mnt_idmapping.c

  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#include <linux/seq_file.h>
 10
 11#include "internal.h"
 12
 13/*
 14 * Outside of this file vfs{g,u}id_t are always created from k{g,u}id_t,
 15 * never from raw values. These are just internal helpers.
 16 */
 17#define VFSUIDT_INIT_RAW(val) (vfsuid_t){ val }
 18#define VFSGIDT_INIT_RAW(val) (vfsgid_t){ val }
 19
 20struct mnt_idmap {
 21	struct uid_gid_map uid_map;
 22	struct uid_gid_map gid_map;
 23	refcount_t count;
 24};
 25
 26/*
 27 * Carries the initial idmapping of 0:0:4294967295 which is an identity
 28 * mapping. This means that {g,u}id 0 is mapped to {g,u}id 0, {g,u}id 1 is
 29 * mapped to {g,u}id 1, [...], {g,u}id 1000 to {g,u}id 1000, [...].
 30 */
 31struct mnt_idmap nop_mnt_idmap = {
 32	.count	= REFCOUNT_INIT(1),
 33};
 34EXPORT_SYMBOL_GPL(nop_mnt_idmap);
 35
 36/*
 37 * Carries the invalid idmapping of a full 0-4294967295 {g,u}id range.
 38 * This means that all {g,u}ids are mapped to INVALID_VFS{G,U}ID.
 39 */
 40struct mnt_idmap invalid_mnt_idmap = {
 41	.count	= REFCOUNT_INIT(1),
 42};
 43EXPORT_SYMBOL_GPL(invalid_mnt_idmap);
 44
 45/**
 46 * initial_idmapping - check whether this is the initial mapping
 47 * @ns: idmapping to check
 48 *
 49 * Check whether this is the initial mapping, mapping 0 to 0, 1 to 1,
 50 * [...], 1000 to 1000 [...].
 51 *
 52 * Return: true if this is the initial mapping, false if not.
 53 */
 54static inline bool initial_idmapping(const struct user_namespace *ns)
 55{
 56	return ns == &init_user_ns;
 57}
 58
 59/**
 60 * make_vfsuid - map a filesystem kuid according to an idmapping
 61 * @idmap: the mount's idmapping
 62 * @fs_userns: the filesystem's idmapping
 63 * @kuid : kuid to be mapped
 64 *
 65 * Take a @kuid and remap it from @fs_userns into @idmap. Use this
 66 * function when preparing a @kuid to be reported to userspace.
 67 *
 68 * If initial_idmapping() determines that this is not an idmapped mount
 69 * we can simply return @kuid unchanged.
 70 * If initial_idmapping() tells us that the filesystem is not mounted with an
 71 * idmapping we know the value of @kuid won't change when calling
 72 * from_kuid() so we can simply retrieve the value via __kuid_val()
 73 * directly.
 74 *
 75 * Return: @kuid mapped according to @idmap.
 76 * If @kuid has no mapping in either @idmap or @fs_userns INVALID_UID is
 77 * returned.
 78 */
 79
 80vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
 81		     struct user_namespace *fs_userns,
 82		     kuid_t kuid)
 83{
 84	uid_t uid;
 85
 86	if (idmap == &nop_mnt_idmap)
 87		return VFSUIDT_INIT(kuid);
 88	if (idmap == &invalid_mnt_idmap)
 89		return INVALID_VFSUID;
 90	if (initial_idmapping(fs_userns))
 91		uid = __kuid_val(kuid);
 92	else
 93		uid = from_kuid(fs_userns, kuid);
 94	if (uid == (uid_t)-1)
 95		return INVALID_VFSUID;
 96	return VFSUIDT_INIT_RAW(map_id_down(&idmap->uid_map, uid));
 97}
 98EXPORT_SYMBOL_GPL(make_vfsuid);
 99
100/**
101 * make_vfsgid - map a filesystem kgid according to an idmapping
102 * @idmap: the mount's idmapping
103 * @fs_userns: the filesystem's idmapping
104 * @kgid : kgid to be mapped
105 *
106 * Take a @kgid and remap it from @fs_userns into @idmap. Use this
107 * function when preparing a @kgid to be reported to userspace.
