[PATCH v2 bpf-next 5/9] bpf, fsverity: Add kfunc bpf_get_fsverity_digest

[Song Liu <song@kernel.org>]

The kfunc can be used to read fsverity_digest, so that we can verify
signature in BPF LSM.

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).

Signed-off-by: Song Liu <song@kernel.org>
---
 fs/verity/fsverity_private.h | 10 +++++
 fs/verity/init.c             |  1 +
 fs/verity/measure.c          | 82 ++++++++++++++++++++++++++++++++++++
 3 files changed, 93 insertions(+)

diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index d071a6e32581..f7124f89ab6f 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -145,4 +145,14 @@ static inline void fsverity_init_signature(void)
 
 void __init fsverity_init_workqueue(void);
 
+/* measure.c */
+
+#ifdef CONFIG_BPF_SYSCALL
+int __init fsverity_init_bpf(void);
+#else
+static inline int fsverity_init_bpf(void)
+{
+}
+#endif
+
 #endif /* _FSVERITY_PRIVATE_H */
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..db8b3472a16c 100644
--- a/fs/verity/measure.c
+++ b/fs/verity/measure.c
@@ -8,6 +8,8 @@
 #include "fsverity_private.h"
 
 #include <linux/uaccess.h>
+#include <linux/bpf.h>
+#include <linux/btf.h>
 
 /**
  * fsverity_ioctl_measure() - get a verity file's digest
@@ -100,3 +102,83 @@ 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_slice_rdwr(digest_ptr, 0, NULL, 0);
+	if (IS_ERR_OR_NULL(arg))
+		return PTR_ERR(arg);
+
+	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();
+
+static int bpf_get_fsverity_digest_filter(const struct bpf_prog *prog, u32 kfunc_id)
+{
+	/* Only allow to attach from LSM hooks, to avoid recursion */
+	return prog->type != BPF_PROG_TYPE_LSM ? -EACCES : 0;
+}
+
+BTF_SET8_START(fsverity_set)
+BTF_ID_FLAGS(func, bpf_get_fsverity_digest)
+BTF_SET8_END(fsverity_set)
+
+static const struct btf_kfunc_id_set bpf_fsverity_set = {
+	.owner = THIS_MODULE,
+	.set = &fsverity_set,
+	.filter = bpf_get_fsverity_digest_filter,
+};
+
+int __init fsverity_init_bpf(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, &bpf_fsverity_set);
+}
+
+#endif /* CONFIG_BPF_SYSCALL */
-- 
2.34.1