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
