linux-kernel - [PATCH bpf-next] libbpf: Set MFD_NOEXEC

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Message-ID: <6bf30e1a22d867af9145aa5e94c3fd9281a1c98d.1735508627.git.dxu@dxuuu.xyz>
Date: Sun, 29 Dec 2024 14:44:33 -0700
From: Daniel Xu <dxu@...uu.xyz>
To: ast@...nel.org,
	andrii@...nel.org,
	daniel@...earbox.net,
	eddyz87@...il.com
Cc: martin.lau@...ux.dev,
	song@...nel.org,
	yonghong.song@...ux.dev,
	john.fastabend@...il.com,
	kpsingh@...nel.org,
	sdf@...ichev.me,
	haoluo@...gle.com,
	jolsa@...nel.org,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next] libbpf: Set MFD_NOEXEC_SEAL when creating memfd

Since 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC"), the
kernel has started printing a warning if neither MFD_NOEXEC_SEAL nor
MFD_EXEC is set in memfd_create().

To avoid this warning (and also be more secure), set MFD_NOEXEC_SEAL by
default. But since libbpf can be running on potentially very old
kernels, leave a fallback for kernels without MFD_NOEXEC_SEAL support.

Signed-off-by: Daniel Xu <dxu@...uu.xyz>
---
 tools/lib/bpf/libbpf.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 66173ddb5a2d..46492cc0927d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1731,12 +1731,24 @@ static int sys_memfd_create(const char *name, unsigned flags)
 #ifndef MFD_CLOEXEC
 #define MFD_CLOEXEC 0x0001U
 #endif
+#ifndef MFD_NOEXEC_SEAL
+#define MFD_NOEXEC_SEAL 0x0008U
+#endif
 
 static int create_placeholder_fd(void)
 {
+	unsigned int flags = MFD_CLOEXEC | MFD_NOEXEC_SEAL;
+	const char *name = "libbpf-placeholder-fd";
 	int fd;
 
-	fd = ensure_good_fd(sys_memfd_create("libbpf-placeholder-fd", MFD_CLOEXEC));
+	fd = ensure_good_fd(sys_memfd_create(name, flags));
+	if (fd >= 0)
+		return fd;
+	else if (errno != EINVAL)
+		return -errno;
+
+	/* Possibly running on kernel without MFD_NOEXEC_SEAL */
+	fd = ensure_good_fd(sys_memfd_create(name, flags & ~MFD_NOEXEC_SEAL));
 	if (fd < 0)
 		return -errno;
 	return fd;
-- 
2.47.1