[Devel] [PATCH RH7] overlayfs: avoid permission check for priveleged processes
Alexander Mikhalitsyn
alexander.mikhalitsyn at virtuozzo.com
Thu Oct 15 16:03:52 MSK 2020
Please, see comments inline
On Wed, 14 Oct 2020 02:05:21 +0300
Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com> wrote:
> Overlayfs temporary override credentials in copy_up function to ones which was
> used to create mount. Unfortunately vfs_setxattr requires CAP_SYS_ADMIN
> capability in current user namespace. This leads to strange situations.
> For example, if overlayfs mount was made inside ve it is impossible to use
> copy_up from init_user_ns even with CAP_SYS_ADMIN. This is because overriden
> credentials are not sufficient in init_user_ns to set xattr to file.
> This is also required for criu since copy_up can be triggered on dump stage:
> reading inotify fhandle from /proc may start copy_up.
>
> Add an option to avoid vfs_setxattr CAP_SYS_ADMIN check if current credentials
> have CAP_SYS_ADMIN in namespace that is recorded in overlayfs mount superblock.
>
> https://jira.sw.ru/browse/PSBM-108122
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> ---
> fs/overlayfs/copy_up.c | 25 +++++++++++++++++++------
> fs/overlayfs/overlayfs.h | 39 ++++++++++++++++++++++++++-------------
> fs/overlayfs/util.c | 32 ++++++++++++++++++++++++++++----
> fs/xattr.c | 2 +-
> 4 files changed, 74 insertions(+), 24 deletions(-)
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 1564a35..d6b285f 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -20,6 +20,7 @@
> #include <linux/fdtable.h>
> #include <linux/ratelimit.h>
> #include <linux/exportfs.h>
> +#include <linux/capability.h>
> #include "overlayfs.h"
>
> #define OVL_COPY_UP_CHUNK_SIZE (1 << 20)
> @@ -321,8 +322,8 @@ out:
> return fh;
> }
>
> -int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
> - struct dentry *upper)
> +int ovl_set_origin_ext(struct dentry *dentry, struct dentry *lower,
> + struct dentry *upper, int propagate_cap)
> {
> const struct ovl_fh *fh = NULL;
> int err;
> @@ -341,8 +342,8 @@ int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
> /*
> * Do not fail when upper doesn't support xattrs.
> */
> - err = ovl_check_setxattr(dentry, upper, OVL_XATTR_ORIGIN, fh,
> - fh ? fh->len : 0, 0);
> + err = ovl_check_setxattr_ext(dentry, upper, OVL_XATTR_ORIGIN, fh,
> + fh ? fh->len : 0, 0, propagate_cap);
> kfree(fh);
>
> return err;
> @@ -433,6 +434,7 @@ struct ovl_copy_up_ctx {
> struct dentry *destdir;
> struct qstr destname;
> struct dentry *workdir;
> + int propagate_cap;
> bool tmpfile;
> bool origin;
> bool indexed;
> @@ -711,7 +713,7 @@ out:
> }
>
> static int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
> - int flags)
> + int flags, int propagate_cap)
> {
> int err;
> struct path parentpath;
> @@ -719,6 +721,7 @@ static int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
> .parent = parent,
> .dentry = dentry,
> .workdir = ovl_workdir(dentry),
> + .propagate_cap = propagate_cap,
Maybe I've missed something but I can't see where this field is used.
> };
>
> if (WARN_ON(!ctx.workdir))
> @@ -768,9 +771,19 @@ static int ovl_copy_up_one(struct dentry *parent, struct dentry *dentry,
> return err;
> }
>
> +static int ovl_can_propagate_cap(struct dentry *dentry)
> +{
> + struct super_block *sb = dentry->d_sb;
> + struct ovl_fs *ofs = sb->s_fs_info;
> + struct user_namespace *ovl_ns = ofs->creator_cred->user_ns;
> +
> + return ns_capable(ovl_ns, CAP_SYS_ADMIN);
> +}
> +
> int ovl_copy_up_flags(struct dentry *dentry, int flags)
> {
> int err = 0;
> + int propagate_cap = ovl_can_propagate_cap(dentry);
> const struct cred *old_cred = ovl_override_creds(dentry->d_sb);
> bool disconnected = (dentry->d_flags & DCACHE_DISCONNECTED);
>
> @@ -815,7 +828,7 @@ int ovl_copy_up_flags(struct dentry *dentry, int flags)
> next = parent;
> }
>
> - err = ovl_copy_up_one(parent, next, flags);
> + err = ovl_copy_up_one(parent, next, flags, propagate_cap);
>
> dput(parent);
> dput(next);
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 7052938..6917acd 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -149,15 +149,6 @@ static inline int ovl_do_symlink(struct inode *dir, struct dentry *dentry,
> return err;
> }
>
> -static inline int ovl_do_setxattr(struct dentry *dentry, const char *name,
> - const void *value, size_t size, int flags)
> -{
> - int err = vfs_setxattr(dentry, name, value, size, flags);
> - pr_debug("setxattr(%pd2, \"%s\", \"%*pE\", %zu, 0x%x) = %i\n",
> - dentry, name, min((int)size, 48), value, size, flags, err);
> - return err;
> -}
> -
> static inline int ovl_do_removexattr(struct dentry *dentry, const char *name)
> {
> int err = vfs_removexattr(dentry, name);
> @@ -245,9 +236,12 @@ int ovl_copy_up_start(struct dentry *dentry);
> void ovl_copy_up_end(struct dentry *dentry);
