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; +} + 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
