Re: [PATCH v2] proc: "mount -o lookup=" support

From: Alexey Gladkov
Date: Wed Jan 19 2022 - 12:04:42 EST

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On Wed, Jan 19, 2022 at 06:48:03PM +0300, Alexey Dobriyan wrote:
> >From 61376c85daab50afb343ce50b5a97e562bc1c8d3 Mon Sep 17 00:00:00 2001
> From: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Date: Mon, 22 Nov 2021 20:41:06 +0300
> Subject: [PATCH 1/1] proc: "mount -o lookup=..." support
>
> Docker implements MaskedPaths configuration option
>
> https://github.com/estesp/docker/blob/9c15e82f19b0ad3c5fe8617a8ec2dddc6639f40a/oci/defaults.go#L97
>
> to disable certain /proc files. It overmounts them with /dev/null.
>
> Implement proper mount option which selectively disables lookup/readdir
> in the top level /proc directory so that MaskedPaths doesn't need
> to be updated as time goes on.
>
> Syntax is
>
> Filter everything
> # mount -t proc -o lookup=/ proc /proc
> # ls /proc
> dr-xr-xr-x 8 root root 0 Nov 22 21:12 995
> lrwxrwxrwx 1 root root 0 Nov 22 21:12 self -> 1163
> lrwxrwxrwx 1 root root 0 Nov 22 21:12 thread-self -> 1163/task/1163
>
> Allow /proc/cpuinfo and /proc/uptime
> # mount -t proc proc -o lookup=cpuinfo/uptime /proc
>
> # ls /proc
> ...
> dr-xr-xr-x 8 root root 0 Nov 22 21:12 995
> -r--r--r-- 1 root root 0 Nov 22 21:12 cpuinfo
> lrwxrwxrwx 1 root root 0 Nov 22 21:12 self -> 1163
> lrwxrwxrwx 1 root root 0 Nov 22 21:12 thread-self -> 1163/task/1163
> -r--r--r-- 1 root root 0 Nov 22 21:12 uptime
>
> Trailing slash is optional but saves 1 allocation.
> Trailing slash is mandatory for "filter everything".
>
> Remounting with lookup= is disabled so that files and dcache entries
> don't stay active while filter list is changed. Users are supposed
> to unmount and mount again with different lookup= set.
> Remount rules may change in the future. (Eric W. Biederman)
>
> Re: speed
> This is the price for filtering, given that lookup= is whitelist it is
> not supposed to be very long. Second, it is one linear memory scan per
> lookup, there are no linked lists. It may be faster than rbtree in fact.
> It consumes 1 allocation per superblock which is list of names itself.
>
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> ---
>
> v2
> documentation!
> descriptive comments!
> disable remount
>
> Documentation/filesystems/proc.rst | 8 ++
> fs/proc/generic.c | 18 ++--
> fs/proc/internal.h | 31 ++++++-
> fs/proc/proc_net.c | 2 +-
> fs/proc/root.c | 127 ++++++++++++++++++++++++++++-
> include/linux/proc_fs.h | 2 +
> 6 files changed, 178 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 8d7f141c6fc7..9a328f0b4346 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -2186,6 +2186,7 @@ The following mount options are supported:
> hidepid= Set /proc/<pid>/ access mode.
> gid= Set the group authorized to learn processes information.
> subset= Show only the specified subset of procfs.
> + lookup= Top-level /proc filter, independent of subset=

Will it be possible to combine lookup= and subset= options when mounting?

> ========= ========================================================
>
> hidepid=off or hidepid=0 means classic mode - everybody may access all
> @@ -2218,6 +2219,13 @@ information about processes information, just add identd to this group.
> subset=pid hides all top level files and directories in the procfs that
> are not related to tasks.
>
> +lookup= mount option makes available only listed files/directories in
> +the top-level /proc directory. Individual names are separated
> +by slash. Empty list is equivalent to subset=pid. lookup= filters before
> +subset= if both options are supplied. lookup= doesn't affect /proc/${pid}
> +directories availability as well as /proc/self and /proc/thread-self
> +symlinks. More fine-grained filtering is not supported at the moment.
