selftests/bpf: add negative tests for new VFS based BPF kfuncs

Add a bunch of negative selftests responsible for asserting that the
BPF verifier successfully rejects a BPF program load when the
underlying BPF program misuses one of the newly introduced VFS based
BPF kfuncs.

The following VFS based BPF kfuncs are extensively tested within this
new selftest:

* struct file *bpf_get_task_exe_file(struct task_struct *);
* void bpf_put_file(struct file *);
* int bpf_path_d_path(struct path *, char *, size_t);

Acked-by: Christian Brauner <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Signed-off-by: Matt Bobrowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/bpf_experimental.h

···
  */
 extern void bpf_throw(u64 cookie) __ksym;
 
+/* Description
+ *	Acquire a reference on the exe_file member field belonging to the
+ *	mm_struct that is nested within the supplied task_struct. The supplied
+ *	task_struct must be trusted/referenced.
+ * Returns
+ *	A referenced file pointer pointing to the exe_file member field of the
+ *	mm_struct nested in the supplied task_struct, or NULL.
+ */
+extern struct file *bpf_get_task_exe_file(struct task_struct *task) __ksym;
+
+/* Description
+ *	Release a reference on the supplied file. The supplied file must be
+ *	acquired.
+ */
+extern void bpf_put_file(struct file *file) __ksym;
+
+/* Description
+ *	Resolve a pathname for the supplied path and store it in the supplied
+ *	buffer. The supplied path must be trusted/referenced.
+ * Returns
+ *	A positive integer corresponding to the length of the resolved pathname,
+ *	including the NULL termination character, stored in the supplied
+ *	buffer. On error, a negative integer is returned.
+ */
+extern int bpf_path_d_path(struct path *path, char *buf, size_t buf__sz) __ksym;
+
 /* This macro must be used to mark the exception callback corresponding to the
  * main program. For example:
  *

tools/testing/selftests/bpf/prog_tests/verifier.c

···
 #include "verifier_value_or_null.skel.h"
 #include "verifier_value_ptr_arith.skel.h"
 #include "verifier_var_off.skel.h"
+#include "verifier_vfs_reject.skel.h"
 #include "verifier_xadd.skel.h"
 #include "verifier_xdp.skel.h"
 #include "verifier_xdp_direct_packet_access.skel.h"
···
 void test_verifier_value_illegal_alu(void)    { RUN(verifier_value_illegal_alu); }
 void test_verifier_value_or_null(void)        { RUN(verifier_value_or_null); }
 void test_verifier_var_off(void)              { RUN(verifier_var_off); }
+void test_verifier_vfs_reject(void)	      { RUN(verifier_vfs_reject); }
 void test_verifier_xadd(void)                 { RUN(verifier_xadd); }
 void test_verifier_xdp(void)                  { RUN(verifier_xdp); }
 void test_verifier_xdp_direct_packet_access(void) { RUN(verifier_xdp_direct_packet_access); }

