[PATCH v4 bpf-next 0/3] bpf: introduce new VFS based BPF kfuncs

[PATCH v4 bpf-next 0/3] bpf: introduce new VFS based BPF kfuncs

[Matt Bobrowski]

G'day!

A respin based off v3, which can be found here [0]. Original
motivations for introducing this suite of BPF kfuncs can be found here
[1].

The primary difference in this version of the patch series is that the
suite of VFS related BPF kfuncs added can be used from both sleepable
and non-sleepable BPF LSM program types. IOW, the KF_SLEEPABLE
annotation has been removed from all of them.

Changes sinve v3:

* KF_SLEEPABLE annotation has been dropped from all newly introduced
  VFS related BPF kfuncs. This includes bpf_get_task_exe_file(),
  bpf_put_file(), and bpf_path_d_path(). Both negative and positive
  selftests backing these new BPF kfuncs have also been updated
  accordingly.

* buf__sz conditional in bpf_path_d_path() has been updated from
  buf__sz <= 0, to !buf__sz.

* Syntax issues as reported so here [2] have been corrected.

[0] https://lore.kernel.org/bpf/20240726085604.2369469-1-mattbobrowski@google.com/
[1] https://lore.kernel.org/bpf/cover.1708377880.git.mattbobrowski@google.com/#t
[2] https://netdev.bots.linux.dev/static/nipa/874023/13742510/checkpatch/stdout

Matt Bobrowski (3):
  bpf: introduce new VFS based BPF kfuncs
  selftests/bpf: add negative tests for new VFS based BPF kfuncs
  selftests/bpf: add positive tests for new VFS based BPF kfuncs

 fs/Makefile                                   |   1 +
 fs/bpf_fs_kfuncs.c                            | 127 ++++++++++++++
 .../testing/selftests/bpf/bpf_experimental.h  |  26 +++
 .../selftests/bpf/prog_tests/verifier.c       |   4 +
 .../selftests/bpf/progs/verifier_vfs_accept.c |  85 +++++++++
 .../selftests/bpf/progs/verifier_vfs_reject.c | 161 ++++++++++++++++++
 6 files changed, 404 insertions(+)
 create mode 100644 fs/bpf_fs_kfuncs.c
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_vfs_accept.c
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_vfs_reject.c

-- 
2.46.0.rc2.264.g509ed76dc8-goog

[PATCH v4 bpf-next 1/3] bpf: introduce new VFS based BPF kfuncs

[Matt Bobrowski]

Add a new variant of bpf_d_path() named bpf_path_d_path() which takes
the form of a BPF kfunc and enforces KF_TRUSTED_ARGS semantics onto
its arguments.

This new d_path() based BPF kfunc variant is intended to address the
legacy bpf_d_path() BPF helper's susceptability to memory corruption
issues [0, 1, 2] by ensuring to only operate on supplied arguments
which are deemed trusted by the BPF verifier. Typically, this means
that only pointers to a struct path which have been referenced counted
may be supplied.

In addition to the new bpf_path_d_path() BPF kfunc, we also add a
KF_ACQUIRE based BPF kfunc bpf_get_task_exe_file() and KF_RELEASE
counterpart BPF kfunc bpf_put_file(). This is so that the new
bpf_path_d_path() BPF kfunc can be used more flexibily from within the
context of a BPF LSM program. It's rather common to ascertain the
backing executable file for the calling process by performing the
following walk current->mm->exe_file while instrumenting a given
operation from the context of the BPF LSM program. However, walking
current->mm->exe_file directly is never deemed to be OK, and doing so
from both inside and outside of BPF LSM program context should be
considered as a bug. Using bpf_get_task_exe_file() and in turn
bpf_put_file() will allow BPF LSM programs to reliably get and put
references to current->mm->exe_file.

As of now, all the newly introduced BPF kfuncs within this patch are
limited to BPF LSM program types. These can be either sleepable or
non-sleepable variants of BPF LSM program types.

