| Message ID | 20250210194512.417339-4-mszeredi@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [1/5] ovl: don't allow datadir only | expand |
> Subject: [PATCH] ovl: don't require metacopy=on for lower -> data redirect Nit: the change does not require metacopy=on nor redirect_dir=follow so maybe some short title like: ovl: relax redirect/metacopy requirements for lower -> data redirect > Allow the special case of a redirect from a lower layer to a data layer > without having to turn on metacopy. This makes the feature work with > userxattr, which in turn allows data layers to be usable in user > namespaces. > > Minimize the risk by only enabling redirect from a single lower layer to a > data layer iff a data layer is specified. The only way to access a data > layer is to enable this, so there's really no reason no to enable this. > > This can be used safely if the lower layer is read-only and the > user.overlay.redirect xattr cannot be modified. > Just need to verify that those assumptions are not broken by upper index with metacopy/redirect - I think the safety of redirect still holds, but not the safety of the verity digest. Thanks, Amir. > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> > --- > Documentation/filesystems/overlayfs.rst | 7 ++++++ > fs/overlayfs/namei.c | 32 ++++++++++++++----------- > fs/overlayfs/params.c | 5 ---- > 3 files changed, 25 insertions(+), 19 deletions(-) > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > index 6245b67ae9e0..5d277d79cf2f 100644 > --- a/Documentation/filesystems/overlayfs.rst > +++ b/Documentation/filesystems/overlayfs.rst > @@ -429,6 +429,13 @@ Only the data of the files in the "data-only" lower layers may be visible > when a "metacopy" file in one of the lower layers above it, has a "redirect" > to the absolute path of the "lower data" file in the "data-only" lower layer. > > +Instead of explicitly enabling "metacopy=on" it is sufficient to specify at > +least one data-only layer to enable redirection of data to a data-only layer. > +In this case other forms of metacopy are rejected. Note: this way data-only > +layers may be used toghether with "userxattr", in which case careful attention > +must be given to privileges needed to change the "user.overlay.redirect" xattr > +to prevent misuse. > + > Since kernel version v6.8, "data-only" lower layers can also be added using > the "datadir+" mount options and the fsconfig syscall from new mount api. > For example:: > diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c > index da322e9768d1..f9dc71b70beb 100644 > --- a/fs/overlayfs/namei.c > +++ b/fs/overlayfs/namei.c > @@ -1042,6 +1042,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, > char *upperredirect = NULL; > bool nextredirect = false; > bool nextmetacopy = false; > + bool check_redirect = (ovl_redirect_follow(ofs) || ofs->numdatalayer); > struct dentry *this; > unsigned int i; > int err; > @@ -1053,7 +1054,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, > .is_dir = false, > .opaque = false, > .stop = false, > - .last = ovl_redirect_follow(ofs) ? false : !ovl_numlower(poe), > + .last = check_redirect ? false : !ovl_numlower(poe), > .redirect = NULL, > .metacopy = 0, > }; > @@ -1141,7 +1142,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, > goto out_put; > } > > - if (!ovl_redirect_follow(ofs)) > + if (!check_redirect) > d.last = i == ovl_numlower(poe) - 1; > else if (d.is_dir || !ofs->numdatalayer) > d.last = lower.layer->idx == ovl_numlower(roe); > @@ -1222,21 +1223,24 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, > } > } > > - /* Defer lookup of lowerdata in data-only layers to first access */ > + /* > + * Defer lookup of lowerdata in data-only layers to first access. > + * Don't require redirect=follow and metacopy=on in this case. > + */ > if (d.metacopy && ctr && ofs->numdatalayer && d.absolute_redirect) { > d.metacopy = 0; > ctr++; > - } > - > - if (nextmetacopy && !ofs->config.metacopy) { > - pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry); > - err = -EPERM; > - goto out_put; > - } > - if (nextredirect && !ovl_redirect_follow(ofs)) { > - pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n", dentry); > - err = -EPERM; > - goto out_put; > + } else { > + if (nextmetacopy && !ofs->config.metacopy) { > + pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry); > + err = -EPERM; > + goto out_put; > + } > + if (nextredirect && !ovl_redirect_follow(ofs)) { > + pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n", dentry); > + err = -EPERM; > + goto out_put; > + } > } > > /* > diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c > index 1115c22deca0..54468b2b0fba 100644 > --- a/fs/overlayfs/params.c > +++ b/fs/overlayfs/params.c > @@ -1000,11 +1000,6 @@ int ovl_fs_params_verify(const struct ovl_fs_context *ctx, > */ > } > > - if (ctx->nr_data > 0 && !config->metacopy) { > - pr_err("lower data-only dirs require metacopy support.\n"); > - return -EINVAL; > - } > - > return 0; > } > > -- > 2.48.1 >
diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index 6245b67ae9e0..5d277d79cf2f 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -429,6 +429,13 @@ Only the data of the files in the "data-only" lower layers may be visible when a "metacopy" file in one of the lower layers above it, has a "redirect" to the absolute path of the "lower data" file in the "data-only" lower layer. +Instead of explicitly enabling "metacopy=on" it is sufficient to specify at +least one data-only layer to enable redirection of data to a data-only layer. +In this case other forms of metacopy are rejected. Note: this way data-only +layers may be used toghether with "userxattr", in which case careful attention +must be given to privileges needed to change the "user.overlay.redirect" xattr +to prevent misuse. + Since kernel version v6.8, "data-only" lower layers can also be added using the "datadir+" mount options and the fsconfig syscall from new mount api. For example:: diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c index da322e9768d1..f9dc71b70beb 100644 --- a/fs/overlayfs/namei.c +++ b/fs/overlayfs/namei.c @@ -1042,6 +1042,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, char *upperredirect = NULL; bool nextredirect = false; bool nextmetacopy = false; + bool check_redirect = (ovl_redirect_follow(ofs) || ofs->numdatalayer); struct dentry *this; unsigned int i; int err; @@ -1053,7 +1054,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, .is_dir = false, .opaque = false, .stop = false, - .last = ovl_redirect_follow(ofs) ? false : !ovl_numlower(poe), + .last = check_redirect ? false : !ovl_numlower(poe), .redirect = NULL, .metacopy = 0, }; @@ -1141,7 +1142,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, goto out_put; } - if (!ovl_redirect_follow(ofs)) + if (!check_redirect) d.last = i == ovl_numlower(poe) - 1; else if (d.is_dir || !ofs->numdatalayer) d.last = lower.layer->idx == ovl_numlower(roe); @@ -1222,21 +1223,24 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry, } } - /* Defer lookup of lowerdata in data-only layers to first access */ + /* + * Defer lookup of lowerdata in data-only layers to first access. + * Don't require redirect=follow and metacopy=on in this case. + */ if (d.metacopy && ctr && ofs->numdatalayer && d.absolute_redirect) { d.metacopy = 0; ctr++; - } - - if (nextmetacopy && !ofs->config.metacopy) { - pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry); - err = -EPERM; - goto out_put; - } - if (nextredirect && !ovl_redirect_follow(ofs)) { - pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n", dentry); - err = -EPERM; - goto out_put; + } else { + if (nextmetacopy && !ofs->config.metacopy) { + pr_warn_ratelimited("refusing to follow metacopy origin for (%pd2)\n", dentry); + err = -EPERM; + goto out_put; + } + if (nextredirect && !ovl_redirect_follow(ofs)) { + pr_warn_ratelimited("refusing to follow redirect for (%pd2)\n", dentry); + err = -EPERM; + goto out_put; + } } /* diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c index 1115c22deca0..54468b2b0fba 100644 --- a/fs/overlayfs/params.c +++ b/fs/overlayfs/params.c @@ -1000,11 +1000,6 @@ int ovl_fs_params_verify(const struct ovl_fs_context *ctx, */ } - if (ctx->nr_data > 0 && !config->metacopy) { - pr_err("lower data-only dirs require metacopy support.\n"); - return -EINVAL; - } - return 0; }
Allow the special case of a redirect from a lower layer to a data layer without having to turn on metacopy. This makes the feature work with userxattr, which in turn allows data layers to be usable in user namespaces. Minimize the risk by only enabling redirect from a single lower layer to a data layer iff a data layer is specified. The only way to access a data layer is to enable this, so there's really no reason no to enable this. This can be used safely if the lower layer is read-only and the user.overlay.redirect xattr cannot be modified. Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> --- Documentation/filesystems/overlayfs.rst | 7 ++++++ fs/overlayfs/namei.c | 32 ++++++++++++++----------- fs/overlayfs/params.c | 5 ---- 3 files changed, 25 insertions(+), 19 deletions(-)