On Fri, Nov 3, 2023 at 2:46 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> Acked-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
> 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");
> +
same as in previous patch, we have dedicated macros for this, please use them
> +/**
> + * 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;
> +
similar nit, you call __bpf_dynptr_size(digest_ptr) many times, let's
use `u32 len = __bpf_dynptr_size(digest_ptr);` ?
> + 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
>
