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Subject[PATCH] 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>
---
fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d80989b6c344..88c4ca7d2782 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);
@@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry,
struct iattr *attr)
struct inode *inode = d_inode(dentry);
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) {
+ struct ctl_table_header *head = grab_header(inode);
+ struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+ 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;
+
+ sysctl_head_finish(head);
+
+ /* 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;
@@ -853,17 +883,8 @@ static int proc_sys_getattr(const struct path
*path, struct kstat *stat,
u32 request_mask, unsigned int query_flags)
{
struct inode *inode = d_inode(path->dentry);
- struct ctl_table_header *head = grab_header(inode);
- struct ctl_table *table = PROC_I(inode)->sysctl_entry;
-
- if (IS_ERR(head))
- return PTR_ERR(head);

generic_fillattr(inode, stat);
- if (table)
- stat->mode = (stat->mode & S_IFMT) | table->mode;
-
- sysctl_head_finish(head);
return 0;
}

--
2.24.0.rc1
From 14ad2d9034ecb43b60f59f6422e597a780c65cd9 Mon Sep 17 00:00:00 2001
From: Topi Miettinen <toiwoton@gmail.com>
Date: Sun, 3 Nov 2019 16:36:43 +0200
Subject: [PATCH] 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 | 41 +++++++++++++++++++++++++++++++----------
1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d80989b6c344..88c4ca7d2782 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);
@@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
struct inode *inode = d_inode(dentry);
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) {
+ struct ctl_table_header *head = grab_header(inode);
+ struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+ 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;
+
+ sysctl_head_finish(head);
+
+ /* 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;
@@ -853,17 +883,8 @@ static int proc_sys_getattr(const struct path *path, struct kstat *stat,
u32 request_mask, unsigned int query_flags)
{
struct inode *inode = d_inode(path->dentry);
- struct ctl_table_header *head = grab_header(inode);
- struct ctl_table *table = PROC_I(inode)->sysctl_entry;
-
- if (IS_ERR(head))
- return PTR_ERR(head);

generic_fillattr(inode, stat);
- if (table)
- stat->mode = (stat->mode & S_IFMT) | table->mode;
-
- sysctl_head_finish(head);
return 0;
}

--
2.24.0.rc1
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