From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f197.google.com (mail-pg1-f197.google.com [209.85.215.197]) by kanga.kvack.org (Postfix) with ESMTP id A2FCF6B0777 for ; Sat, 10 Nov 2018 01:05:20 -0500 (EST) Received: by mail-pg1-f197.google.com with SMTP id h10so199032pgv.20 for ; Fri, 09 Nov 2018 22:05:20 -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 t62-v6sor11961661pfk.5.2018.11.09.22.05.18 for (Google Transport Security); Fri, 09 Nov 2018 22:05:18 -0800 (PST) Content-Type: text/plain; charset=utf-8 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: <20181110032005.GA22238@google.com> Date: Fri, 9 Nov 2018 22:05:14 -0800 Content-Transfer-Encoding: quoted-printable Message-Id: <69CE06CC-E47C-4992-848A-66EB23EE6C74@amacapital.net> References: <20181108041537.39694-1-joel@joelfernandes.org> <20181110032005.GA22238@google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Joel Fernandes Cc: Jann Horn , 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 7:20 PM, Joel Fernandes wrote:= >=20 >> On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote: >>> On Fri, Nov 9, 2018 at 10:06 PM Jann Horn wrote: >>> 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 driver= s >>>> 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, unsi= gned int seals) >>>> } >>>> } >>>>=20 >>>> + if ((seals & F_SEAL_FUTURE_WRITE) && >>>> + !(*file_seals & F_SEAL_FUTURE_WRITE)) { >>>> + /* >>>> + * The FUTURE_WRITE seal also prevents growing and shri= nking >>>> + * 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. >>=20 >> My favorite approach would be to forbid open() on memfds, hope that >> nobody notices the tiny API break, and then add an ioctl for "reopen >> this memfd with reduced permissions" - but that's just my personal >> opinion. >=20 > I did something along these lines and it fixes the issue, but I forbid ope= n > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its n= ot > an ABI break because this is a brand new seal. That seems the least intrus= ive > solution and it works. Do you mind testing it and I'll add your and Tested= -by > to the new fix? The patch is based on top of this series. >=20 > ---8<----------- > From: "Joel Fernandes (Google)" > Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed m= emfd >=20 > Jann Horn found that reopening an F_SEAL_FUTURE_WRITE sealed memfd > through /proc/self/fd/N symlink as writeable succeeds. The simplest fix > without causing ABI breakages and ugly VFS hacks is to simply deny all > opens on F_SEAL_FUTURE_WRITE sealed fds. >=20 > Reported-by: Jann Horn > Signed-off-by: Joel Fernandes (Google) > --- > mm/shmem.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) >=20 > diff --git a/mm/shmem.c b/mm/shmem.c > index 446942677cd4..5b378c486b8f 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -3611,7 +3611,25 @@ static const struct address_space_operations shmem_= aops =3D { > .error_remove_page =3D generic_error_remove_page, > }; >=20 > +/* Could arrive here for memfds opened through /proc/ */ > +int shmem_open(struct inode *inode, struct file *file) > +{ > + struct shmem_inode_info *info =3D SHMEM_I(inode); > + > + /* > + * memfds for which future writes have been prevented > + * should not be reopened, say, through /proc/pid/fd/N > + * symlinks otherwise it can cause a sealed memfd to be > + * promoted to writable. > + */ > + if (info->seals & F_SEAL_FUTURE_WRITE) > + return -EACCES; > + > + return 0; > +} The result of this series is very warty. We have a concept of seals, and the= y all work similarly, except the new future write seal. That one: - causes a probably-observable effect in the file mode in F_GETFL. - causes reopen to fail. - does *not* affect other struct files that may already exist on the same in= ode. - mysteriously malfunctions if you try to set it again on another struct fil= e that already exists - probably is insecure when used on hugetlbfs. I see two reasonable solutions: 1. Don=E2=80=99t fiddle with the struct file at all. Instead make the inode f= lag work by itself. 2. Don=E2=80=99t call it a =E2=80=9Cseal=E2=80=9D. Instead fix the /proc ho= le and add an API to drop write access on an existing struct file. I personally prefer #2. > + > static const struct file_operations shmem_file_operations =3D { > + .open =3D shmem_open, > .mmap =3D shmem_mmap, > .get_unmapped_area =3D shmem_get_unmapped_area, > #ifdef CONFIG_TMPFS > --=20 > 2.19.1.930.g4563a0d9d0-goog >=20