[0] https://lore.kernel.org/bpf/CAG48ez0ppjcT=QxU-jtCUfb5xQb3mLr=5FcwddF_VKfEBPs_Dg@mail.gmail.com/
[1] https://lore.kernel.org/bpf/20230606181714.532998-1-jolsa@kernel.org/
[2] https://lore.kernel.org/bpf/20220219113744.1852259-1-memxor@gmail.com/

Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 fs/Makefile        |   1 +
 fs/bpf_fs_kfuncs.c | 127 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 128 insertions(+)
 create mode 100644 fs/bpf_fs_kfuncs.c

diff --git a/fs/Makefile b/fs/Makefile
index 6ecc9b0a53f2..61679fd587b7 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -129,3 +129,4 @@ obj-$(CONFIG_EFIVAR_FS)		+= efivarfs/
 obj-$(CONFIG_EROFS_FS)		+= erofs/
 obj-$(CONFIG_VBOXSF_FS)		+= vboxsf/
 obj-$(CONFIG_ZONEFS_FS)		+= zonefs/
+obj-$(CONFIG_BPF_LSM)		+= bpf_fs_kfuncs.o
diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
new file mode 100644
index 000000000000..2a66331d8921
--- /dev/null
+++ b/fs/bpf_fs_kfuncs.c
@@ -0,0 +1,127 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Google LLC. */
+
+#include <linux/bpf.h>
+#include <linux/btf.h>
+#include <linux/btf_ids.h>
+#include <linux/dcache.h>
+#include <linux/err.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include <linux/init.h>
+#include <linux/mm.h>
+#include <linux/path.h>
+#include <linux/sched.h>
+
+__bpf_kfunc_start_defs();
+
+/**
+ * bpf_get_task_exe_file - get a reference on the exe_file struct file member of
+ *                         the mm_struct that is nested within the supplied
+ *                         task_struct
+ * @task: task_struct of which the nested mm_struct exe_file member to get a
+ * reference on
+ *
+ * Get a reference on the exe_file struct file member field of the mm_struct
+ * nested within the supplied *task*. The referenced file pointer acquired by
+ * this BPF kfunc must be released using bpf_put_file(). Failing to call
+ * bpf_put_file() on the returned referenced struct file pointer that has been
+ * acquired by this BPF kfunc will result in the BPF program being rejected by
+ * the BPF verifier.
+ *
+ * This BPF kfunc may only be called from BPF LSM programs.
+ *
+ * Internally, this BPF kfunc leans on get_task_exe_file(), such that calling
+ * bpf_get_task_exe_file() would be analogous to calling get_task_exe_file()
+ * directly in kernel context.
+ *
+ * Return: A referenced struct file pointer to the exe_file member of the
+ * mm_struct that is nested within the supplied *task*. On error, NULL is
+ * returned.
+ */
+__bpf_kfunc struct file *bpf_get_task_exe_file(struct task_struct *task)
+{
+	return get_task_exe_file(task);
+}
+
+/**
+ * bpf_put_file - put a reference on the supplied file
+ * @file: file to put a reference on
+ *
+ * Put a reference on the supplied *file*. Only referenced file pointers may be
+ * passed to this BPF kfunc. Attempting to pass an unreferenced file pointer, or
+ * any other arbitrary pointer for that matter, will result in the BPF program
+ * being rejected by the BPF verifier.
+ *
+ * This BPF kfunc may only be called from BPF LSM programs.
+ */
+__bpf_kfunc void bpf_put_file(struct file *file)
+{
+	fput(file);
+}
+
+/**
+ * bpf_path_d_path - resolve the pathname for the supplied path
+ * @path: path to resolve the pathname for
+ * @buf: buffer to return the resolved pathname in
+ * @buf__sz: length of the supplied buffer
+ *
+ * Resolve the pathname for the supplied *path* and store it in *buf*. This BPF
+ * kfunc is the safer variant of the legacy bpf_d_path() helper and should be
+ * used in place of bpf_d_path() whenever possible. It enforces KF_TRUSTED_ARGS
+ * semantics, meaning that the supplied *path* must itself hold a valid
+ * reference, or else the BPF program will be outright rejected by the BPF
+ * verifier.
+ *
+ * This BPF kfunc may only be called from BPF LSM programs.
+ *
+ * Return: A positive integer corresponding to the length of the resolved
+ * pathname in *buf*, including the NUL termination character. On error, a
+ * negative integer is returned.
+ */
+__bpf_kfunc int bpf_path_d_path(struct path *path, char *buf, size_t buf__sz)
+{
+	int len;
+	char *ret;
+
+	if (!buf__sz)
+		return -EINVAL;
+
+	ret = d_path(path, buf, buf__sz);
+	if (IS_ERR(ret))
+		return PTR_ERR(ret);
+
+	len = buf + buf__sz - ret;
+	memmove(buf, ret, len);
+	return len;
+}
+
+__bpf_kfunc_end_defs();
+
+BTF_KFUNCS_START(bpf_fs_kfunc_set_ids)
+BTF_ID_FLAGS(func, bpf_get_task_exe_file,
+	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_path_d_path, KF_TRUSTED_ARGS)
+BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
+
+static int bpf_fs_kfuncs_filter(const struct bpf_prog *prog, u32 kfunc_id)
+{
+	if (!btf_id_set8_contains(&bpf_fs_kfunc_set_ids, kfunc_id) ||
+	    prog->type == BPF_PROG_TYPE_LSM)
+		return 0;
+	return -EACCES;
+}
+
+static const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set = &bpf_fs_kfunc_set_ids,
+	.filter = bpf_fs_kfuncs_filter,
+};
+
+static int __init bpf_fs_kfuncs_init(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fs_kfunc_set);
+}
+
+late_initcall(bpf_fs_kfuncs_init);
-- 
2.46.0.rc2.264.g509ed76dc8-goog

