| Message ID | 20250219-work-overlayfs-v3-1-46af55e4ceda@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | ovl: add override_creds mount option | expand |
On Wed, Feb 19, 2025 at 11:02 AM Christian Brauner <brauner@kernel.org> wrote: > > Currently overlayfs uses the mounter's credentials for it's > override_creds() calls. That provides a consistent permission model. > > This patches allows a caller to instruct overlayfs to use its > credentials instead. The caller must be located in the same user > namespace hierarchy as the user namespace the overlayfs instance will be > mounted in. This provides a consistent and simple security model. > > With this it is possible to e.g., mount an overlayfs instance where the > mounter must have CAP_SYS_ADMIN but the credentials used for > override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage of > custom fs{g,u}id different from the callers and other tweaks. > > Signed-off-by: Christian Brauner <brauner@kernel.org> > --- > Documentation/filesystems/overlayfs.rst | 24 +++++++++++++++++++----- > fs/overlayfs/params.c | 25 +++++++++++++++++++++++++ > fs/overlayfs/super.c | 16 +++++++++++++++- > 3 files changed, 59 insertions(+), 6 deletions(-) > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > index 6245b67ae9e0..2db379b4b31e 100644 > --- a/Documentation/filesystems/overlayfs.rst > +++ b/Documentation/filesystems/overlayfs.rst > @@ -292,13 +292,27 @@ rename or unlink will of course be noticed and handled). > Permission model > ---------------- > > +An overlay filesystem stashes credentials that will be used when > +accessing lower or upper filesystems. > + > +In the old mount api the credentials of the task calling mount(2) are > +stashed. In the new mount api the credentials of the task creating the > +superblock through FSCONFIG_CMD_CREATE command of fsconfig(2) are > +stashed. > + > +Starting with kernel v6.15 it is possible to use the "override_creds" > +mount option which will cause the credentials of the calling task to be > +recorded. Note that "override_creds" is only meaningful when used with > +the new mount api as the old mount api combines setting options and > +superblock creation in a single mount(2) syscall. > + > Permission checking in the overlay filesystem follows these principles: > > 1) permission check SHOULD return the same result before and after copy up > > 2) task creating the overlay mount MUST NOT gain additional privileges > > - 3) non-mounting task MAY gain additional privileges through the overlay, > + 3) task[*] MAY gain additional privileges through the overlay, > compared to direct access on underlying lower or upper filesystems > > This is achieved by performing two permission checks on each access: > @@ -306,7 +320,7 @@ This is achieved by performing two permission checks on each access: > a) check if current task is allowed access based on local DAC (owner, > group, mode and posix acl), as well as MAC checks > > - b) check if mounting task would be allowed real operation on lower or > + b) check if stashed credentials would be allowed real operation on lower or > upper layer based on underlying filesystem permissions, again including > MAC checks > > @@ -315,10 +329,10 @@ are copied up. On the other hand it can result in server enforced > permissions (used by NFS, for example) being ignored (3). > > Check (b) ensures that no task gains permissions to underlying layers that > -the mounting task does not have (2). This also means that it is possible > +the stashed credentials do not have (2). This also means that it is possible > to create setups where the consistency rule (1) does not hold; normally, > -however, the mounting task will have sufficient privileges to perform all > -operations. > +however, the stashed credentials will have sufficient privileges to > +perform all operations. > > Another way to demonstrate this model is drawing parallels between:: > > diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c > index 1115c22deca0..6a94a56f14fb 100644 > --- a/fs/overlayfs/params.c > +++ b/fs/overlayfs/params.c > @@ -59,6 +59,7 @@ enum ovl_opt { > Opt_metacopy, > Opt_verity, > Opt_volatile, > + Opt_override_creds, > }; > > static const struct constant_table ovl_parameter_bool[] = { > @@ -155,6 +156,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = { > fsparam_enum("metacopy", Opt_metacopy, ovl_parameter_bool), > fsparam_enum("verity", Opt_verity, ovl_parameter_verity), > fsparam_flag("volatile", Opt_volatile), > + fsparam_flag_no("override_creds", Opt_override_creds), > {} > }; > > @@ -662,6 +664,29 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param) > case Opt_userxattr: > config->userxattr = true; > break; > + case Opt_override_creds: { > + const struct cred *cred = NULL; > + > + if (result.negated) { > + swap(cred, ofs->creator_cred); > + put_cred(cred); > + break; > + } > + > + if (!current_in_userns(fc->user_ns)) { > + err = -EINVAL; > + break; > + } > + > + cred = prepare_creds(); > + if (cred) > + swap(cred, ofs->creator_cred); > + else > + err = -EINVAL; Did you mean ENOMEM? > + > + put_cred(cred); > + break; > + } > default: > pr_err("unrecognized mount option \"%s\" or missing value\n", > param->key); > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > index 86ae6f6da36b..cf0d8f1b6710 100644 > --- a/fs/overlayfs/super.c > +++ b/fs/overlayfs/super.c > @@ -1305,6 +1305,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) > { > struct ovl_fs *ofs = sb->s_fs_info; > struct ovl_fs_context *ctx = fc->fs_private; > + const struct cred *old_cred = NULL; > struct dentry *root_dentry; > struct ovl_entry *oe; > struct ovl_layer *layers; > @@ -1318,10 +1319,15 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) > sb->s_d_op = &ovl_dentry_operations; > > err = -ENOMEM; > - ofs->creator_cred = cred = prepare_creds(); > + if (!ofs->creator_cred) > + ofs->creator_cred = cred = prepare_creds(); > + else > + cred = (struct cred *)ofs->creator_cred; > if (!cred) > goto out_err; > > + old_cred = ovl_override_creds(sb); > + > err = ovl_fs_params_verify(ctx, &ofs->config); > if (err) > goto out_err; > @@ -1481,11 +1487,19 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) > > sb->s_root = root_dentry; > > + ovl_revert_creds(old_cred); > return 0; > > out_free_oe: > ovl_free_entry(oe); > out_err: > + /* > + * Revert creds before calling ovl_free_fs() which will call > + * put_cred() and put_cred() requires that the cred's that are > + * put are current->cred. > + */ > + if (old_cred) > + ovl_revert_creds(old_cred); Did you mean that cred's that are NOT current->cred? With those fixes you may add: Reviewed-by: Amir Goldstein <amir73il@gmail.com> Thanks, Amir.
On Wed, Feb 19, 2025 at 12:44:45PM +0100, Amir Goldstein wrote: > On Wed, Feb 19, 2025 at 11:02 AM Christian Brauner <brauner@kernel.org> wrote: > > > > Currently overlayfs uses the mounter's credentials for it's > > override_creds() calls. That provides a consistent permission model. > > > > This patches allows a caller to instruct overlayfs to use its > > credentials instead. The caller must be located in the same user > > namespace hierarchy as the user namespace the overlayfs instance will be > > mounted in. This provides a consistent and simple security model. > > > > With this it is possible to e.g., mount an overlayfs instance where the > > mounter must have CAP_SYS_ADMIN but the credentials used for > > override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage of > > custom fs{g,u}id different from the callers and other tweaks. > > > > Signed-off-by: Christian Brauner <brauner@kernel.org> > > --- > > Documentation/filesystems/overlayfs.rst | 24 +++++++++++++++++++----- > > fs/overlayfs/params.c | 25 +++++++++++++++++++++++++ > > fs/overlayfs/super.c | 16 +++++++++++++++- > > 3 files changed, 59 insertions(+), 6 deletions(-) > > > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > > index 6245b67ae9e0..2db379b4b31e 100644 > > --- a/Documentation/filesystems/overlayfs.rst > > +++ b/Documentation/filesystems/overlayfs.rst > > @@ -292,13 +292,27 @@ rename or unlink will of course be noticed and handled). > > Permission model > > ---------------- > > > > +An overlay filesystem stashes credentials that will be used when > > +accessing lower or upper filesystems. > > + > > +In the old mount api the credentials of the task calling mount(2) are > > +stashed. In the new mount api the credentials of the task creating the > > +superblock through FSCONFIG_CMD_CREATE command of fsconfig(2) are > > +stashed. > > + > > +Starting with kernel v6.15 it is possible to use the "override_creds" > > +mount option which will cause the credentials of the calling task to be > > +recorded. Note that "override_creds" is only meaningful when used with > > +the new mount api as the old mount api combines setting options and > > +superblock creation in a single mount(2) syscall. > > + > > Permission checking in the overlay filesystem follows these principles: > > > > 1) permission check SHOULD return the same result before and after copy up > > > > 2) task creating the overlay mount MUST NOT gain additional privileges > > > > - 3) non-mounting task MAY gain additional privileges through the overlay, > > + 3) task[*] MAY gain additional privileges through the overlay, > > compared to direct access on underlying lower or upper filesystems > > > > This is achieved by performing two permission checks on each access: > > @@ -306,7 +320,7 @@ This is achieved by performing two permission checks on each access: > > a) check if current task is allowed access based on local DAC (owner, > > group, mode and posix acl), as well as MAC checks > > > > - b) check if mounting task would be allowed real operation on lower or > > + b) check if stashed credentials would be allowed real operation on lower or > > upper layer based on underlying filesystem permissions, again including > > MAC checks > > > > @@ -315,10 +329,10 @@ are copied up. On the other hand it can result in server enforced > > permissions (used by NFS, for example) being ignored (3). > > > > Check (b) ensures that no task gains permissions to underlying layers that > > -the mounting task does not have (2). This also means that it is possible > > +the stashed credentials do not have (2). This also means that it is possible > > to create setups where the consistency rule (1) does not hold; normally, > > -however, the mounting task will have sufficient privileges to perform all > > -operations. > > +however, the stashed credentials will have sufficient privileges to > > +perform all operations. > > > > Another way to demonstrate this model is drawing parallels between:: > > > > diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c > > index 1115c22deca0..6a94a56f14fb 100644 > > --- a/fs/overlayfs/params.c > > +++ b/fs/overlayfs/params.c > > @@ -59,6 +59,7 @@ enum ovl_opt { > > Opt_metacopy, > > Opt_verity, > > Opt_volatile, > > + Opt_override_creds, > > }; > > > > static const struct constant_table ovl_parameter_bool[] = { > > @@ -155,6 +156,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = { > > fsparam_enum("metacopy", Opt_metacopy, ovl_parameter_bool), > > fsparam_enum("verity", Opt_verity, ovl_parameter_verity), > > fsparam_flag("volatile", Opt_volatile), > > + fsparam_flag_no("override_creds", Opt_override_creds), > > {} > > }; > > > > @@ -662,6 +664,29 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param) > > case Opt_userxattr: > > config->userxattr = true; > > break; > > + case Opt_override_creds: { > > + const struct cred *cred = NULL; > > + > > + if (result.negated) { > > + swap(cred, ofs->creator_cred); > > + put_cred(cred); > > + break; > > + } > > + > > + if (!current_in_userns(fc->user_ns)) { > > + err = -EINVAL; > > + break; > > + } > > + > > + cred = prepare_creds(); > > + if (cred) > > + swap(cred, ofs->creator_cred); > > + else > > + err = -EINVAL; > > Did you mean ENOMEM? Yes, thanks for spotting that. Fixed in-tree. > > > + > > + put_cred(cred); > > + break; > > + } > > default: > > pr_err("unrecognized mount option \"%s\" or missing value\n", > > param->key); > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > > index 86ae6f6da36b..cf0d8f1b6710 100644 > > --- a/fs/overlayfs/super.c > > +++ b/fs/overlayfs/super.c > > @@ -1305,6 +1305,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) > > { > > struct ovl_fs *ofs = sb->s_fs_info; > > struct ovl_fs_context *ctx = fc->fs_private; > > + const struct cred *old_cred = NULL; > > struct dentry *root_dentry; > > struct ovl_entry *oe; > > struct ovl_layer *layers; > > @@ -1318,10 +1319,15 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) > > sb->s_d_op = &ovl_dentry_operations; > > > > err = -ENOMEM; > > - ofs->creator_cred = cred = prepare_creds(); > > + if (!ofs->creator_cred) > > + ofs->creator_cred = cred = prepare_creds(); > > + else > > + cred = (struct cred *)ofs->creator_cred; > > if (!cred) > > goto out_err; > > > > + old_cred = ovl_override_creds(sb); > > + > > err = ovl_fs_params_verify(ctx, &ofs->config); > > if (err) > > goto out_err; > > @@ -1481,11 +1487,19 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) > > > > sb->s_root = root_dentry; > > > > + ovl_revert_creds(old_cred); > > return 0; > > > > out_free_oe: > > ovl_free_entry(oe); > > out_err: > > + /* > > + * Revert creds before calling ovl_free_fs() which will call > > + * put_cred() and put_cred() requires that the cred's that are > > + * put are current->cred. > > + */ > > + if (old_cred) > > + ovl_revert_creds(old_cred); > > Did you mean that cred's that are NOT current->cred? Fixed. > > With those fixes you may add: > > Reviewed-by: Amir Goldstein <amir73il@gmail.com> > > Thanks, > Amir.
diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index 6245b67ae9e0..2db379b4b31e 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -292,13 +292,27 @@ rename or unlink will of course be noticed and handled). Permission model ---------------- +An overlay filesystem stashes credentials that will be used when +accessing lower or upper filesystems. + +In the old mount api the credentials of the task calling mount(2) are +stashed. In the new mount api the credentials of the task creating the +superblock through FSCONFIG_CMD_CREATE command of fsconfig(2) are +stashed. + +Starting with kernel v6.15 it is possible to use the "override_creds" +mount option which will cause the credentials of the calling task to be +recorded. Note that "override_creds" is only meaningful when used with +the new mount api as the old mount api combines setting options and +superblock creation in a single mount(2) syscall. + Permission checking in the overlay filesystem follows these principles: 1) permission check SHOULD return the same result before and after copy up 2) task creating the overlay mount MUST NOT gain additional privileges - 3) non-mounting task MAY gain additional privileges through the overlay, + 3) task[*] MAY gain additional privileges through the overlay, compared to direct access on underlying lower or upper filesystems This is achieved by performing two permission checks on each access: @@ -306,7 +320,7 @@ This is achieved by performing two permission checks on each access: a) check if current task is allowed access based on local DAC (owner, group, mode and posix acl), as well as MAC checks - b) check if mounting task would be allowed real operation on lower or + b) check if stashed credentials would be allowed real operation on lower or upper layer based on underlying filesystem permissions, again including MAC checks @@ -315,10 +329,10 @@ are copied up. On the other hand it can result in server enforced permissions (used by NFS, for example) being ignored (3). Check (b) ensures that no task gains permissions to underlying layers that -the mounting task does not have (2). This also means that it is possible +the stashed credentials do not have (2). This also means that it is possible to create setups where the consistency rule (1) does not hold; normally, -however, the mounting task will have sufficient privileges to perform all -operations. +however, the stashed credentials will have sufficient privileges to +perform all operations. Another way to demonstrate this model is drawing parallels between:: diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c index 1115c22deca0..6a94a56f14fb 100644 --- a/fs/overlayfs/params.c +++ b/fs/overlayfs/params.c @@ -59,6 +59,7 @@ enum ovl_opt { Opt_metacopy, Opt_verity, Opt_volatile, + Opt_override_creds, }; static const struct constant_table ovl_parameter_bool[] = { @@ -155,6 +156,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = { fsparam_enum("metacopy", Opt_metacopy, ovl_parameter_bool), fsparam_enum("verity", Opt_verity, ovl_parameter_verity), fsparam_flag("volatile", Opt_volatile), + fsparam_flag_no("override_creds", Opt_override_creds), {} }; @@ -662,6 +664,29 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param) case Opt_userxattr: config->userxattr = true; break; + case Opt_override_creds: { + const struct cred *cred = NULL; + + if (result.negated) { + swap(cred, ofs->creator_cred); + put_cred(cred); + break; + } + + if (!current_in_userns(fc->user_ns)) { + err = -EINVAL; + break; + } + + cred = prepare_creds(); + if (cred) + swap(cred, ofs->creator_cred); + else + err = -EINVAL; + + put_cred(cred); + break; + } default: pr_err("unrecognized mount option \"%s\" or missing value\n", param->key); diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c index 86ae6f6da36b..cf0d8f1b6710 100644 --- a/fs/overlayfs/super.c +++ b/fs/overlayfs/super.c @@ -1305,6 +1305,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) { struct ovl_fs *ofs = sb->s_fs_info; struct ovl_fs_context *ctx = fc->fs_private; + const struct cred *old_cred = NULL; struct dentry *root_dentry; struct ovl_entry *oe; struct ovl_layer *layers; @@ -1318,10 +1319,15 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) sb->s_d_op = &ovl_dentry_operations; err = -ENOMEM; - ofs->creator_cred = cred = prepare_creds(); + if (!ofs->creator_cred) + ofs->creator_cred = cred = prepare_creds(); + else + cred = (struct cred *)ofs->creator_cred; if (!cred) goto out_err; + old_cred = ovl_override_creds(sb); + err = ovl_fs_params_verify(ctx, &ofs->config); if (err) goto out_err; @@ -1481,11 +1487,19 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc) sb->s_root = root_dentry; + ovl_revert_creds(old_cred); return 0; out_free_oe: ovl_free_entry(oe); out_err: + /* + * Revert creds before calling ovl_free_fs() which will call + * put_cred() and put_cred() requires that the cred's that are + * put are current->cred. + */ + if (old_cred) + ovl_revert_creds(old_cred); ovl_free_fs(ofs); sb->s_fs_info = NULL; return err;
Currently overlayfs uses the mounter's credentials for it's override_creds() calls. That provides a consistent permission model. This patches allows a caller to instruct overlayfs to use its credentials instead. The caller must be located in the same user namespace hierarchy as the user namespace the overlayfs instance will be mounted in. This provides a consistent and simple security model. With this it is possible to e.g., mount an overlayfs instance where the mounter must have CAP_SYS_ADMIN but the credentials used for override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage of custom fs{g,u}id different from the callers and other tweaks. Signed-off-by: Christian Brauner <brauner@kernel.org> --- Documentation/filesystems/overlayfs.rst | 24 +++++++++++++++++++----- fs/overlayfs/params.c | 25 +++++++++++++++++++++++++ fs/overlayfs/super.c | 16 +++++++++++++++- 3 files changed, 59 insertions(+), 6 deletions(-)