20 > int main(void) { > int fd =3D syscall(__NR_memfd_create, "testfd", 0); > if (fd =3D=3D -1) err(1, "memfd"); > char path[100]; > sprintf(path, "/proc/self/fd/%d", fd); > int fd2 =3D open(path, O_RDWR); > if (fd2 =3D=3D -1) err(1, "reopen"); > printf("reopen successful: %d\n", fd2); > } > $ gcc -o memfd memfd.c > $ ./memfd > reopen successful: 4 > $ >=20 > That aside: I wonder whether a better API would be something that > allows you to create a new readonly file descriptor, instead of > fiddling with the writability of an existing fd. Every now and then I try to write a patch to prevent using proc to reopen a f= ile with greater permission than the original open. I like your idea to have a clean way to reopen a a memfd with reduced permis= sions. But I would make it a syscall instead and maybe make it only work for= memfd at first. And the proc issue would need to be fixed, too.=