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

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