108 *
109 * If initial_idmapping() determines that this is not an idmapped mount
110 * we can simply return @kgid unchanged.
111 * If initial_idmapping() tells us that the filesystem is not mounted with an
112 * idmapping we know the value of @kgid won't change when calling
113 * from_kgid() so we can simply retrieve the value via __kgid_val()
114 * directly.
115 *
116 * Return: @kgid mapped according to @idmap.
117 * If @kgid has no mapping in either @idmap or @fs_userns INVALID_GID is
118 * returned.
119 */
120vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
121		     struct user_namespace *fs_userns, kgid_t kgid)
122{
123	gid_t gid;
124
125	if (idmap == &nop_mnt_idmap)
126		return VFSGIDT_INIT(kgid);
127	if (idmap == &invalid_mnt_idmap)
128		return INVALID_VFSGID;
129	if (initial_idmapping(fs_userns))
130		gid = __kgid_val(kgid);
131	else
132		gid = from_kgid(fs_userns, kgid);
133	if (gid == (gid_t)-1)
134		return INVALID_VFSGID;
135	return VFSGIDT_INIT_RAW(map_id_down(&idmap->gid_map, gid));
136}
137EXPORT_SYMBOL_GPL(make_vfsgid);
138
139/**
140 * from_vfsuid - map a vfsuid into the filesystem idmapping
141 * @idmap: the mount's idmapping
142 * @fs_userns: the filesystem's idmapping
143 * @vfsuid : vfsuid to be mapped
144 *
145 * Map @vfsuid into the filesystem idmapping. This function has to be used in
146 * order to e.g. write @vfsuid to inode->i_uid.
147 *
148 * Return: @vfsuid mapped into the filesystem idmapping
149 */
150kuid_t from_vfsuid(struct mnt_idmap *idmap,
151		   struct user_namespace *fs_userns, vfsuid_t vfsuid)
152{
153	uid_t uid;
154
155	if (idmap == &nop_mnt_idmap)
156		return AS_KUIDT(vfsuid);
157	if (idmap == &invalid_mnt_idmap)
158		return INVALID_UID;
159	uid = map_id_up(&idmap->uid_map, __vfsuid_val(vfsuid));
160	if (uid == (uid_t)-1)
161		return INVALID_UID;
162	if (initial_idmapping(fs_userns))
163		return KUIDT_INIT(uid);
164	return make_kuid(fs_userns, uid);
165}
166EXPORT_SYMBOL_GPL(from_vfsuid);
167
168/**
169 * from_vfsgid - map a vfsgid into the filesystem idmapping
170 * @idmap: the mount's idmapping
171 * @fs_userns: the filesystem's idmapping
172 * @vfsgid : vfsgid to be mapped
173 *
174 * Map @vfsgid into the filesystem idmapping. This function has to be used in
175 * order to e.g. write @vfsgid to inode->i_gid.
176 *
177 * Return: @vfsgid mapped into the filesystem idmapping
178 */
179kgid_t from_vfsgid(struct mnt_idmap *idmap,
180		   struct user_namespace *fs_userns, vfsgid_t vfsgid)
181{
182	gid_t gid;
183
184	if (idmap == &nop_mnt_idmap)
185		return AS_KGIDT(vfsgid);
186	if (idmap == &invalid_mnt_idmap)
187		return INVALID_GID;
188	gid = map_id_up(&idmap->gid_map, __vfsgid_val(vfsgid));
189	if (gid == (gid_t)-1)
190		return INVALID_GID;
191	if (initial_idmapping(fs_userns))
192		return KGIDT_INIT(gid);
193	return make_kgid(fs_userns, gid);
194}
195EXPORT_SYMBOL_GPL(from_vfsgid);
196
197#ifdef CONFIG_MULTIUSER
198/**
199 * vfsgid_in_group_p() - check whether a vfsuid matches the caller's groups
200 * @vfsgid: the mnt gid to match
201 *
202 * This function can be used to determine whether @vfsuid matches any of the
203 * caller's groups.
204 *
205 * Return: 1 if vfsuid matches caller's groups, 0 if not.
