[v3,1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
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Message ID 20181018065908.254389-1-joel@joelfernandes.org
State New
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Series
  • [v3,1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
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Commit Message

Joel Fernandes Oct. 18, 2018, 6:59 a.m. UTC
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@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>
---
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(-)

Comments

Valdis Kl ē tnieks Oct. 19, 2018, 5:32 p.m. UTC | #1
On Wed, 17 Oct 2018 23:59:07 -0700, "Joel Fernandes (Google)" said:
> 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:

What is supposed to happen if some other process has an already existing R/W
mmap of the region?  (For that matter, the test program doesn't seem to
actually test that the existing mmap region remains writable?)
Joel Fernandes Oct. 19, 2018, 5:57 p.m. UTC | #2
On Fri, Oct 19, 2018 at 10:32 AM,  <valdis.kletnieks@vt.edu> wrote:
> On Wed, 17 Oct 2018 23:59:07 -0700, "Joel Fernandes (Google)" said:
>> 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:
>
> What is supposed to happen if some other process has an already existing R/W
> mmap of the region?  (For that matter, the test program doesn't seem to
> actually test that the existing mmap region remains writable?)
>

Why would it not remain writable? We don't change anything in the
mapping that prevents it from being writable, in the patch.

We do test that existing writable mmaps can continue to exist after
the seal is set, in a way, because we test that setting of the seal
succeeds.

I could test that processor stores can continue to happen my doing a
memset into the existing map, but I feel that is like testing 2+2 = 4,
in a way ;-) Do you really think its worth testing? If you do, then I
could add a test for that.

- Joel
Valdis Kl ē tnieks Oct. 19, 2018, 6:49 p.m. UTC | #3
On Fri, 19 Oct 2018 10:57:31 -0700, Joel Fernandes said:
> On Fri, Oct 19, 2018 at 10:32 AM,  <valdis.kletnieks@vt.edu> wrote:
> > What is supposed to happen if some other process has an already existing R/W
> > mmap of the region?  (For that matter, the test program doesn't seem to
> > actually test that the existing mmap region remains writable?)

> Why would it not remain writable? We don't change anything in the
> mapping that prevents it from being writable, in the patch.

OK, if the meaning here is "if another process races and gets its own R/W mmap
before we seal our mmap, it's OK".  Seems like somewhat shaky security-wise - a
possibly malicious process can fail to get a R/W map because we just sealed it,
but if it had done the attempt a few milliseconds earlier it would have its own
R/W mmap to do as it pleases...

On the other hand, decades of trying have proven that trying to do any sort
of revoke() is a lot harder to do than it looks...

> We do test that existing writable mmaps can continue to exist after
> the seal is set, in a way, because we test that setting of the seal
> succeeds.

Well, if the semantics are "We don't bother trying to deal with existing R/W
maps", then it doesn't really matter - I was thinking along the lines of "If we're
revoking other R/W accesses, we should test that we didn't nuke *this* one in
the bargain"....
Joel Fernandes Oct. 19, 2018, 7:32 p.m. UTC | #4
On Fri, Oct 19, 2018 at 02:49:11PM -0400, valdis.kletnieks@vt.edu wrote:
> On Fri, 19 Oct 2018 10:57:31 -0700, Joel Fernandes said:
> > On Fri, Oct 19, 2018 at 10:32 AM,  <valdis.kletnieks@vt.edu> wrote:
> > > What is supposed to happen if some other process has an already existing R/W
> > > mmap of the region?  (For that matter, the test program doesn't seem to
> > > actually test that the existing mmap region remains writable?)
> 
> > Why would it not remain writable? We don't change anything in the
> > mapping that prevents it from being writable, in the patch.
> 
> OK, if the meaning here is "if another process races and gets its own R/W mmap
> before we seal our mmap, it's OK".  Seems like somewhat shaky security-wise - a
> possibly malicious process can fail to get a R/W map because we just sealed it,
> but if it had done the attempt a few milliseconds earlier it would have its own
> R/W mmap to do as it pleases...
> 
> On the other hand, decades of trying have proven that trying to do any sort
> of revoke() is a lot harder to do than it looks...
> 

No it is not a security issue. The issue you bring up can happen even with
the existing F_SEAL_WRITE where someone else races to mmap it.

And if someone else could race and do an mmap on the memfd, then they somehow
goes the fd at which point that is a security issue anyway. That is the whole
point of memfd, that it can be securely sent over IPC to another process.
Also, before sending it to the receiving/racing process, the memfd would have
already been sealed with the F_SEAL_FUTURE_WRITE so there is no question of a
race on the receiving side.

- Joel
Joel Fernandes Oct. 22, 2018, 11:49 p.m. UTC | #5
On Wed, Oct 17, 2018 at 11:59:07PM -0700, 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 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@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>

Apologies for the follow-up. Now that merge window has opened, just checking
if this patch (which IMO has been beaten to death) can make it for 4.20?  Its
pretty much completed and is well tested at this point (tests are in 2/2).
Then I can move onto other memfd enhancements I'm planning.

thanks,

 - Joel

Patch
diff mbox series

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;