[PATCH v3 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd

[Joel Fernandes (Google)]

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. The following program shows the seal
working in action:

 #include <stdio.h>
 #include <errno.h>
 #include <sys/mman.h>
 #include <linux/memfd.h>
 #include <linux/fcntl.h>
 #include <asm/unistd.h>
 #include <unistd.h>
 #define F_SEAL_FUTURE_WRITE 0x0010
 #define REGION_SIZE (5 * 1024 * 1024)

int memfd_create_region(const char *name, size_t size)
{
    int ret;
    int fd = syscall(__NR_memfd_create, name, MFD_ALLOW_SEALING);
    if (fd < 0) return fd;
    ret = ftruncate(fd, size);
    if (ret < 0) { close(fd); return ret; }
    return fd;
}

int main() {
    int ret, fd;
    void *addr, *addr2, *addr3, *addr1;
    ret = memfd_create_region("test_region", REGION_SIZE);
    printf("ret=%d\n", ret);
    fd = ret;

    // Create map
    addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr == MAP_FAILED)
	    printf("map 0 failed\n");
    else
	    printf("map 0 passed\n");

    if ((ret = write(fd, "test", 4)) != 4)
	    printf("write failed even though no future-write seal "
		   "(ret=%d errno =%d)\n", ret, errno);
    else
	    printf("write passed\n");

    addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr1 == MAP_FAILED)
	    perror("map 1 prot-write failed even though no seal\n");
    else
	    printf("map 1 prot-write passed as expected\n");

    ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE |
				 F_SEAL_GROW |
				 F_SEAL_SHRINK);
    if (ret == -1)
	    printf("fcntl failed, errno: %d\n", errno);
    else
	    printf("future-write seal now active\n");

    if ((ret = write(fd, "test", 4)) != 4)
	    printf("write failed as expected due to future-write seal\n");
    else
	    printf("write passed (unexpected)\n");

    addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if (addr2 == MAP_FAILED)
	    perror("map 2 prot-write failed as expected due to seal\n");
    else
	    printf("map 2 passed\n");

    addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0);
    if (addr3 == MAP_FAILED)
	    perror("map 3 failed\n");
    else
	    printf("map 3 prot-read passed as expected\n");
}

The output of running this program is as follows:
ret=3
map 0 passed
write passed
map 1 prot-write passed as expected
future-write seal now active
write failed as expected due to future-write seal
map 2 prot-write failed as expected due to seal
: Permission denied
map 3 prot-read passed as expected

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>
---
v1->v2: No change, just added selftests to the series. manpages are
ready and I'll submit them once the patches are accepted.

v2->v3: Updated commit message to have more support code (John Stultz)
 	Renamed seal from F_SEAL_FS_WRITE to F_SEAL_FUTURE_WRITE
						(Christoph Hellwig)
	Allow for this seal only if grow/shrink seals are also
	either previous set, or are requested along with this seal.
						(Christoph Hellwig)
	Added locking to synchronize access to file->f_mode.
						(Christoph Hellwig)

 include/uapi/linux/fcntl.h |  1 +
 mm/memfd.c                 | 22 +++++++++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 6448cdd9a350..a2f8658f1c55 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -41,6 +41,7 @@
 #define F_SEAL_SHRINK	0x0002	/* prevent file from shrinking */
 #define F_SEAL_GROW	0x0004	/* prevent file from growing */
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
+#define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/memfd.c b/mm/memfd.c
index 2bb5e257080e..5ba9804e9515 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
 #define F_ALL_SEALS (F_SEAL_SEAL | \
 		     F_SEAL_SHRINK | \
 		     F_SEAL_GROW | \
-		     F_SEAL_WRITE)
+		     F_SEAL_WRITE | \
+		     F_SEAL_FUTURE_WRITE)
 
 static int memfd_add_seals(struct file *file, unsigned int seals)
 {
@@ -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);
+	}
+
 	*file_seals |= seals;
 	error = 0;
 
-- 
2.19.1.331.ge82ca0e54c-goog