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@google.com wrote:
On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google) joel@joelfernandes.org 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@google.com Cc: john.stultz@linaro.org Cc: tkjos@google.com Cc: gregkh@linuxfoundation.org Cc: hch@infradead.org Reviewed-by: John Stultz john.stultz@linaro.org Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org
[...]
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@joelfernandes.org 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@google.com Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org --- 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