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

[David Hildenbrand <david@redhat.com>]

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@mail.gmail.com
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   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