bpf: allow fs kfuncs for binfmt_misc_ops programs

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>

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index f1863a891db641b2e615adf1f1d0906c6b319417..5b7d03e4fc6d2a8ca39fda570db4cdb039e151cb 100644 (file)
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -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);