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Subject[PATCH] proc: Allow restricting permissions in /proc/sys
Date
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@gmail.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@gmail.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@gmail.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
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