linux-ext4 - Re: [PATCH v14 03/22] vfs: Add MAY_DELETE_SELF and MAY_DELETE

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Date:	Fri, 6 Nov 2015 14:26:45 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Andreas Gruenbacher <agruenba@...hat.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	Dave Chinner <david@...morbit.com>, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v14 03/22] vfs: Add MAY_DELETE_SELF and MAY_DELETE_CHILD permission flags

On Nov 5, 2015, at 4:39 AM, Andreas Gruenbacher <agruenba@...hat.com> wrote:
> 
> Normally, deleting a file requires MAY_WRITE access to the parent
> directory.  With richacls, a file may be deleted with MAY_DELETE_CHILD access
> to the parent directory or with MAY_DELETE_SELF access to the file.
> 
> To support that, pass the MAY_DELETE_CHILD mask flag to inode_permission()
> when checking for delete access inside a directory, and MAY_DELETE_SELF
> when checking for delete access to a file itelf.
> 
> The MAY_DELETE_SELF permission overrides the sticky directory check.
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
> Reviewed-by: J. Bruce Fields <bfields@...hat.com>
> ---
> fs/namei.c         | 21 ++++++++++++---------
> include/linux/fs.h |  2 ++
> 2 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 0259392..2eab19e 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -453,9 +453,9 @@ static int sb_permission(struct super_block *sb, struct inode *inode, int mask)
>  * this, letting us set arbitrary permissions for filesystem access without
>  * changing the "normal" UIDs which are used for other things.
>  *
> - * MAY_WRITE must be set in @mask whenever MAY_APPEND, MAY_CREATE_FILE, or
> - * MAY_CREATE_DIR are set.  That way, file systems that don't support these
> - * permissions will check for MAY_WRITE instead.
> + * MAY_WRITE must be set in @mask whenever MAY_APPEND, MAY_CREATE_FILE,
> + * MAY_CREATE_DIR, or MAY_DELETE_CHILD are set.  That way, file systems that
> + * don't support these permissions will check for MAY_WRITE instead.
>  */
> int inode_permission(struct inode *inode, int mask)
> {
> @@ -2555,7 +2555,7 @@ static int may_delete(struct inode *dir, struct dentry *victim,
> 		      bool isdir, bool replace)
> {
> 	struct inode *inode = d_backing_inode(victim);
> -	int error, mask = MAY_WRITE | MAY_EXEC;
> +	int error, mask = MAY_EXEC;
> 
> 	if (d_is_negative(victim))
> 		return -ENOENT;
> @@ -2565,15 +2565,18 @@ static int may_delete(struct inode *dir, struct dentry *victim,
> 	audit_inode_child(dir, victim, AUDIT_TYPE_CHILD_DELETE);
> 
> 	if (replace)
> -		mask |= isdir ? MAY_CREATE_DIR : MAY_CREATE_FILE;
> -	error = inode_permission(dir, mask);
> +		mask |= MAY_WRITE | (isdir ? MAY_CREATE_DIR : MAY_CREATE_FILE);
> +	error = inode_permission(dir, mask | MAY_WRITE | MAY_DELETE_CHILD);
> +	if (!error && check_sticky(dir, inode))
> +		error = -EPERM;
> +	if (error && IS_RICHACL(inode) &&
> +	    inode_permission(inode, MAY_DELETE_SELF) == 0)
> +		error = 0;

This looks like a serious bug, as it is overriding other errors returned
from inode_permission() such as -EROFS from sb_permission() or even from
generic_permission->acl_permission_check().  Clearing the error returned
by an earlier check doesn't seem safe, only new errors should be added.

The call to inode_permission(inode) is also duplicating the sb_permission()
check from inode_permission(dir), so at most should be __inode_permission().

It looks like this would be correct if you check MAY_DELETE_SELF together
with check_sticky():

	if (!error && check_sticky(dir, inode) &&
	    !__inode_permission(inode, MAY_DELETE_SELF))
		error = -EPERM;

I ommitted the IS_RICHACL() check here, since that should probably be in
__inode_permission() when MAY_DELETE_SELF is passed?

Cheers, Andreas

> 	if (error)
> 		return error;
> 	if (IS_APPEND(dir))
> 		return -EPERM;

> -
> -	if (check_sticky(dir, inode) || IS_APPEND(inode) ||
> -	    IS_IMMUTABLE(inode) || IS_SWAPFILE(inode))
> +	if (IS_APPEND(inode) || IS_IMMUTABLE(inode) || IS_SWAPFILE(inode))
> 		return -EPERM;

> 	if (isdir) {
> 		if (!d_is_dir(victim))
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index d6e2330..402acd7 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -84,6 +84,8 @@ typedef void (dax_iodone_t)(struct buffer_head *bh_map, int uptodate);
> #define MAY_NOT_BLOCK		0x00000080
> #define MAY_CREATE_FILE		0x00000100
> #define MAY_CREATE_DIR		0x00000200
> +#define MAY_DELETE_CHILD	0x00000400
> +#define MAY_DELETE_SELF		0x00000800
> 
> /*
>  * flags in file.f_mode.  Note that FMODE_READ and FMODE_WRITE must correspond
> --
> 2.5.0
> 


Cheers, Andreas






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