> +
> Chapter 5: Filesystem behavior
> ==============================
>
> diff --git a/fs/proc/generic.c b/fs/proc/generic.c
> index 5b78739e60e4..4d04f8d89cdc 100644
> --- a/fs/proc/generic.c
> +++ b/fs/proc/generic.c
> @@ -282,7 +282,7 @@ struct dentry *proc_lookup(struct inode *dir, struct dentry *dentry,
> * for success..
> */
> int proc_readdir_de(struct file *file, struct dir_context *ctx,
> - struct proc_dir_entry *de)
> + struct proc_dir_entry *de, const struct proc_lookup_list *ll)
> {
> int i;
>
> @@ -307,12 +307,15 @@ int proc_readdir_de(struct file *file, struct dir_context *ctx,
> struct proc_dir_entry *next;
> pde_get(de);
> read_unlock(&proc_subdir_lock);
> - if (!dir_emit(ctx, de->name, de->namelen,
> - de->low_ino, de->mode >> 12)) {
> - pde_put(de);
> - return 0;
> +
> + if (in_lookup_list(ll, de->name, de->namelen)) {
> + if (!dir_emit(ctx, de->name, de->namelen, de->low_ino, de->mode >> 12)) {
> + pde_put(de);
> + return 0;
> + }
> + ctx->pos++;
> }
> - ctx->pos++;
> +
> read_lock(&proc_subdir_lock);
> next = pde_subdir_next(de);
> pde_put(de);
> @@ -330,7 +333,8 @@ int proc_readdir(struct file *file, struct dir_context *ctx)
> if (fs_info->pidonly == PROC_PIDONLY_ON)
> return 1;
>
> - return proc_readdir_de(file, ctx, PDE(inode));
> + return proc_readdir_de(file, ctx, PDE(inode),
> + PDE(inode) == &proc_root ? fs_info->lookup_list : NULL);
> }
>
> /*
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index 03415f3fb3a8..e74acb437c56 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -190,7 +190,7 @@ struct proc_dir_entry *proc_register(struct proc_dir_entry *dir,
> extern struct dentry *proc_lookup(struct inode *, struct dentry *, unsigned int);
> struct dentry *proc_lookup_de(struct inode *, struct dentry *, struct proc_dir_entry *);
> extern int proc_readdir(struct file *, struct dir_context *);
> -int proc_readdir_de(struct file *, struct dir_context *, struct proc_dir_entry *);
> +int proc_readdir_de(struct file *, struct dir_context *, struct proc_dir_entry *, const struct proc_lookup_list *);
>
> static inline void pde_get(struct proc_dir_entry *pde)
> {
> @@ -318,3 +318,32 @@ static inline void pde_force_lookup(struct proc_dir_entry *pde)
> /* /proc/net/ entries can be changed under us by setns(CLONE_NEWNET) */
> pde->proc_dops = &proc_net_dentry_ops;
> }
> +
> +/*
> + * Pascal strings stiched together making filtering memory access pattern linear.
