1 // SPDX-License-Identifier: GPL-2.0
2 /* Copyright (c) 2022 Christian Brauner <brauner@kernel.org> */
3
4 #include <linux/cred.h>
5 #include <linux/fs.h>
6 #include <linux/mnt_idmapping.h>
7 #include <linux/slab.h>
8 #include <linux/user_namespace.h>
9
10 #include "internal.h"
11
12 struct mnt_idmap {
13 struct user_namespace *owner;
14 refcount_t count;
15 };
16
17 /*
18 * Carries the initial idmapping of 0:0:4294967295 which is an identity
19 * mapping. This means that {g,u}id 0 is mapped to {g,u}id 0, {g,u}id 1 is
20 * mapped to {g,u}id 1, [...], {g,u}id 1000 to {g,u}id 1000, [...].
21 */
22 struct mnt_idmap nop_mnt_idmap = {
23 .owner = &init_user_ns,
24 .count = REFCOUNT_INIT(1),
25 };
26 EXPORT_SYMBOL_GPL(nop_mnt_idmap);
27
28 /**
29 * check_fsmapping - check whether an mount idmapping is allowed
30 * @idmap: idmap of the relevent mount
31 * @sb: super block of the filesystem
32 *
33 * Return: true if @idmap is allowed, false if not.
34 */
check_fsmapping(const struct mnt_idmap * idmap,const struct super_block * sb)35 bool check_fsmapping(const struct mnt_idmap *idmap,
36 const struct super_block *sb)
37 {
38 return idmap->owner != sb->s_user_ns;
39 }
40
41 /**
42 * initial_idmapping - check whether this is the initial mapping
43 * @ns: idmapping to check
44 *
45 * Check whether this is the initial mapping, mapping 0 to 0, 1 to 1,
46 * [...], 1000 to 1000 [...].
47 *
48 * Return: true if this is the initial mapping, false if not.
49 */
initial_idmapping(const struct user_namespace * ns)50 static inline bool initial_idmapping(const struct user_namespace *ns)
51 {
52 return ns == &init_user_ns;
53 }
54
55 /**
56 * no_idmapping - check whether we can skip remapping a kuid/gid
57 * @mnt_userns: the mount's idmapping
58 * @fs_userns: the filesystem's idmapping
59 *
60 * This function can be used to check whether a remapping between two
61 * idmappings is required.
62 * An idmapped mount is a mount that has an idmapping attached to it that
63 * is different from the filsystem's idmapping and the initial idmapping.
64 * If the initial mapping is used or the idmapping of the mount and the
65 * filesystem are identical no remapping is required.
66 *
67 * Return: true if remapping can be skipped, false if not.
68 */
no_idmapping(const struct user_namespace * mnt_userns,const struct user_namespace * fs_userns)69 static inline bool no_idmapping(const struct user_namespace *mnt_userns,
70 const struct user_namespace *fs_userns)
71 {
72 return initial_idmapping(mnt_userns) || mnt_userns == fs_userns;
73 }
74
75 /**
76 * make_vfsuid - map a filesystem kuid according to an idmapping
77 * @idmap: the mount's idmapping
78 * @fs_userns: the filesystem's idmapping
79 * @kuid : kuid to be mapped
80 *
81 * Take a @kuid and remap it from @fs_userns into @idmap. Use this
82 * function when preparing a @kuid to be reported to userspace.
83 *
84 * If no_idmapping() determines that this is not an idmapped mount we can
85 * simply return @kuid unchanged.
86 * If initial_idmapping() tells us that the filesystem is not mounted with an
87 * idmapping we know the value of @kuid won't change when calling
88 * from_kuid() so we can simply retrieve the value via __kuid_val()
89 * directly.
90 *
91 * Return: @kuid mapped according to @idmap.
92 * If @kuid has no mapping in either @idmap or @fs_userns INVALID_UID is
93 * returned.
94 */
95
make_vfsuid(struct mnt_idmap * idmap,struct user_namespace * fs_userns,kuid_t kuid)96 vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
97 struct user_namespace *fs_userns,
98 kuid_t kuid)
99 {
100 uid_t uid;
101 struct user_namespace *mnt_userns = idmap->owner;
102
103 if (no_idmapping(mnt_userns, fs_userns))
104 return VFSUIDT_INIT(kuid);
105 if (initial_idmapping(fs_userns))
106 uid = __kuid_val(kuid);
107 else
108 uid = from_kuid(fs_userns, kuid);
109 if (uid == (uid_t)-1)
110 return INVALID_VFSUID;
111 return VFSUIDT_INIT(make_kuid(mnt_userns, uid));
112 }
113 EXPORT_SYMBOL_GPL(make_vfsuid);
114
115 /**
116 * make_vfsgid - map a filesystem kgid according to an idmapping
117 * @idmap: the mount's idmapping
118 * @fs_userns: the filesystem's idmapping
119 * @kgid : kgid to be mapped
120 *
121 * Take a @kgid and remap it from @fs_userns into @idmap. Use this
122 * function when preparing a @kgid to be reported to userspace.
123 *
124 * If no_idmapping() determines that this is not an idmapped mount we can
125 * simply return @kgid unchanged.
126 * If initial_idmapping() tells us that the filesystem is not mounted with an
127 * idmapping we know the value of @kgid won't change when calling
128 * from_kgid() so we can simply retrieve the value via __kgid_val()
129 * directly.
130 *
131 * Return: @kgid mapped according to @idmap.
132 * If @kgid has no mapping in either @idmap or @fs_userns INVALID_GID is
133 * returned.