[PATCH v4 bpf-next 2/3] selftests/bpf: add negative tests for new VFS based BPF kfuncs

[Matt Bobrowski]

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 <brauner@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 .../testing/selftests/bpf/bpf_experimental.h  |  26 +++
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../selftests/bpf/progs/verifier_vfs_reject.c | 161 ++++++++++++++++++
 3 files changed, 189 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_vfs_reject.c

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index 828556cdc2f0..b0668f29f7b3 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -195,6 +195,32 @@ extern void bpf_iter_task_vma_destroy(struct bpf_iter_task_vma *it) __ksym;
  */
 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:
  *
diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index 67a49d12472c..14d74ba2188e 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -85,6 +85,7 @@
 #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"
@@ -205,6 +206,7 @@ void test_verifier_value(void)                { RUN(verifier_value); }
 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); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_vfs_reject.c b/tools/testing/selftests/bpf/progs/verifier_vfs_reject.c
new file mode 100644
index 000000000000..d6d3f4fcb24c
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_vfs_reject.c
@@ -0,0 +1,161 @@
+// 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";
-- 
2.46.0.rc2.264.g509ed76dc8-goog

[PATCH v4 bpf-next 3/3] selftests/bpf: add positive tests for new VFS based BPF kfuncs

[Matt Bobrowski]

Add a bunch of positive selftests which extensively cover the various
contexts and parameters in which the new VFS based BPF kfuncs may be
used from.

