bpf, fsverity: Add kfunc bpf_get_fsverity_digest

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>
Link: https://lore.kernel.org/r/20231129234417.856536-3-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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,

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)

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 +102,85 @@ int fsverity_get_digest(struct inode *inode,
 	return hash_alg->digest_size;
 }
 EXPORT_SYMBOL_GPL(fsverity_get_digest);
+
+#ifdef CONFIG_BPF_SYSCALL
+
+/* bpf kfuncs */
+__bpf_kfunc_start_defs();
+
+/**
+ * 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);
+	u32 dynptr_sz = __bpf_dynptr_size(digest_ptr);
+	struct fsverity_digest *arg;
+	const struct fsverity_info *vi;
+	const struct fsverity_hash_alg *hash_alg;
+	int out_digest_sz;
+
+	if (dynptr_sz < sizeof(struct fsverity_digest))
+		return -EINVAL;
+
+	arg = __bpf_dynptr_data_rw(digest_ptr, dynptr_sz);
+	if (!arg)
+		return -EINVAL;
+
+	if (!IS_ALIGNED((uintptr_t)arg, __alignof__(*arg)))
+		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 = dynptr_sz - 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;
+}
+
+__bpf_kfunc_end_defs();
+
+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 */