bpf: introduce new VFS based BPF kfuncs

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>
Link: https://lore.kernel.org/r/20240731110833.1834742-2-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Matt Bobrowski 2024-07-31 11:08:31 +00:00 committed by Alexei Starovoitov
parent 3d650ab5e7
commit d08e2045eb
2 changed files with 124 additions and 0 deletions

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@ -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

123
fs/bpf_fs_kfuncs.c Normal file
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@ -0,0 +1,123 @@
// 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/fs.h>
#include <linux/file.h>
#include <linux/mm.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);