[04/11] fs: Don't remove suid for CAP_FSETID for userns root

Submitted by Dongsu Park on Dec. 22, 2017, 2:32 p.m.

Details

Message ID ddf1fb9b5001e633e0022dee7fecb0ef431e851f.1512041070.git.dongsu@kinvolk.io
State New
Series "FUSE mounts from non-init user namespaces"
Headers show

Commit Message

Dongsu Park Dec. 22, 2017, 2:32 p.m.
From: Seth Forshee <seth.forshee@canonical.com>

Expand the check in should_remove_suid() to keep privileges for
CAP_FSETID in s_user_ns rather than init_user_ns.

Patch v4 is available: https://patchwork.kernel.org/patch/8944621/

--EWB Changed from ns_capable(sb->s_user_ns, ) to capable_wrt_inode_uidgid

Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
---
 fs/inode.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Patch hide | download patch | download mbox

diff --git a/fs/inode.c b/fs/inode.c
index fd401028..6459a437 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1749,7 +1749,8 @@  EXPORT_SYMBOL(touch_atime);
  */
 int should_remove_suid(struct dentry *dentry)
 {
-	umode_t mode = d_inode(dentry)->i_mode;
+	struct inode *inode = d_inode(dentry);
+	umode_t mode = inode->i_mode;
 	int kill = 0;
 
 	/* suid always must be killed */
@@ -1763,7 +1764,8 @@  int should_remove_suid(struct dentry *dentry)
 	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
 		kill |= ATTR_KILL_SGID;
 
-	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
+	if (unlikely(kill && !capable_wrt_inode_uidgid(inode, CAP_FSETID) &&
+		     S_ISREG(mode)))
 		return kill;
 
 	return 0;

Comments

Serge E. Hallyn Dec. 23, 2017, 3:26 a.m.
On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
> From: Seth Forshee <seth.forshee@canonical.com>
> 
> Expand the check in should_remove_suid() to keep privileges for

I realize this description came from Seth, but reading it now,
'Expand' seems wrong.  Expanding a check brings to my mind making
it stricter, not looser.  How about 'Relax the check' ?

> CAP_FSETID in s_user_ns rather than init_user_ns.
> 
> Patch v4 is available: https://patchwork.kernel.org/patch/8944621/
> 
> --EWB Changed from ns_capable(sb->s_user_ns, ) to capable_wrt_inode_uidgid

Why exactly?

This is wrong, because capable_wrt_inode_uidgid() does a check
against current_user_ns, not the  inode->i_sb->s_user_ns

> 
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Serge Hallyn <serge@hallyn.com>
> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
> ---
>  fs/inode.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index fd401028..6459a437 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1749,7 +1749,8 @@ EXPORT_SYMBOL(touch_atime);
>   */
>  int should_remove_suid(struct dentry *dentry)
>  {
> -	umode_t mode = d_inode(dentry)->i_mode;
> +	struct inode *inode = d_inode(dentry);
> +	umode_t mode = inode->i_mode;
>  	int kill = 0;
>  
>  	/* suid always must be killed */
> @@ -1763,7 +1764,8 @@ int should_remove_suid(struct dentry *dentry)
>  	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
>  		kill |= ATTR_KILL_SGID;
>  
> -	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
> +	if (unlikely(kill && !capable_wrt_inode_uidgid(inode, CAP_FSETID) &&
> +		     S_ISREG(mode)))
>  		return kill;
>  
>  	return 0;
> -- 
> 2.13.6
Dongsu Park Dec. 23, 2017, 12:38 p.m.
Hi,

On Sat, Dec 23, 2017 at 4:26 AM, Serge E. Hallyn <serge@hallyn.com> wrote:
> On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
>> From: Seth Forshee <seth.forshee@canonical.com>
>>
>> Expand the check in should_remove_suid() to keep privileges for
>
> I realize this description came from Seth, but reading it now,
> 'Expand' seems wrong.  Expanding a check brings to my mind making
> it stricter, not looser.  How about 'Relax the check' ?

Makes sense. Will do.

>> CAP_FSETID in s_user_ns rather than init_user_ns.
>>
>> Patch v4 is available: https://patchwork.kernel.org/patch/8944621/
>>
>> --EWB Changed from ns_capable(sb->s_user_ns, ) to capable_wrt_inode_uidgid
>
> Why exactly?
>
> This is wrong, because capable_wrt_inode_uidgid() does a check
> against current_user_ns, not the  inode->i_sb->s_user_ns

Ah. I see.
I suppose it was changed probably for the privileged_wrt_inode_uidgid()
called by capable_wrt_inode_uidgid(). But as you pointed out, that checks
against current_user_ns, which is wrong. I would just create another
wrapper like capable_userns_wrt_inode_uidgid(), which takes an
additional parameter of (struct user_namespace *), to be able to check for
both ns_capable() and privileged_wrt_inode_uidgid().

Thanks,
Dongsu

>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Cc: Serge Hallyn <serge@hallyn.com>
>> Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
>> Signed-off-by: Dongsu Park <dongsu@kinvolk.io>
>> ---
>>  fs/inode.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/inode.c b/fs/inode.c
>> index fd401028..6459a437 100644
>> --- a/fs/inode.c
>> +++ b/fs/inode.c
>> @@ -1749,7 +1749,8 @@ EXPORT_SYMBOL(touch_atime);
>>   */
>>  int should_remove_suid(struct dentry *dentry)
>>  {
>> -     umode_t mode = d_inode(dentry)->i_mode;
>> +     struct inode *inode = d_inode(dentry);
>> +     umode_t mode = inode->i_mode;
>>       int kill = 0;
>>
>>       /* suid always must be killed */
>> @@ -1763,7 +1764,8 @@ int should_remove_suid(struct dentry *dentry)
>>       if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
>>               kill |= ATTR_KILL_SGID;
>>
>> -     if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
>> +     if (unlikely(kill && !capable_wrt_inode_uidgid(inode, CAP_FSETID) &&
>> +                  S_ISREG(mode)))
>>               return kill;
>>
>>       return 0;
>> --
>> 2.13.6