Again, the following VFS based BPF kfuncs are thoroughly 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 <brauner@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 .../selftests/bpf/prog_tests/verifier.c       |  2 +
 .../selftests/bpf/progs/verifier_vfs_accept.c | 85 +++++++++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_vfs_accept.c

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index 14d74ba2188e..f8f546eba488 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -85,6 +85,7 @@
 #include "verifier_value_or_null.skel.h"
 #include "verifier_value_ptr_arith.skel.h"
 #include "verifier_var_off.skel.h"
+#include "verifier_vfs_accept.skel.h"
 #include "verifier_vfs_reject.skel.h"
 #include "verifier_xadd.skel.h"
 #include "verifier_xdp.skel.h"
@@ -206,6 +207,7 @@ void test_verifier_value(void)                { RUN(verifier_value); }
 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_accept(void)	      { RUN(verifier_vfs_accept); }
 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); }
diff --git a/tools/testing/selftests/bpf/progs/verifier_vfs_accept.c b/tools/testing/selftests/bpf/progs/verifier_vfs_accept.c
new file mode 100644
index 000000000000..a7c0a553aa50
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/verifier_vfs_accept.c
@@ -0,0 +1,85 @@
+// 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 "bpf_misc.h"
+#include "bpf_experimental.h"
+
+static char buf[64];
+
+SEC("lsm.s/file_open")
+__success
+int BPF_PROG(get_task_exe_file_and_put_kfunc_from_current_sleepable)
+{
+	struct file *acquired;
+
+	acquired = bpf_get_task_exe_file(bpf_get_current_task_btf());
+	if (!acquired)
+		return 0;
+
+	bpf_put_file(acquired);
+	return 0;
+}
+
+SEC("lsm/file_open")
+__success
+int BPF_PROG(get_task_exe_file_and_put_kfunc_from_current_non_sleepable, struct file *file)
+{
+	struct file *acquired;
+
+	acquired = bpf_get_task_exe_file(bpf_get_current_task_btf());
+	if (!acquired)
+		return 0;
+
+	bpf_put_file(acquired);
+	return 0;
+}
+
+SEC("lsm.s/task_alloc")
+__success
+int BPF_PROG(get_task_exe_file_and_put_kfunc_from_argument,
+	     struct task_struct *task)
+{
+	struct file *acquired;
+
+	acquired = bpf_get_task_exe_file(task);
+	if (!acquired)
+		return 0;
+
+	bpf_put_file(acquired);
+	return 0;
+}
+
+SEC("lsm.s/inode_getattr")
+__success
+int BPF_PROG(path_d_path_from_path_argument, struct path *path)
+{
+	int ret;
+
+	ret = bpf_path_d_path(path, buf, sizeof(buf));
+	__sink(ret);
+	return 0;
+}
+
+SEC("lsm.s/file_open")
+__success
+int BPF_PROG(path_d_path_from_file_argument, struct file *file)
+{
+	int ret;
+	struct path *path;
+
+	/* The f_path member is a path which is embedded directly within a
+	 * file. Therefore, a pointer to such embedded members are still
+	 * recognized by the BPF verifier as being PTR_TRUSTED as it's
+	 * essentially PTR_TRUSTED w/ a non-zero fixed offset.
+	 */
+	path = &file->f_path;
+	ret = bpf_path_d_path(path, buf, sizeof(buf));
+	__sink(ret);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.46.0.rc2.264.g509ed76dc8-goog

Re: [PATCH v4 bpf-next 1/3] bpf: introduce new VFS based BPF kfuncs

[Song Liu]