> + *
> + * "mount -t proc -o lookup=/" results in
> + *
> + * (u8[]){
> + * 0
> + * }
> + *
> + * "mount -t proc -o lookup=cpuinfo/uptime/" results in
> + *
> + * (u8[]){
> + * 7, 'c', 'p', 'u', 'i', 'n', 'f', 'o',
> + * 6, 'u', 'p', 't', 'i', 'm', 'e',
> + * 0
> + * }
> + */
> +struct proc_lookup_list {
> + u8 len;
> + char str[];
> +};
> +
> +static inline struct proc_lookup_list *lookup_list_next(const struct proc_lookup_list *ll)
> +{
> + return (struct proc_lookup_list *)((void *)ll + 1 + ll->len);
> +}
> +
> +bool in_lookup_list(const struct proc_lookup_list *ll, const char *str, unsigned int len);
> diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
> index 15c2e55d2ed2..7941df2d3d74 100644
> --- a/fs/proc/proc_net.c
> +++ b/fs/proc/proc_net.c
> @@ -321,7 +321,7 @@ static int proc_tgid_net_readdir(struct file *file, struct dir_context *ctx)
> ret = -EINVAL;
> net = get_proc_task_net(file_inode(file));
> if (net != NULL) {
> - ret = proc_readdir_de(file, ctx, net->proc_net);
> + ret = proc_readdir_de(file, ctx, net->proc_net, NULL);
> put_net(net);
> }
> return ret;
> diff --git a/fs/proc/root.c b/fs/proc/root.c
> index c7e3b1350ef8..8000558d7d2c 100644
> --- a/fs/proc/root.c
> +++ b/fs/proc/root.c
> @@ -35,18 +35,22 @@ struct proc_fs_context {
> enum proc_hidepid hidepid;
> int gid;
> enum proc_pidonly pidonly;
> + struct proc_lookup_list *lookup_list;
> + unsigned int lookup_list_len;
> };
>
> enum proc_param {
> Opt_gid,
> Opt_hidepid,
> Opt_subset,
> + Opt_lookup,
> };
>
> static const struct fs_parameter_spec proc_fs_parameters[] = {
> fsparam_u32("gid", Opt_gid),
> fsparam_string("hidepid", Opt_hidepid),
> fsparam_string("subset", Opt_subset),
> + fsparam_string("lookup", Opt_lookup),
> {}
> };
>
> @@ -112,6 +116,65 @@ static int proc_parse_subset_param(struct fs_context *fc, char *value)
> return 0;
> }
>
> +static int proc_parse_lookup_param(struct fs_context *fc, char *str0)
> +{
> + struct proc_fs_context *ctx = fc->fs_private;
> + struct proc_lookup_list *ll;
> + char *str;
> + const char *slash;
> + const char *src;
> + unsigned int len;
> + int rv;
> +
> + /* Force trailing slash, simplify loops below. */
> + len = strlen(str0);
> + if (len > 0 && str0[len - 1] == '/') {
> + str = str0;
> + } else {
> + str = kmalloc(len + 2, GFP_KERNEL);
> + if (!str) {
> + rv = -ENOMEM;
> + goto out;
> + }
> + memcpy(str, str0, len);
> + str[len] = '/';
> + str[len + 1] = '\0';
> + }
> +
> + len = 0;
> + for (src = str; (slash = strchr(src, '/')); src = slash + 1) {
> + if (slash - src >= 256) {
> + rv = -EINVAL;
> + goto out_free_str;
> + }
> + len += 1 + (slash - src);
> + }
> + len += 1;
> +
> + ctx->lookup_list = ll = kmalloc(len, GFP_KERNEL);
> + ctx->lookup_list_len = len;
> + if (!ll) {
> + rv = -ENOMEM;
> + goto out_free_str;
> + }
> +
> + for (src = str; (slash = strchr(src, '/')); src = slash + 1) {
> + ll->len = slash - src;
> + memcpy(ll->str, src, ll->len);
> + ll = lookup_list_next(ll);
> + }
> + ll->len = 0;
> +
> + rv = 0;
> +
> +out_free_str:
> + if (str != str0) {
> + kfree(str);
> + }
> +out:
> + return rv;
> +}
> +
> static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
> {
> struct proc_fs_context *ctx = fc->fs_private;
> @@ -137,6 +200,11 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
> return -EINVAL;
> break;
>
> + case Opt_lookup:
> + if (proc_parse_lookup_param(fc, param->string) < 0)
> + return -EINVAL;
> + break;
> +
> default:
> return -EINVAL;
> }
> @@ -157,6 +225,10 @@ static void proc_apply_options(struct proc_fs_info *fs_info,
> fs_info->hide_pid = ctx->hidepid;
> if (ctx->mask & (1 << Opt_subset))
> fs_info->pidonly = ctx->pidonly;
> + if (ctx->mask & (1 << Opt_lookup)) {
> + fs_info->lookup_list = ctx->lookup_list;
> + ctx->lookup_list = NULL;
> + }
> }
>
> static int proc_fill_super(struct super_block *s, struct fs_context *fc)
> @@ -218,6 +290,14 @@ static int proc_reconfigure(struct fs_context *fc)
> struct super_block *sb = fc->root->d_sb;
> struct proc_fs_info *fs_info = proc_sb_info(sb);
>
> + /*
> + * "Hide everything" lookup filter is not a problem as only
> + * /proc/${pid}, /proc/self and /proc/thread-self are accessible.
