• Antonio Murdaca's avatar
    ovl: override creds with the ones from the superblock mounter · 3fe6e52f
    Antonio Murdaca authored
    In user namespace the whiteout creation fails with -EPERM because the
    current process isn't capable(CAP_SYS_ADMIN) when setting xattr.
    
    A simple reproducer:
    
    $ mkdir upper lower work merged lower/dir
    $ sudo mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merged
    $ unshare -m -p -f -U -r bash
    
    Now as root in the user namespace:
    
    \# touch merged/dir/{1,2,3} # this will force a copy up of lower/dir
    \# rm -fR merged/*
    
    This ends up failing with -EPERM after the files in dir has been
    correctly deleted:
    
    unlinkat(4, "2", 0)                     = 0
    unlinkat(4, "1", 0)                     = 0
    unlinkat(4, "3", 0)                     = 0
    close(4)                                = 0
    unlinkat(AT_FDCWD, "merged/dir", AT_REMOVEDIR) = -1 EPERM (Operation not
    permitted)
    
    Interestingly, if you don't place files in merged/dir you can remove it,
    meaning if upper/dir does not exist, creating the char device file works
    properly in that same location.
    
    This patch uses ovl_sb_creator_cred() to get the cred struct from the
    superblock mounter and override the old cred with these new ones so that
    the whiteout creation is possible because overlay is wrong in assuming that
    the creds it will get with prepare_creds will be in the initial user
    namespace.  The old cap_raise game is removed in favor of just overriding
    the old cred struct.
    
    This patch also drops from ovl_copy_up_one() the following two lines:
    
    override_cred->fsuid = stat->uid;
    override_cred->fsgid = stat->gid;
    
    This is because the correct uid and gid are taken directly with the stat
    struct and correctly set with ovl_set_attr().
    Signed-off-by: default avatarAntonio Murdaca <runcom@redhat.com>
    Signed-off-by: default avatarMiklos Szeredi <mszeredi@redhat.com>
    3fe6e52f
super.c 25.9 KB