Re: [PATCH v14 03/22] vfs: Add MAY_DELETE_SELF and MAY_DELETE_CHILD permission flags

Andreas Dilger
2015-11-06T21:26:45Z

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On Nov 5, 2015, at 4:39 AM, Andreas Gruenbacher <agruenba@redhat.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@redhat.com>
> Reviewed-by: J. Bruce Fields <bfields@redhat.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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