> + */
> + if (fs_info->lookup_list && fs_info->lookup_list->len > 0) {
> + return invalfc(fc, "'-o remount,lookup=' is unsupported, unmount and mount instead");
> + }
> +
> sync_filesystem(sb);
>
> proc_apply_options(fs_info, fc, current_user_ns());
> @@ -234,11 +314,34 @@ static void proc_fs_context_free(struct fs_context *fc)
> struct proc_fs_context *ctx = fc->fs_private;
>
> put_pid_ns(ctx->pid_ns);
> + kfree(ctx->lookup_list);
> kfree(ctx);
> }
>
> +static int proc_fs_context_dup(struct fs_context *fc, struct fs_context *src_fc)
> +{
> + struct proc_fs_context *src = fc->fs_private;
> + struct proc_fs_context *dst;
> +
> + dst = kmemdup(src, sizeof(struct proc_fs_context), GFP_KERNEL);
> + if (!dst) {
> + return -ENOMEM;
> + }
> +
> + dst->lookup_list = kmemdup(dst->lookup_list, dst->lookup_list_len, GFP_KERNEL);
> + if (!dst->lookup_list) {
> + kfree(dst);
> + return -ENOMEM;
> + }
> + get_pid_ns(dst->pid_ns);
> +
> + fc->fs_private = dst;
> + return 0;
> +}
> +
> static const struct fs_context_operations proc_fs_context_ops = {
> .free = proc_fs_context_free,
> + .dup = proc_fs_context_dup,
> .parse_param = proc_parse_param,
> .get_tree = proc_get_tree,
> .reconfigure = proc_reconfigure,
> @@ -274,6 +377,7 @@ static void proc_kill_sb(struct super_block *sb)
>
> kill_anon_super(sb);
> put_pid_ns(fs_info->pid_ns);
> + kfree(fs_info->lookup_list);
> kfree(fs_info);
> }
>
> @@ -317,12 +421,33 @@ static int proc_root_getattr(struct user_namespace *mnt_userns,
> return 0;
> }
>
> +bool in_lookup_list(const struct proc_lookup_list *ll, const char *str, unsigned int len)
> +{
> + if (ll) {
> + for (; ll->len > 0; ll = lookup_list_next(ll)) {
> + if (ll->len == len && strncmp(ll->str, str, len) == 0) {
> + return true;
> + }
> + }
> + return false;
> + } else {
> + return true;
> + }
> +}
> +
> static struct dentry *proc_root_lookup(struct inode * dir, struct dentry * dentry, unsigned int flags)
> {
> + struct proc_fs_info *proc_sb = proc_sb_info(dir->i_sb);
> +
> if (!proc_pid_lookup(dentry, flags))
> return NULL;
>
> - return proc_lookup(dir, dentry, flags);
> + if (in_lookup_list(proc_sb->lookup_list, dentry->d_name.name, dentry->d_name.len)) {
> + return proc_lookup(dir, dentry, flags);
> + } else {
> + return NULL;
> + }
> +
> }
>
> static int proc_root_readdir(struct file *file, struct dir_context *ctx)
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index 069c7fd95396..d2c067560bf9 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -10,6 +10,7 @@
> #include <linux/fs.h>
>
> struct proc_dir_entry;
> +struct proc_lookup_list;
> struct seq_file;
> struct seq_operations;
>
> @@ -65,6 +66,7 @@ struct proc_fs_info {
> kgid_t pid_gid;
> enum proc_hidepid hide_pid;
> enum proc_pidonly pidonly;
> + const struct proc_lookup_list *lookup_list;
> };
>
> static inline struct proc_fs_info *proc_sb_info(struct super_block *sb)
> --
> 2.31.1
>

--
Rgrds, legion

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