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Message-ID: <201911121523.9C097E7D2C@keescook>
Date: Tue, 12 Nov 2019 15:25:23 -0800
From: Kees Cook <keescook@...omium.org>
To: Topi Miettinen <toiwoton@...il.com>
Cc: Luis Chamberlain <mcgrof@...nel.org>,
Alexey Dobriyan <adobriyan@...il.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"open list:FILESYSTEMS (VFS and infrastructure)"
<linux-fsdevel@...r.kernel.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] proc: Allow restricting permissions in /proc/sys
Ah! I see the v2 here now. :) Can you please include that in your
Subject next time, as "[PATCH v2] proc: Allow restricting permissions
in /proc/sys"? Also, can you adjust your MUA to not send a duplicate
attachment? The patch inline is fine.
Please CC akpm as well, since I think this should likely go through the
-mm tree.
Eric, do you have any other thoughts on this?
Thanks!
-Kees
On Mon, Nov 04, 2019 at 02:07:29PM +0200, Topi Miettinen wrote:
> Several items in /proc/sys need not be accessible to unprivileged
> tasks. Let the system administrator change the permissions, but only
> to more restrictive modes than what the sysctl tables allow.
>
> Signed-off-by: Topi Miettinen <toiwoton@...il.com>
> ---
> v2: actually keep track of changed permissions instead of relying on inode
> cache
> ---
> fs/proc/proc_sysctl.c | 42 ++++++++++++++++++++++++++++++++++++++----
> include/linux/sysctl.h | 1 +
> 2 files changed, 39 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index d80989b6c344..1f75382c49fd 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int
> mask)
> if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
> return -EACCES;
>
> + error = generic_permission(inode, mask);
> + if (error)
> + return error;
> +
> head = grab_header(inode);
> if (IS_ERR(head))
> return PTR_ERR(head);
> @@ -835,17 +839,46 @@ static int proc_sys_permission(struct inode *inode,
> int mask)
> static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
> {
> struct inode *inode = d_inode(dentry);
> + struct ctl_table_header *head = grab_header(inode);
> + struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> int error;
>
> - if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
> + if (attr->ia_valid & (ATTR_UID | ATTR_GID))
> return -EPERM;
>
> + if (attr->ia_valid & ATTR_MODE) {
> + umode_t max_mode = 0777; /* Only these bits may change */
> +
> + if (IS_ERR(head))
> + return PTR_ERR(head);
> +
> + if (!table) /* global root - r-xr-xr-x */
> + max_mode &= ~0222;
> + else /*
> + * Don't allow permissions to become less
> + * restrictive than the sysctl table entry
> + */
> + max_mode &= table->mode;
> +
> + /* Execute bits only allowed for directories */
> + if (!S_ISDIR(inode->i_mode))
> + max_mode &= ~0111;
> +
> + if (attr->ia_mode & ~S_IFMT & ~max_mode)
> + return -EPERM;
> + }
> +
> error = setattr_prepare(dentry, attr);
> if (error)
> return error;
>
> setattr_copy(inode, attr);
> mark_inode_dirty(inode);
> +
> + if (table)
> + table->current_mode = inode->i_mode;
> + sysctl_head_finish(head);
> +
> return 0;
> }
>
> @@ -861,7 +894,7 @@ static int proc_sys_getattr(const struct path *path,
> struct kstat *stat,
>
> generic_fillattr(inode, stat);
> if (table)
> - stat->mode = (stat->mode & S_IFMT) | table->mode;
> + stat->mode = (stat->mode & S_IFMT) | table->current_mode;
>
> sysctl_head_finish(head);
> return 0;
> @@ -981,7 +1014,7 @@ static struct ctl_dir *new_dir(struct ctl_table_set
> *set,
> memcpy(new_name, name, namelen);
> new_name[namelen] = '\0';
> table[0].procname = new_name;
> - table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
> + table[0].current_mode = table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
> init_header(&new->header, set->dir.header.root, set, node, table);
>
> return new;
> @@ -1155,6 +1188,7 @@ static int sysctl_check_table(const char *path, struct
> ctl_table *table)
> if ((table->mode & (S_IRUGO|S_IWUGO)) != table->mode)
> err |= sysctl_err(path, table, "bogus .mode 0%o",
> table->mode);
> + table->current_mode = table->mode;
> }
> return err;
> }
> @@ -1192,7 +1226,7 @@ static struct ctl_table_header *new_links(struct
> ctl_dir *dir, struct ctl_table
> int len = strlen(entry->procname) + 1;
> memcpy(link_name, entry->procname, len);
> link->procname = link_name;
> - link->mode = S_IFLNK|S_IRWXUGO;
> + link->current_mode = link->mode = S_IFLNK|S_IRWXUGO;
> link->data = link_root;
> link_name += len;
> }
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 6df477329b76..7c519c35bf9c 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -126,6 +126,7 @@ struct ctl_table
> void *data;
> int maxlen;
> umode_t mode;
> + umode_t current_mode;
> struct ctl_table *child; /* Deprecated */
> proc_handler *proc_handler; /* Callback for text formatting */
> struct ctl_table_poll *poll;
> --
> 2.24.0.rc1
>
> From 3cde64e0aa2734c335355ee6d0d9f12c1f1e8a87 Mon Sep 17 00:00:00 2001
> From: Topi Miettinen <toiwoton@...il.com>
> Date: Sun, 3 Nov 2019 16:36:43 +0200
> Subject: [PATCH] proc: Allow restricting permissions in /proc/sys
>
> Several items in /proc/sys need not be accessible to unprivileged
> tasks. Let the system administrator change the permissions, but only
> to more restrictive modes than what the sysctl tables allow.
