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

[Miklos Szeredi]

On Sat, Dec 23, 2017 at 1:38 PM, Dongsu Park  wrote:
> Hi,
>
> On Sat, Dec 23, 2017 at 4:26 AM, Serge E. Hallyn  wrote:
>> On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
>>> From: Seth Forshee 
>>>
>>> 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

I'm thoroughly confused.   s_user_ns is supposed to be about the
usernamespace the filesystem perceives to be in, right?  How does that
come into play when checking permissions to do something?

Thanks,
Miklos

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

[Miklos Szeredi]

On Sat, Dec 23, 2017 at 1:38 PM, Dongsu Park  wrote:
> Hi,
>
> On Sat, Dec 23, 2017 at 4:26 AM, Serge E. Hallyn  wrote:
>> On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
>>> From: Seth Forshee 
>>>
>>> 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

I'm thoroughly confused.   s_user_ns is supposed to be about the
usernamespace the filesystem perceives to be in, right?  How does that
come into play when checking permissions to do something?

Thanks,
Miklos

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

[Dongsu Park]

Hi,

On Sat, Dec 23, 2017 at 4:26 AM, Serge E. Hallyn  wrote:
> On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
>> From: Seth Forshee 
>>
>> 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-fsde...@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: Alexander Viro 
>> Cc: Serge Hallyn 
>> Signed-off-by: Seth Forshee 
>> Signed-off-by: Dongsu Park 
>> ---
>>  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

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

[Dongsu Park]

Hi,

On Sat, Dec 23, 2017 at 4:26 AM, Serge E. Hallyn  wrote:
> On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
>> From: Seth Forshee 
>>
>> 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-fsde...@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: Alexander Viro 
>> Cc: Serge Hallyn 
>> Signed-off-by: Seth Forshee 
>> Signed-off-by: Dongsu Park 
>> ---
>>  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

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

[Serge E. Hallyn]

On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
> From: Seth Forshee 
> 
> 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-fsde...@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Alexander Viro 
> Cc: Serge Hallyn 
> Signed-off-by: Seth Forshee 
> Signed-off-by: Dongsu Park 
> ---
>  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

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

[Serge E. Hallyn]

On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
> From: Seth Forshee 
> 
> 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-fsde...@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: Alexander Viro 
> Cc: Serge Hallyn 
> Signed-off-by: Seth Forshee 
> Signed-off-by: Dongsu Park 
> ---
>  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