134 */
make_vfsgid(struct mnt_idmap * idmap,struct user_namespace * fs_userns,kgid_t kgid)135 vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
136 struct user_namespace *fs_userns, kgid_t kgid)
137 {
138 gid_t gid;
139 struct user_namespace *mnt_userns = idmap->owner;
140
141 if (no_idmapping(mnt_userns, fs_userns))
142 return VFSGIDT_INIT(kgid);
143 if (initial_idmapping(fs_userns))
144 gid = __kgid_val(kgid);
145 else
146 gid = from_kgid(fs_userns, kgid);
147 if (gid == (gid_t)-1)
148 return INVALID_VFSGID;
149 return VFSGIDT_INIT(make_kgid(mnt_userns, gid));
150 }
151 EXPORT_SYMBOL_GPL(make_vfsgid);
152
153 /**
154 * from_vfsuid - map a vfsuid into the filesystem idmapping
155 * @idmap: the mount's idmapping
156 * @fs_userns: the filesystem's idmapping
157 * @vfsuid : vfsuid to be mapped
158 *
159 * Map @vfsuid into the filesystem idmapping. This function has to be used in
160 * order to e.g. write @vfsuid to inode->i_uid.
161 *
162 * Return: @vfsuid mapped into the filesystem idmapping
163 */
from_vfsuid(struct mnt_idmap * idmap,struct user_namespace * fs_userns,vfsuid_t vfsuid)164 kuid_t from_vfsuid(struct mnt_idmap *idmap,
165 struct user_namespace *fs_userns, vfsuid_t vfsuid)
166 {
167 uid_t uid;
168 struct user_namespace *mnt_userns = idmap->owner;
169
170 if (no_idmapping(mnt_userns, fs_userns))
171 return AS_KUIDT(vfsuid);
172 uid = from_kuid(mnt_userns, AS_KUIDT(vfsuid));
173 if (uid == (uid_t)-1)
174 return INVALID_UID;
175 if (initial_idmapping(fs_userns))
176 return KUIDT_INIT(uid);
177 return make_kuid(fs_userns, uid);
178 }
179 EXPORT_SYMBOL_GPL(from_vfsuid);
180
181 /**
182 * from_vfsgid - map a vfsgid into the filesystem idmapping
183 * @idmap: the mount's idmapping
184 * @fs_userns: the filesystem's idmapping
185 * @vfsgid : vfsgid to be mapped
186 *
187 * Map @vfsgid into the filesystem idmapping. This function has to be used in
188 * order to e.g. write @vfsgid to inode->i_gid.
189 *
190 * Return: @vfsgid mapped into the filesystem idmapping
191 */
from_vfsgid(struct mnt_idmap * idmap,struct user_namespace * fs_userns,vfsgid_t vfsgid)192 kgid_t from_vfsgid(struct mnt_idmap *idmap,
193 struct user_namespace *fs_userns, vfsgid_t vfsgid)
194 {
195 gid_t gid;
196 struct user_namespace *mnt_userns = idmap->owner;
197
198 if (no_idmapping(mnt_userns, fs_userns))
199 return AS_KGIDT(vfsgid);
200 gid = from_kgid(mnt_userns, AS_KGIDT(vfsgid));
201 if (gid == (gid_t)-1)
202 return INVALID_GID;
203 if (initial_idmapping(fs_userns))
204 return KGIDT_INIT(gid);
205 return make_kgid(fs_userns, gid);
206 }
207 EXPORT_SYMBOL_GPL(from_vfsgid);
208
209 #ifdef CONFIG_MULTIUSER
210 /**
211 * vfsgid_in_group_p() - check whether a vfsuid matches the caller's groups
212 * @vfsgid: the mnt gid to match
213 *
214 * This function can be used to determine whether @vfsuid matches any of the
215 * caller's groups.
216 *
217 * Return: 1 if vfsuid matches caller's groups, 0 if not.
218 */
vfsgid_in_group_p(vfsgid_t vfsgid)219 int vfsgid_in_group_p(vfsgid_t vfsgid)
220 {
221 return in_group_p(AS_KGIDT(vfsgid));
222 }
223 #else
vfsgid_in_group_p(vfsgid_t vfsgid)224 int vfsgid_in_group_p(vfsgid_t vfsgid)
225 {
226 return 1;
227 }
228 #endif
229 EXPORT_SYMBOL_GPL(vfsgid_in_group_p);
230
alloc_mnt_idmap(struct user_namespace * mnt_userns)231 struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns)
232 {
233 struct mnt_idmap *idmap;
234
235 idmap = kzalloc(sizeof(struct mnt_idmap), GFP_KERNEL_ACCOUNT);
236 if (!idmap)
237 return ERR_PTR(-ENOMEM);
238
239 idmap->owner = get_user_ns(mnt_userns);
240 refcount_set(&idmap->count, 1);
241 return idmap;
242 }
243
244 /**
245 * mnt_idmap_get - get a reference to an idmapping
246 * @idmap: the idmap to bump the reference on
247 *
248 * If @idmap is not the @nop_mnt_idmap bump the reference count.
249 *
250 * Return: @idmap with reference count bumped if @not_mnt_idmap isn't passed.
251 */
mnt_idmap_get(struct mnt_idmap * idmap)252 struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap)
253 {
254 if (idmap != &nop_mnt_idmap)
255 refcount_inc(&idmap->count);
256
257 return idmap;
258 }
259
260 /**
261 * mnt_idmap_put - put a reference to an idmapping
262 * @idmap: the idmap to put the reference on
263 *
264 * If this is a non-initial idmapping, put the reference count when a mount is
265 * released and free it if we're the last user.
266 */
mnt_idmap_put(struct mnt_idmap * idmap)267 void mnt_idmap_put(struct mnt_idmap *idmap)
268 {
269 if (idmap != &nop_mnt_idmap && refcount_dec_and_test(&idmap->count)) {
270 put_user_ns(idmap->owner);
271 kfree(idmap);
272 }
273 }
274