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SubjectRe: [PATCH] Allow restricting permissions in /proc/sys
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On 13.11.2019 6.50, Andy Lutomirski wrote:
> On Tue, Nov 12, 2019 at 3:22 PM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
>>
>> [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:
>>> 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;
>
> 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.

>>>
>>> + 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.

>>> +
>>> + 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

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