Re: [PATCH v1 2/3] selftests/memfd_secret: add vmsplice() test

[David Hildenbrand]

On 26.03.24 07:17, Mike Rapoport wrote:
> Hi David,
>
> On Mon, Mar 25, 2024 at 02:41:13PM +0100, David Hildenbrand wrote:
> > Let's add a simple reproducer for a scneario where GUP-fast could succeed
> > on secretmem folios, making vmsplice() succeed instead of failing. The
> > reproducer is based on a reproducer [1] by Miklos Szeredi.
> >
> > Perform the ftruncate() only once, and check the return value.
> >
> > For some reason, vmsplice() reliably fails (making the test succeed) when
> > we move the test_vmsplice() call after test_process_vm_read() /
> > test_ptrace().
>
> That's because ftruncate() call was in test_remote_access() and you need it
> to mmap secretmem.

I don't think that's the reason. I reshuffled the code a couple of times
without luck.

And in fact, even executing the vmsplice() test twice results in the
second iteration succeeding on an old kernel (6.7.4-200.fc39.x86_64).

ok 1 mlock limit is respected
ok 2 file IO is blocked as expected
not ok 3 vmsplice is blocked as expected
ok 4 vmsplice is blocked as expected
ok 5 process_vm_read is blocked as expected
ok 6 ptrace is blocked as expected

Note that the mmap()+memset() succeeded. So the secretmem pages should be in the page table.


Even weirder, if I simply mmap()+memset()+munmap() secretmem *once*, the test passes

diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
index 0acbdcf8230e..7a973ec6ac8f 100644
--- a/tools/testing/selftests/mm/memfd_secret.c
+++ b/tools/testing/selftests/mm/memfd_secret.c
@@ -96,6 +96,14 @@ static void test_vmsplice(int fd)
                return;
        }
 
+       mem = mmap(NULL, page_size, prot, mode, fd, 0);
+       if (mem == MAP_FAILED) {
+               fail("Unable to mmap secret memory\n");
+               goto close_pipe;
+       }
+       memset(mem, PATTERN, page_size);
+       munmap(mem, page_size);
+
        mem = mmap(NULL, page_size, prot, mode, fd, 0);
        if (mem == MAP_FAILED) {
                fail("Unable to mmap secret memory\n");

ok 1 mlock limit is respected
ok 2 file IO is blocked as expected
ok 3 vmsplice is blocked as expected
ok 4 process_vm_read is blocked as expected
ok 5 ptrace is blocked as expected


.. could it be that munmap()+mmap() will end up turning these pages into LRU pages?

I am 100% sure that is happening -- likely, because VM_LOCKED is involved,
because on the patched kernel, I see the following:

ok 1 mlock limit is respected
ok 2 file IO is blocked as expected
ok 3 vmsplice is blocked as expected
not ok 4 vmsplice is blocked as expected
ok 5 process_vm_read is blocked as expected
ok 6 ptrace is blocked as expected


At this point, I think we should remove the LRU test for secretmem.

I'll adjust patch #1 and extend this test to cover that case as well.

> > Properly cleaning up in test_remote_access(), which is not
> > part of this change, won't change that behavior. Therefore, run the
> > vmsplice() test for now first -- something is a bit off once we involve
> > fork().
> >
> > [1] https://lkml.kernel.org/r/CAJfpegt3UCsMmxd0taOY11Uaw5U=eS1fE5dn0wZX3HF0oy8-oQ@xxxxxxxxxxxxxx
> >
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   tools/testing/selftests/mm/memfd_secret.c | 44 +++++++++++++++++++++--
> >   1 file changed, 42 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
> > index 9b298f6a04b3..0acbdcf8230e 100644
> > --- a/tools/testing/selftests/mm/memfd_secret.c
> > +++ b/tools/testing/selftests/mm/memfd_secret.c
> > @@ -20,6 +20,7 @@
> >   #include <unistd.h>
> >   #include <errno.h>
> >   #include <stdio.h>
> > +#include <fcntl.h>
> >   
> >   #include "../kselftest.h"
> >   
> > @@ -83,6 +84,43 @@ static void test_mlock_limit(int fd)
> >   	pass("mlock limit is respected\n");
> >   }
> >   
> > +static void test_vmsplice(int fd)
> > +{
> > +	ssize_t transferred;
> > +	struct iovec iov;
> > +	int pipefd[2];
> > +	char *mem;
> > +
> > +	if (pipe(pipefd)) {
> > +		fail("pipe failed: %s\n", strerror(errno));
> > +		return;
> > +	}
> > +
> > +	mem = mmap(NULL, page_size, prot, mode, fd, 0);
> > +	if (mem == MAP_FAILED) {
> > +		fail("Unable to mmap secret memory\n");
> > +		goto close_pipe;
> > +	}
> > +
> > +	/*
> > +	 * vmsplice() may use GUP-fast, which must also fail. Prefault the
> > +	 * page table, so GUP-fast could find it.
> > +	 */
> > +	memset(mem, PATTERN, page_size);
> > +
> > +	iov.iov_base = mem;
> > +	iov.iov_len = page_size;
> > +	transferred = vmsplice(pipefd[1], &iov, 1, 0);
> > +
> > +	ksft_test_result(transferred < 0 && errno == EFAULT,
> > +			 "vmsplice is blocked as expected\n");
>
> The same message will be printed on success and on failure.
>
> I think
>
> 	if (transferred < 0 && errno == EFAULT)
> 		pass("vmsplice is blocked as expected");
> 	else
> 		fail("vmsplice: unexpected memory acccess");
>
> is clearer than feeding different strings to ksft_test_result().

Can do, thanks!

--
Cheers,

David / dhildenb