> On Nov 9, 2018, at 7:20 PM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > >> On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote: >>> On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <jannh@xxxxxxxxxx> wrote: >>> On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google) >>> <joel@xxxxxxxxxxxxxxxxx> 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. >>>> >>>> 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 >>>> >>>> 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. >>> >>> 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... >>> >>>> The following program shows the seal >>>> working in action: >>> [...] >>>> Cc: jreck@xxxxxxxxxx >>>> Cc: john.stultz@xxxxxxxxxx >>>> Cc: tkjos@xxxxxxxxxx >>>> Cc: gregkh@xxxxxxxxxxxxxxxxxxx >>>> Cc: hch@xxxxxxxxxxxxx >>>> Reviewed-by: John Stultz <john.stultz@xxxxxxxxxx> >>>> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> >>>> --- >>> [...] >>>> 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, unsigned int seals) >>>> } >>>> } >>>> >>>> + if ((seals & F_SEAL_FUTURE_WRITE) && >>>> + !(*file_seals & F_SEAL_FUTURE_WRITE)) { >>>> + /* >>>> + * The FUTURE_WRITE seal also prevents growing and shrinking >>>> + * so we need them to be already set, or requested now. >>>> + */ >>>> + int test_seals = (seals | *file_seals) & >>>> + (F_SEAL_GROW | F_SEAL_SHRINK); >>>> + >>>> + if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) { >>>> + error = -EINVAL; >>>> + goto unlock; >>>> + } >>>> + >>>> + spin_lock(&file->f_lock); >>>> + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE); >>>> + spin_unlock(&file->f_lock); >>>> + } >>> >>> 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? >>> >>> $ cat memfd.c >>> #define _GNU_SOURCE >>> #include <unistd.h> >>> #include <sys/syscall.h> >>> #include <printf.h> >>> #include <fcntl.h> >>> #include <err.h> >>> #include <stdio.h> >>> >>> int main(void) { >>> int fd = syscall(__NR_memfd_create, "testfd", 0); >>> if (fd == -1) err(1, "memfd"); >>> char path[100]; >>> sprintf(path, "/proc/self/fd/%d", fd); >>> int fd2 = open(path, O_RDWR); >>> if (fd2 == -1) err(1, "reopen"); >>> printf("reopen successful: %d\n", fd2); >>> } >>> $ gcc -o memfd memfd.c >>> $ ./memfd >>> reopen successful: 4 >>> $ >>> >>> 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. >> >> 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. > > I did something along these lines and it fixes the issue, but I forbid open > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its not > an ABI break because this is a brand new seal. That seems the least intrusive > 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. > > ---8<----------- > From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> > Subject: [PATCH] mm/memfd: Fix possible promotion to writeable of sealed memfd > > 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. > > Reported-by: Jann Horn <jannh@xxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > mm/shmem.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > 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 = { > .error_remove_page = generic_error_remove_page, > }; > > +/* Could arrive here for memfds opened through /proc/ */ > +int shmem_open(struct inode *inode, struct file *file) > +{ > + struct shmem_inode_info *info = 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 they 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 inode. - mysteriously malfunctions if you try to set it again on another struct file that already exists - probably is insecure when used on hugetlbfs. I see two reasonable solutions: 1. Don’t fiddle with the struct file at all. Instead make the inode flag work by itself. 2. Don’t call it a “seal”. Instead fix the /proc hole 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 = { > + .open = shmem_open, > .mmap = shmem_mmap, > .get_unmapped_area = shmem_get_unmapped_area, > #ifdef CONFIG_TMPFS > -- > 2.19.1.930.g4563a0d9d0-goog >
