[PATCH 19/34] namei: add lookup helpers with idmapped mounts aware permission checking

Christian Brauner
2020-10-29T00:36:33Z

The lookup_one_len(), lookup_one_len_unlocked(), and lookup_positive_unlocked()
helpers are used by filesystems targeted in this first iteration to lookup
dentries if the caller is privileged over the inode of the base dentry. Add
three new helpers lookup_one_len_mapped(), lookup_one_len_mapped_unlocked(),
and lookup_one_len_mapped_unlocked() to handle idmapped mounts. If the inode is
accessed through an idmapped mount it is mapped according to the mount's user
namespace. Afterwards the permissions checks are identical to non-idmapped
mounts. If the initial user namespace is passed all mapping operations are a
nop so non-idmapped mounts will not see a change in behavior and will also not
see any performance impact. It also means that the non-idmapped-mount aware
helpers can be implemented on top of their idmapped-mount aware counterparts by
passing the initial user namespace.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
fs/namei.c | 47 ++++++++++++++++++++++++++++++++-----------
include/linux/namei.h | 6 ++++++
2 files changed, 41 insertions(+), 12 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index a8a3de936cfc..7901ea09e80e 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2517,8 +2517,9 @@ int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
}
EXPORT_SYMBOL(vfs_path_lookup);

-static int lookup_one_len_common(const char *name, struct dentry *base,
- int len, struct qstr *this)
+static int lookup_one_len_common(const char *name, struct dentry *base, int len,
+ struct qstr *this,
+ struct user_namespace *mnt_user_ns)
{
this->name = name;
this->len = len;
@@ -2546,7 +2547,7 @@ static int lookup_one_len_common(const char *name, struct dentry *base,
return err;
}

- return inode_permission(base->d_inode, MAY_EXEC);
+ return mapped_inode_permission(mnt_user_ns, base->d_inode, MAY_EXEC);
}

/**
@@ -2570,7 +2571,7 @@ struct dentry *try_lookup_one_len(const char *name, struct dentry *base, int len

WARN_ON_ONCE(!inode_is_locked(base->d_inode));

- err = lookup_one_len_common(name, base, len, &this);
+ err = lookup_one_len_common(name, base, len, &this, &init_user_ns);
if (err)
return ERR_PTR(err);

@@ -2589,7 +2590,8 @@ EXPORT_SYMBOL(try_lookup_one_len);
*
* The caller must hold base->i_mutex.
*/
-struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
+struct dentry *lookup_one_len_mapped(const char *name, struct dentry *base, int len,
+ struct user_namespace *mnt_user_ns)
{
struct dentry *dentry;
struct qstr this;
@@ -2597,13 +2599,19 @@ struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)

WARN_ON_ONCE(!inode_is_locked(base->d_inode));

- err = lookup_one_len_common(name, base, len, &this);
+ err = lookup_one_len_common(name, base, len, &this, mnt_user_ns);
if (err)
return ERR_PTR(err);

dentry = lookup_dcache(&this, base, 0);
return dentry ? dentry : __lookup_slow(&this, base, 0);
}
+EXPORT_SYMBOL(lookup_one_len_mapped);
+
+struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
+{
+ return lookup_one_len_mapped(name, base, len, &init_user_ns);
+}
EXPORT_SYMBOL(lookup_one_len);

/**
@@ -2618,14 +2626,14 @@ EXPORT_SYMBOL(lookup_one_len);
* Unlike lookup_one_len, it should be called without the parent
* i_mutex held, and will take the i_mutex itself if necessary.
*/
-struct dentry *lookup_one_len_unlocked(const char *name,
- struct dentry *base, int len)
+struct dentry *lookup_one_len_mapped_unlocked(const char *name, struct dentry *base,
+ int len, struct user_namespace *mnt_user_ns)
{
struct qstr this;
int err;
struct dentry *ret;

- err = lookup_one_len_common(name, base, len, &this);
+ err = lookup_one_len_common(name, base, len, &this, mnt_user_ns);
if (err)
return ERR_PTR(err);

@@ -2634,6 +2642,13 @@ struct dentry *lookup_one_len_unlocked(const char *name,
ret = lookup_slow(&this, base, 0);
return ret;
}
+EXPORT_SYMBOL(lookup_one_len_mapped_unlocked);
+
+struct dentry *lookup_one_len_unlocked(const char *name,
+ struct dentry *base, int len)
+{
+ return lookup_one_len_mapped_unlocked(name, base, len, &init_user_ns);
+}
EXPORT_SYMBOL(lookup_one_len_unlocked);

/*
@@ -2644,16 +2659,24 @@ EXPORT_SYMBOL(lookup_one_len_unlocked);
* need to be very careful; pinned positives have ->d_inode stable, so
* this one avoids such problems.
*/
-struct dentry *lookup_positive_unlocked(const char *name,
- struct dentry *base, int len)
+struct dentry *lookup_positive_mapped_unlocked(const char *name,
+ struct dentry *base, int len,
+ struct user_namespace *mnt_user_ns)
{
- struct dentry *ret = lookup_one_len_unlocked(name, base, len);
+ struct dentry *ret = lookup_one_len_mapped_unlocked(name, base, len, mnt_user_ns);
if (!IS_ERR(ret) && d_flags_negative(smp_load_acquire(&ret->d_flags))) {
dput(ret);
ret = ERR_PTR(-ENOENT);
}
return ret;
}
+EXPORT_SYMBOL(lookup_positive_mapped_unlocked);
+
+struct dentry *lookup_positive_unlocked(const char *name,
+ struct dentry *base, int len)
+{
+ return lookup_positive_mapped_unlocked(name, base, len, &init_user_ns);
+}
EXPORT_SYMBOL(lookup_positive_unlocked);

#ifdef CONFIG_UNIX98_PTYS
diff --git a/include/linux/namei.h b/include/linux/namei.h
index a4bb992623c4..42dbe4c2653a 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -68,8 +68,14 @@ extern struct dentry *kern_path_locked(const char *, struct path *);

extern struct dentry *try_lookup_one_len(const char *, struct dentry *, int);
extern struct dentry *lookup_one_len(const char *, struct dentry *, int);
+extern struct dentry *lookup_one_len_mapped(const char *, struct dentry *, int,
+ struct user_namespace *);
extern struct dentry *lookup_one_len_unlocked(const char *, struct dentry *, int);
+extern struct dentry *lookup_one_len_mapped_unlocked(const char *, struct dentry *,
+ int, struct user_namespace *);
extern struct dentry *lookup_positive_unlocked(const char *, struct dentry *, int);
+extern struct dentry *lookup_positive_mapped_unlocked(const char *, struct dentry *,
+ int, struct user_namespace *);

extern int follow_down_one(struct path *);
extern int follow_down(struct path *);
--
2.29.0