[PATCH v2 1/2] memfd: fix MFD_NOEXEC_SEAL to be non-sealable by default

[jeffxu]

From: Jeff Xu <jeffxu@xxxxxxxxxx>

By default, memfd_create() creates a non-sealable MFD, unless the
MFD_ALLOW_SEALING flag is set.

When the MFD_NOEXEC_SEAL flag is initially introduced, the MFD created
with that flag is sealable, even though MFD_ALLOW_SEALING is not set.
This patch changes MFD_NOEXEC_SEAL to be non-sealable by default,
unless MFD_ALLOW_SEALING is explicitly set.

This is a non-backward compatible change. However, as MFD_NOEXEC_SEAL
is new, we expect not many applications will rely on the nature of
MFD_NOEXEC_SEAL being sealable. In most cases, the application already
sets MFD_ALLOW_SEALING if they need a sealable MFD.

Additionally, this enhances the useability of  pid namespace sysctl
vm.memfd_noexec. When vm.memfd_noexec equals 1 or 2, the kernel will
add MFD_NOEXEC_SEAL if mfd_create does not specify MFD_EXEC or
MFD_NOEXEC_SEAL, and the addition of MFD_NOEXEC_SEAL enables the MFD
to be sealable. This means, any application that does not desire this
behavior will be unable to utilize vm.memfd_noexec = 1 or 2 to
migrate/enforce non-executable MFD. This adjustment ensures that
applications can anticipate that the sealable characteristic will
remain unmodified by vm.memfd_noexec.

This patch was initially developed by Barnabás Pőcze, and Barnabás
used Debian Code Search and GitHub to try to find potential breakages
and could only find a single one. Dbus-broker's memfd_create() wrapper
is aware of this implicit `MFD_ALLOW_SEALING` behavior, and tries to
work around it [1]. This workaround will break. Luckily, this only
affects the test suite, it does not affect
the normal operations of dbus-broker. There is a PR with a fix[2]. In
addition, David Rheinsberg also raised similar fix in [3]

[1]: https://github.com/bus1/dbus-broker/blob/9eb0b7e5826fc76cad7b025bc46f267d4a8784cb/src/util/misc.c#L114
[2]: https://github.com/bus1/dbus-broker/pull/366
[3]: https://lore.kernel.org/lkml/20230714114753.170814-1-david@xxxxxxxxxxxx/

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 105ff5339f498a ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC")
Signed-off-by: Barnabás Pőcze <pobrn@xxxxxxxxxxxxxx>
Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxx>
Reviewed-by: David Rheinsberg <david@xxxxxxxxxxxx>
---
 mm/memfd.c                                 |  9 ++++----
 tools/testing/selftests/memfd/memfd_test.c | 26 +++++++++++++++++++++-
 2 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/mm/memfd.c b/mm/memfd.c
index 7d8d3ab3fa37..8b7f6afee21d 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -356,12 +356,11 @@ SYSCALL_DEFINE2(memfd_create,
 
 		inode->i_mode &= ~0111;
 		file_seals = memfd_file_seals_ptr(file);
-		if (file_seals) {
-			*file_seals &= ~F_SEAL_SEAL;
+		if (file_seals)
 			*file_seals |= F_SEAL_EXEC;
-		}
-	} else if (flags & MFD_ALLOW_SEALING) {
-		/* MFD_EXEC and MFD_ALLOW_SEALING are set */
+	}
+
+	if (flags & MFD_ALLOW_SEALING) {
 		file_seals = memfd_file_seals_ptr(file);
 		if (file_seals)
 			*file_seals &= ~F_SEAL_SEAL;
diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index 95af2d78fd31..8579a93d006b 100644
--- a/tools/testing/selftests/memfd/memfd_test.c
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -1151,7 +1151,7 @@ static void test_noexec_seal(void)
 			    mfd_def_size,
 			    MFD_CLOEXEC | MFD_NOEXEC_SEAL);
 	mfd_assert_mode(fd, 0666);
-	mfd_assert_has_seals(fd, F_SEAL_EXEC);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL | F_SEAL_EXEC);
 	mfd_fail_chmod(fd, 0777);
 	close(fd);
 }
@@ -1169,6 +1169,14 @@ static void test_sysctl_sysctl0(void)
 	mfd_assert_has_seals(fd, 0);
 	mfd_assert_chmod(fd, 0644);
 	close(fd);
+
+	fd = mfd_assert_new("kern_memfd_sysctl_0_dfl",
+			    mfd_def_size,
+			    MFD_CLOEXEC);
+	mfd_assert_mode(fd, 0777);
+	mfd_assert_has_seals(fd, F_SEAL_SEAL);
+	mfd_assert_chmod(fd, 0644);
+	close(fd);
 }
 
 static void test_sysctl_set_sysctl0(void)
@@ -1206,6 +1214,14 @@ static void test_sysctl_sysctl1(void)
 	mfd_assert_has_seals(fd, F_SEAL_EXEC);
 	mfd_fail_chmod(fd, 0777);
 	close(fd);
+
+	fd = mfd_assert_new("kern_memfd_sysctl_1_noexec_nosealable",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_NOEXEC_SEAL);
+	mfd_assert_mode(fd, 0666);
+	mfd_assert_has_seals(fd, F_SEAL_EXEC | F_SEAL_SEAL);
+	mfd_fail_chmod(fd, 0777);
+	close(fd);
 }
 
 static void test_sysctl_set_sysctl1(void)
@@ -1238,6 +1254,14 @@ static void test_sysctl_sysctl2(void)
 	mfd_assert_has_seals(fd, F_SEAL_EXEC);
 	mfd_fail_chmod(fd, 0777);
 	close(fd);
+
+	fd = mfd_assert_new("kern_memfd_sysctl_2_noexec_notsealable",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_NOEXEC_SEAL);
+	mfd_assert_mode(fd, 0666);
+	mfd_assert_has_seals(fd, F_SEAL_EXEC | F_SEAL_SEAL);
+	mfd_fail_chmod(fd, 0777);
+	close(fd);
 }
 
 static void test_sysctl_set_sysctl2(void)
-- 
2.45.1.288.g0e0cd299f1-goog