[PATCH v8 bpf-next 5/9] bpf, fsverity: Add kfunc bpf_get_fsverity_digest

Song Liu
2023-11-02T20:16:59Z

fsverity provides fast and reliable hash of files, namely fsverity_digest.
The digest can be used by security solutions to verify file contents.

Add new kfunc bpf_get_fsverity_digest() so that we can access fsverity from
BPF LSM programs. This kfunc is added to fs/verity/measure.c because some
data structure used in the function is private to fsverity
(fs/verity/fsverity_private.h).

To avoid recursion, bpf_get_fsverity_digest is only allowed in BPF LSM
programs.

Signed-off-by: Song Liu <song@kernel.org>
Acked-by: Eric Biggers <ebiggers@google.com>
---
fs/verity/fsverity_private.h | 10 +++++
fs/verity/init.c | 1 +
fs/verity/measure.c | 85 ++++++++++++++++++++++++++++++++++++
3 files changed, 96 insertions(+)

diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index d071a6e32581..a6a6b2749241 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -100,6 +100,16 @@ fsverity_msg(const struct inode *inode, const char *level,
#define fsverity_err(inode, fmt, ...) \
fsverity_msg((inode), KERN_ERR, fmt, ##__VA_ARGS__)

+/* measure.c */
+
+#ifdef CONFIG_BPF_SYSCALL
+void __init fsverity_init_bpf(void);
+#else
+static inline void fsverity_init_bpf(void)
+{
+}
+#endif
+
/* open.c */

int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
diff --git a/fs/verity/init.c b/fs/verity/init.c
index a29f062f6047..1e207c0f71de 100644
--- a/fs/verity/init.c
+++ b/fs/verity/init.c
@@ -69,6 +69,7 @@ static int __init fsverity_init(void)
fsverity_init_workqueue();
fsverity_init_sysctl();
fsverity_init_signature();
+ fsverity_init_bpf();
return 0;
}
late_initcall(fsverity_init)
diff --git a/fs/verity/measure.c b/fs/verity/measure.c
index eec5956141da..a08177eda96e 100644
--- a/fs/verity/measure.c
+++ b/fs/verity/measure.c
@@ -7,6 +7,8 @@

#include "fsverity_private.h"

+#include <linux/bpf.h>
+#include <linux/btf.h>
#include <linux/uaccess.h>

/**
@@ -100,3 +102,86 @@ int fsverity_get_digest(struct inode *inode,
return hash_alg->digest_size;
}
EXPORT_SYMBOL_GPL(fsverity_get_digest);
+
+#ifdef CONFIG_BPF_SYSCALL
+
+/* bpf kfuncs */
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+ "kfuncs which will be used in BPF programs");
+
+/**
+ * bpf_get_fsverity_digest: read fsverity digest of file
+ * @file: file to get digest from
+ * @digest_ptr: (out) dynptr for struct fsverity_digest
+ *
+ * Read fsverity_digest of *file* into *digest_ptr*.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_get_fsverity_digest(struct file *file, struct bpf_dynptr_kern *digest_ptr)
+{
+ const struct inode *inode = file_inode(file);
+ struct fsverity_digest *arg;
+ const struct fsverity_info *vi;
+ const struct fsverity_hash_alg *hash_alg;
+ int out_digest_sz;
+
+ arg = __bpf_dynptr_data_rw(digest_ptr, __bpf_dynptr_size(digest_ptr));
+ if (!arg)
+ return -EINVAL;
+
+ if (!IS_ALIGNED((uintptr_t)arg, __alignof__(*arg)))
+ return -EINVAL;
+
+ if (__bpf_dynptr_size(digest_ptr) < sizeof(struct fsverity_digest))
+ return -EINVAL;
+
+ vi = fsverity_get_info(inode);
+ if (!vi)
+ return -ENODATA; /* not a verity file */
+
+ hash_alg = vi->tree_params.hash_alg;
+
+ arg->digest_algorithm = hash_alg - fsverity_hash_algs;
+ arg->digest_size = hash_alg->digest_size;
+
+ out_digest_sz = __bpf_dynptr_size(digest_ptr) - sizeof(struct fsverity_digest);
+
+ /* copy digest */
+ memcpy(arg->digest, vi->file_digest, min_t(int, hash_alg->digest_size, out_digest_sz));
+
+ /* fill the extra buffer with zeros */
+ if (out_digest_sz > hash_alg->digest_size)
+ memset(arg->digest + arg->digest_size, 0, out_digest_sz - hash_alg->digest_size);
+
+ return 0;
+}
+
+__diag_pop();
+
+BTF_SET8_START(fsverity_set_ids)
+BTF_ID_FLAGS(func, bpf_get_fsverity_digest, KF_TRUSTED_ARGS)
+BTF_SET8_END(fsverity_set_ids)
+
+static int bpf_get_fsverity_digest_filter(const struct bpf_prog *prog, u32 kfunc_id)
+{
+ if (!btf_id_set8_contains(&fsverity_set_ids, kfunc_id))
+ return 0;
+
+ /* Only allow to attach from LSM hooks, to avoid recursion */
+ return prog->type != BPF_PROG_TYPE_LSM ? -EACCES : 0;
+}
+
+static const struct btf_kfunc_id_set bpf_fsverity_set = {
+ .owner = THIS_MODULE,
+ .set = &fsverity_set_ids,
+ .filter = bpf_get_fsverity_digest_filter,
+};
+
+void __init fsverity_init_bpf(void)
+{
+ register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fsverity_set);
+}
+
+#endif /* CONFIG_BPF_SYSCALL */
--
2.34.1