On Wed, Jul 31, 2024 at 4:09 AM Matt Bobrowski <mattbobrowski@google.com> wrote:
>
> Add a new variant of bpf_d_path() named bpf_path_d_path() which takes
> the form of a BPF kfunc and enforces KF_TRUSTED_ARGS semantics onto
> its arguments.
>
> This new d_path() based BPF kfunc variant is intended to address the
> legacy bpf_d_path() BPF helper's susceptability to memory corruption
> issues [0, 1, 2] by ensuring to only operate on supplied arguments
> which are deemed trusted by the BPF verifier. Typically, this means
> that only pointers to a struct path which have been referenced counted
> may be supplied.
>
> In addition to the new bpf_path_d_path() BPF kfunc, we also add a
> KF_ACQUIRE based BPF kfunc bpf_get_task_exe_file() and KF_RELEASE
> counterpart BPF kfunc bpf_put_file(). This is so that the new
> bpf_path_d_path() BPF kfunc can be used more flexibily from within the
> context of a BPF LSM program. It's rather common to ascertain the
> backing executable file for the calling process by performing the
> following walk current->mm->exe_file while instrumenting a given
> operation from the context of the BPF LSM program. However, walking
> current->mm->exe_file directly is never deemed to be OK, and doing so
> from both inside and outside of BPF LSM program context should be
> considered as a bug. Using bpf_get_task_exe_file() and in turn
> bpf_put_file() will allow BPF LSM programs to reliably get and put
> references to current->mm->exe_file.
>
> As of now, all the newly introduced BPF kfuncs within this patch are
> limited to BPF LSM program types. These can be either sleepable or
> non-sleepable variants of BPF LSM program types.
>
> [0] https://lore.kernel.org/bpf/CAG48ez0ppjcT=QxU-jtCUfb5xQb3mLr=5FcwddF_VKfEBPs_Dg@mail.gmail.com/
> [1] https://lore.kernel.org/bpf/20230606181714.532998-1-jolsa@kernel.org/
> [2] https://lore.kernel.org/bpf/20220219113744.1852259-1-memxor@gmail.com/
>
> Acked-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>

Acked-by: Song Liu <song@kernel.org>

with one nitpic below

> ---
>  fs/Makefile        |   1 +
>  fs/bpf_fs_kfuncs.c | 127 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 128 insertions(+)
>  create mode 100644 fs/bpf_fs_kfuncs.c
>
> diff --git a/fs/Makefile b/fs/Makefile
> index 6ecc9b0a53f2..61679fd587b7 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -129,3 +129,4 @@ obj-$(CONFIG_EFIVAR_FS)             += efivarfs/
>  obj-$(CONFIG_EROFS_FS)         += erofs/
>  obj-$(CONFIG_VBOXSF_FS)                += vboxsf/
>  obj-$(CONFIG_ZONEFS_FS)                += zonefs/
> +obj-$(CONFIG_BPF_LSM)          += bpf_fs_kfuncs.o
> diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
> new file mode 100644
> index 000000000000..2a66331d8921
> --- /dev/null
> +++ b/fs/bpf_fs_kfuncs.c
> @@ -0,0 +1,127 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2024 Google LLC. */
> +
> +#include <linux/bpf.h>
> +#include <linux/btf.h>
> +#include <linux/btf_ids.h>
> +#include <linux/dcache.h>
> +#include <linux/err.h>
> +#include <linux/fs.h>
> +#include <linux/file.h>
> +#include <linux/init.h>
> +#include <linux/mm.h>
> +#include <linux/path.h>
> +#include <linux/sched.h>

It appears we don't need to include all these headers. With my
daily config, #include <linux/bpf.h> alone is sufficient.

Thanks,
Song

Re: [PATCH v4 bpf-next 1/3] bpf: introduce new VFS based BPF kfuncs

[Alexei Starovoitov]

On Wed, Jul 31, 2024 at 6:16 PM Song Liu <song@kernel.org> wrote:
>
> > --- /dev/null
> > +++ b/fs/bpf_fs_kfuncs.c
> > @@ -0,0 +1,127 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2024 Google LLC. */
> > +
> > +#include <linux/bpf.h>
> > +#include <linux/btf.h>
> > +#include <linux/btf_ids.h>
> > +#include <linux/dcache.h>
> > +#include <linux/err.h>
> > +#include <linux/fs.h>
> > +#include <linux/file.h>
> > +#include <linux/init.h>
> > +#include <linux/mm.h>
> > +#include <linux/path.h>
> > +#include <linux/sched.h>
>
> It appears we don't need to include all these headers. With my
> daily config, #include <linux/bpf.h> alone is sufficient.

In general it's a good idea to include necessary headers and
not rely on implicit recursive includes, but in this case the
list is indeed excessive.
I trimmed it a bit while applying.

Thanks everyone!