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bpf: allow fs kfuncs for binfmt_misc_ops programs

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The fs kfuncs are currently exclusive to LSM programs. A binfmt_misc
handler needs a subset of them to do anything interesting: computing an
interpreter relative to the binary's location wants bpf_path_d_path()
on bprm->file->f_path from the load program, and matching on per-binary
metadata wants bpf_get_file_xattr() and friends right from the match
program.

Register the fs kfunc set for struct_ops programs as well and extend
the filter to admit binfmt_misc_ops programs. The xattr setters stay
exclusive to LSM programs: a binary type handler decides how to run a
binary, it has no business modifying filesystem state.

This only takes effect in builds that have the fs kfunc set at all,
i.e. CONFIG_BPF_LSM. Without it a binfmt_misc handler is limited to
bprm fields and the file-backed dynptr, which are provided by the
common kfunc set.

Link: https://lore.kernel.org/20260704211409.1978485-1-farid.m.zakaria@gmail.com
Link: https://patch.msgid.link/20260714-work-bpf-binfmt_misc-v2-5-57b7529c002c@kernel.org
Reviewed-by: Farid Zakaria <farid.m.zakaria@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Christian Brauner 2026-07-14 21:58:10 +02:00
parent ceb912149e
commit 7bddf0e9f1
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
1 changed files with 25 additions and 3 deletions
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28
fs/bpf_fs_kfuncs.c
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@ -1,6 +1,7 @@
// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2024 Google LLC. */
#include <linux/binfmt_misc.h>
#include <linux/bpf.h>
#include <linux/bpf_lsm.h>
#include <linux/btf.h>
@ -392,10 +393,25 @@ BTF_ID_FLAGS(func, bpf_remove_dentry_xattr, KF_SLEEPABLE)
BTF_ID_FLAGS(func, bpf_real_data_inode, KF_SLEEPABLE | KF_RET_NULL)
BTF_KFUNCS_END(bpf_fs_kfunc_set_ids)
/* Side-effecting kfuncs that stay exclusive to LSM programs. */
BTF_SET_START(bpf_fs_kfunc_lsm_only_ids)
BTF_ID(func, bpf_set_dentry_xattr)
BTF_ID(func, bpf_remove_dentry_xattr)
BTF_SET_END(bpf_fs_kfunc_lsm_only_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)
if (!btf_id_set8_contains(&bpf_fs_kfunc_set_ids, kfunc_id))
return 0;
if (prog->type == BPF_PROG_TYPE_LSM)
return 0;
if (prog->type != BPF_PROG_TYPE_STRUCT_OPS)
return -EACCES;
/* ->st_ops is unset during the cfg pass; enforced once it is set. */
if (!prog->aux->st_ops)
return 0;
if (bpf_prog_is_binfmt_misc_ops(prog) &&
!btf_id_set_contains(&bpf_fs_kfunc_lsm_only_ids, kfunc_id))
return 0;
return -EACCES;
}
@ -438,7 +454,13 @@ static const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
static int __init bpf_fs_kfuncs_init(void)
{
return register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fs_kfunc_set);
int ret;
ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fs_kfunc_set);
if (ret || !IS_ENABLED(CONFIG_BINFMT_MISC_BPF))
return ret;
return register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
&bpf_fs_kfunc_set);
}
late_initcall(bpf_fs_kfuncs_init);