> bool ovl_check_origin_xattr(struct dentry *dentry);
> bool ovl_check_dir_xattr(struct dentry *dentry, const char *name);
> -int ovl_check_setxattr(struct dentry *dentry, struct dentry *upperdentry,
> +int ovl_check_setxattr_ext(struct dentry *dentry, struct dentry *upperdentry,
> const char *name, const void *value, size_t size,
> - int xerr);
> + int xerr, int propagate_cap);
> +int ovl_do_setxattr_ext(struct dentry *dentry, const char *name,
> + const void *value, size_t size, int flags,
> + int propagate_cap);
> int ovl_set_impure(struct dentry *dentry, struct dentry *upperdentry);
> void ovl_set_flag(unsigned long flag, struct inode *inode);
> void ovl_clear_flag(unsigned long flag, struct inode *inode);
> @@ -279,6 +273,19 @@ static inline unsigned int ovl_xino_bits(struct super_block *sb)
> return ofs->xino_bits;
> }
>
> +static inline int ovl_check_setxattr(struct dentry *dentry,
> + struct dentry *upperdentry, const char *name,
> + const void *value, size_t size, int xerr)
> +{
> + return ovl_check_setxattr_ext(dentry, upperdentry, name, value, size,
> + xerr, 0);
> +}
> +
> +static inline int ovl_do_setxattr(struct dentry *dentry, const char *name,
> + const void *value, size_t size, int flags)
> +{
> + return ovl_do_setxattr_ext(dentry, name, value, size, flags, 0);
> +}
>
> /* namei.c */
> int ovl_check_fh_len(struct ovl_fh *fh, int fh_len);
> @@ -400,8 +407,14 @@ int ovl_copy_up_flags(struct dentry *dentry, int flags);
> int ovl_copy_xattr(struct dentry *old, struct dentry *new);
> int ovl_set_attr(struct dentry *upper, struct kstat *stat);
> struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper);
> -int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
> - struct dentry *upper);
> +int ovl_set_origin_ext(struct dentry *dentry, struct dentry *lower,
> + struct dentry *upper, int propagate_cap);
> +
> +static inline int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
> + struct dentry *upper)
> +{
> + return ovl_set_origin_ext(dentry, lower, upper, 0);
> +}
>
> /* export.c */
> extern const struct export_operations ovl_export_operations;
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 52a5116..30c10f1 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -419,9 +419,32 @@ bool ovl_check_dir_xattr(struct dentry *dentry, const char *name)
> return false;
> }
>
> -int ovl_check_setxattr(struct dentry *dentry, struct dentry *upperdentry,
> - const char *name, const void *value, size_t size,
> - int xerr)
> +int ovl_do_setxattr_ext(struct dentry *dentry, const char *name,
> + const void *value, size_t size, int flags, int propagate_cap)
> +{
> + int err = vfs_setxattr(dentry, name, value, size, flags);
> +
> + if (propagate_cap && err == -EPERM) {
> + struct inode *ino = dentry->d_inode;
> +
> + if (IS_IMMUTABLE(ino) || IS_APPEND(ino))
> + goto out;
> +
> + inode_lock(ino);
> + err = __vfs_setxattr_noperm(dentry, name, value, size, flags);
Speaking honestly, this is looks as very unsafe solution because
you have exported function that was "private" and as I can see you
have copied some checks from xattr_permission function (fs/xattr.c) also.
What if design will change and we will need to have some extra checks
here?
As I can see in xattr_permission function we have also:
/*
* Updating an xattr will likely cause i_uid and i_gid
* to be writen back improperly if their true value is
* unknown to the vfs.
*/
if (HAS_UNMAPPED_ID(inode))
return -EPERM;
Do we need to have this check here too?
> + inode_unlock(ino);
> + }
> +
> +out:
> + pr_debug("setxattr(%pd2, \"%s\", \"%*pE\", %zu, 0x%x) = %i, propagate_cap = %d\n",
> + dentry, name, min((int)size, 48), value, size, flags, err,
> + propagate_cap);
> + return err;
> +}
> +
> +int ovl_check_setxattr_ext(struct dentry *dentry, struct dentry *upperdentry,
> + const char *name, const void *value, size_t size,
> + int xerr, int propagate_cap)
> {
> int err;
> struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
> @@ -429,7 +452,8 @@ int ovl_check_setxattr(struct dentry *dentry, struct dentry *upperdentry,
> if (ofs->noxattr)
> return xerr;
>
> - err = ovl_do_setxattr(upperdentry, name, value, size, 0);
> + err = ovl_do_setxattr_ext(upperdentry, name, value, size, 0,
> + propagate_cap);
>
> if (err == -EOPNOTSUPP) {
> pr_warn("overlayfs: cannot set %s xattr on upper\n", name);
> diff --git a/fs/xattr.c b/fs/xattr.c
> index 9c24acc..c164e83 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -124,7 +124,7 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
>
> return error;
> }
> -
> +EXPORT_SYMBOL_GPL(__vfs_setxattr_noperm);
>
> int
> vfs_setxattr(struct dentry *dentry, const char *name, const void *value,
> --
> 1.8.3.1
>
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--
Regards,
Alex.
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