>
> Signed-off-by: Topi Miettinen <toiwoton@...il.com>
> ---
> fs/proc/proc_sysctl.c | 42 ++++++++++++++++++++++++++++++++++++++----
> include/linux/sysctl.h | 1 +
> 2 files changed, 39 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index d80989b6c344..1f75382c49fd 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int mask)
> if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
> return -EACCES;
>
> + error = generic_permission(inode, mask);
> + if (error)
> + return error;
> +
> head = grab_header(inode);
> if (IS_ERR(head))
> return PTR_ERR(head);
> @@ -835,17 +839,46 @@ static int proc_sys_permission(struct inode *inode, int mask)
> static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
> {
> struct inode *inode = d_inode(dentry);
> + struct ctl_table_header *head = grab_header(inode);
> + struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> int error;
>
> - if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
> + if (attr->ia_valid & (ATTR_UID | ATTR_GID))
> return -EPERM;
>
> + if (attr->ia_valid & ATTR_MODE) {
> + umode_t max_mode = 0777; /* Only these bits may change */
> +
> + if (IS_ERR(head))
> + return PTR_ERR(head);
> +
> + if (!table) /* global root - r-xr-xr-x */
> + max_mode &= ~0222;
> + else /*
> + * Don't allow permissions to become less
> + * restrictive than the sysctl table entry
> + */
> + max_mode &= table->mode;
> +
> + /* Execute bits only allowed for directories */
> + if (!S_ISDIR(inode->i_mode))
> + max_mode &= ~0111;
> +
> + if (attr->ia_mode & ~S_IFMT & ~max_mode)
> + return -EPERM;
> + }
> +
> error = setattr_prepare(dentry, attr);
> if (error)
> return error;
>
> setattr_copy(inode, attr);
> mark_inode_dirty(inode);
> +
> + if (table)
> + table->current_mode = inode->i_mode;
> + sysctl_head_finish(head);
> +
> return 0;
> }
>
> @@ -861,7 +894,7 @@ static int proc_sys_getattr(const struct path *path, struct kstat *stat,
>
> generic_fillattr(inode, stat);
> if (table)
> - stat->mode = (stat->mode & S_IFMT) | table->mode;
> + stat->mode = (stat->mode & S_IFMT) | table->current_mode;
>
> sysctl_head_finish(head);
> return 0;
> @@ -981,7 +1014,7 @@ static struct ctl_dir *new_dir(struct ctl_table_set *set,
> memcpy(new_name, name, namelen);
> new_name[namelen] = '\0';
> table[0].procname = new_name;
> - table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
> + table[0].current_mode = table[0].mode = S_IFDIR|S_IRUGO|S_IXUGO;
> init_header(&new->header, set->dir.header.root, set, node, table);
>
> return new;
> @@ -1155,6 +1188,7 @@ static int sysctl_check_table(const char *path, struct ctl_table *table)
> if ((table->mode & (S_IRUGO|S_IWUGO)) != table->mode)
> err |= sysctl_err(path, table, "bogus .mode 0%o",
> table->mode);
> + table->current_mode = table->mode;
> }
> return err;
> }
> @@ -1192,7 +1226,7 @@ static struct ctl_table_header *new_links(struct ctl_dir *dir, struct ctl_table
> int len = strlen(entry->procname) + 1;
> memcpy(link_name, entry->procname, len);
> link->procname = link_name;
> - link->mode = S_IFLNK|S_IRWXUGO;
> + link->current_mode = link->mode = S_IFLNK|S_IRWXUGO;
> link->data = link_root;
> link_name += len;
> }
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 6df477329b76..7c519c35bf9c 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -126,6 +126,7 @@ struct ctl_table
> void *data;
> int maxlen;
> umode_t mode;
> + umode_t current_mode;
> struct ctl_table *child; /* Deprecated */
> proc_handler *proc_handler; /* Callback for text formatting */
> struct ctl_table_poll *poll;
> --
> 2.24.0.rc1
>
--
Kees Cook
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