Re: [PATCH] Allow restricting permissions in /proc/sys

On 13.11.2019 6.50, Andy Lutomirski wrote:

On Tue, Nov 12, 2019 at 3:22 PM Christian Brauner
[off-list ref] wrote:

quoted

[Cc+ linux-api@vger.kernel.org]

since that's potentially relevant to quite a few people.

On Sun, Nov 03, 2019 at 04:55:48PM +0200, Topi Miettinen wrote:

quoted

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 <redacted>
---
  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;

Supporting at least ATTR_GID would make this much more useful.

Yes, also XATTR/ACL support would be useful. But so far I've tried to allow only tightening of permissions.

quoted

quoted

+       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;

Style nit: please put braces around multi-line if and else branches,
even if they're only multi-line because of comments.

OK, thanks.

quoted

quoted

+
+               sysctl_head_finish(head);
+
+               /* Execute bits only allowed for directories */
+               if (!S_ISDIR(inode->i_mode))
+                       max_mode &= ~0111;

Why is this needed?

In general, /proc/sys does not allow executable permissions for the files, so I've continued this policy.

-Topi