tools/testing/selftests/bpf/progs/verifier_vfs_reject.c

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Google LLC. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <linux/limits.h>
+
+#include "bpf_misc.h"
+#include "bpf_experimental.h"
+
+static char buf[PATH_MAX];
+
+SEC("lsm.s/file_open")
+__failure __msg("Possibly NULL pointer passed to trusted arg0")
+int BPF_PROG(get_task_exe_file_kfunc_null)
+{
+	struct file *acquired;
+
+	/* Can't pass a NULL pointer to bpf_get_task_exe_file(). */
+	acquired = bpf_get_task_exe_file(NULL);
+	if (!acquired)
+		return 0;
+
+	bpf_put_file(acquired);
+	return 0;
+}
+
+SEC("lsm.s/inode_getxattr")
+__failure __msg("arg#0 pointer type STRUCT task_struct must point to scalar, or struct with scalar")
+int BPF_PROG(get_task_exe_file_kfunc_fp)
+{
+	u64 x;
+	struct file *acquired;
+	struct task_struct *task;
+
+	task = (struct task_struct *)&x;
+	/* Can't pass random frame pointer to bpf_get_task_exe_file(). */
+	acquired = bpf_get_task_exe_file(task);
+	if (!acquired)
+		return 0;
+
+	bpf_put_file(acquired);
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("R1 must be referenced or trusted")
+int BPF_PROG(get_task_exe_file_kfunc_untrusted)
+{
+	struct file *acquired;
+	struct task_struct *parent;
+
+	/* Walking a trusted struct task_struct returned from
+	 * bpf_get_current_task_btf() yields an untrusted pointer.
+	 */
+	parent = bpf_get_current_task_btf()->parent;
+	/* Can't pass untrusted pointer to bpf_get_task_exe_file(). */
+	acquired = bpf_get_task_exe_file(parent);
+	if (!acquired)
+		return 0;
+
+	bpf_put_file(acquired);
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("Unreleased reference")
+int BPF_PROG(get_task_exe_file_kfunc_unreleased)
+{
+	struct file *acquired;
+
+	acquired = bpf_get_task_exe_file(bpf_get_current_task_btf());
+	if (!acquired)
+		return 0;
+
+	/* Acquired but never released. */
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("release kernel function bpf_put_file expects")
+int BPF_PROG(put_file_kfunc_unacquired, struct file *file)
+{
+	/* Can't release an unacquired pointer. */
+	bpf_put_file(file);
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("Possibly NULL pointer passed to trusted arg0")
+int BPF_PROG(path_d_path_kfunc_null)
+{
+	/* Can't pass NULL value to bpf_path_d_path() kfunc. */
+	bpf_path_d_path(NULL, buf, sizeof(buf));
+	return 0;
+}
+
+SEC("lsm.s/task_alloc")
+__failure __msg("R1 must be referenced or trusted")
+int BPF_PROG(path_d_path_kfunc_untrusted_from_argument, struct task_struct *task)
+{
+	struct path *root;
+
+	/* Walking a trusted argument typically yields an untrusted
+	 * pointer. This is one example of that.
+	 */
+	root = &task->fs->root;
+	bpf_path_d_path(root, buf, sizeof(buf));
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("R1 must be referenced or trusted")
+int BPF_PROG(path_d_path_kfunc_untrusted_from_current)
+{
+	struct path *pwd;
+	struct task_struct *current;
+
+	current = bpf_get_current_task_btf();
+	/* Walking a trusted pointer returned from bpf_get_current_task_btf()
+	 * yields an untrusted pointer.
+	 */
+	pwd = &current->fs->pwd;
+	bpf_path_d_path(pwd, buf, sizeof(buf));
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("kernel function bpf_path_d_path args#0 expected pointer to STRUCT path but R1 has a pointer to STRUCT file")
+int BPF_PROG(path_d_path_kfunc_type_mismatch, struct file *file)
+{
+	bpf_path_d_path((struct path *)&file->f_task_work, buf, sizeof(buf));
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__failure __msg("invalid access to map value, value_size=4096 off=0 size=8192")
+int BPF_PROG(path_d_path_kfunc_invalid_buf_sz, struct file *file)
+{
+	/* bpf_path_d_path() enforces a constraint on the buffer size supplied
+	 * by the BPF LSM program via the __sz annotation. buf here is set to
+	 * PATH_MAX, so let's ensure that the BPF verifier rejects BPF_PROG_LOAD
+	 * attempts if the supplied size and the actual size of the buffer
+	 * mismatches.
+	 */
+	bpf_path_d_path(&file->f_path, buf, PATH_MAX * 2);
+	return 0;
+}
+
+SEC("fentry/vfs_open")
+__failure __msg("calling kernel function bpf_path_d_path is not allowed")
+int BPF_PROG(path_d_path_kfunc_non_lsm, struct path *path, struct file *f)
+{
+	/* Calling bpf_path_d_path() from a non-LSM BPF program isn't permitted.
+	 */
+	bpf_path_d_path(path, buf, sizeof(buf));
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";