206 */
207int vfsgid_in_group_p(vfsgid_t vfsgid)
208{
209	return in_group_p(AS_KGIDT(vfsgid));
210}
211#else
212int vfsgid_in_group_p(vfsgid_t vfsgid)
213{
214	return 1;
215}
216#endif
217EXPORT_SYMBOL_GPL(vfsgid_in_group_p);
218
219static int copy_mnt_idmap(struct uid_gid_map *map_from,
220			  struct uid_gid_map *map_to)
221{
222	struct uid_gid_extent *forward, *reverse;
223	u32 nr_extents = READ_ONCE(map_from->nr_extents);
224	/* Pairs with smp_wmb() when writing the idmapping. */
225	smp_rmb();
226
227	/*
228	 * Don't blindly copy @map_to into @map_from if nr_extents is
229	 * smaller or equal to UID_GID_MAP_MAX_BASE_EXTENTS. Since we
230	 * read @nr_extents someone could have written an idmapping and
231	 * then we might end up with inconsistent data. So just don't do
232	 * anything at all.
233	 */
234	if (nr_extents == 0)
235		return -EINVAL;
236
237	/*
238	 * Here we know that nr_extents is greater than zero which means
239	 * a map has been written. Since idmappings can't be changed
240	 * once they have been written we know that we can safely copy
241	 * from @map_to into @map_from.
242	 */
243
244	if (nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
245		*map_to = *map_from;
246		return 0;
247	}
248
249	forward = kmemdup_array(map_from->forward, nr_extents,
250				sizeof(struct uid_gid_extent),
251				GFP_KERNEL_ACCOUNT);
252	if (!forward)
253		return -ENOMEM;
254
255	reverse = kmemdup_array(map_from->reverse, nr_extents,
256				sizeof(struct uid_gid_extent),
257				GFP_KERNEL_ACCOUNT);
258	if (!reverse) {
259		kfree(forward);
260		return -ENOMEM;
261	}
262
263	/*
264	 * The idmapping isn't exposed anywhere so we don't need to care
265	 * about ordering between extent pointers and @nr_extents
266	 * initialization.
267	 */
268	map_to->forward = forward;
269	map_to->reverse = reverse;
270	map_to->nr_extents = nr_extents;
271	return 0;
272}
273
274static void free_mnt_idmap(struct mnt_idmap *idmap)
275{
276	if (idmap->uid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
277		kfree(idmap->uid_map.forward);
278		kfree(idmap->uid_map.reverse);
279	}
280	if (idmap->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
281		kfree(idmap->gid_map.forward);
282		kfree(idmap->gid_map.reverse);
283	}
284	kfree(idmap);
285}
286
287struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns)
288{
289	struct mnt_idmap *idmap;
290	int ret;
291
292	idmap = kzalloc(sizeof(struct mnt_idmap), GFP_KERNEL_ACCOUNT);
293	if (!idmap)
294		return ERR_PTR(-ENOMEM);
295
296	refcount_set(&idmap->count, 1);
297	ret = copy_mnt_idmap(&mnt_userns->uid_map, &idmap->uid_map);
298	if (!ret)
299		ret = copy_mnt_idmap(&mnt_userns->gid_map, &idmap->gid_map);
300	if (ret) {
301		free_mnt_idmap(idmap);
302		idmap = ERR_PTR(ret);
303	}
304	return idmap;
305}
306
307/**
308 * mnt_idmap_get - get a reference to an idmapping
309 * @idmap: the idmap to bump the reference on
310 *
311 * If @idmap is not the @nop_mnt_idmap bump the reference count.
312 *
313 * Return: @idmap with reference count bumped if @not_mnt_idmap isn't passed.
314 */
315struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap)
316{
317	if (idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap)
318		refcount_inc(&idmap->count);
319
320	return idmap;
321}
322EXPORT_SYMBOL_GPL(mnt_idmap_get);
323
324/**
325 * mnt_idmap_put - put a reference to an idmapping
326 * @idmap: the idmap to put the reference on
327 *
328 * If this is a non-initial idmapping, put the reference count when a mount is
329 * released and free it if we're the last user.
330 */
331void mnt_idmap_put(struct mnt_idmap *idmap)
332{
333	if (idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap &&
334	    refcount_dec_and_test(&idmap->count))
335		free_mnt_idmap(idmap);
336}
337EXPORT_SYMBOL_GPL(mnt_idmap_put);
338
339int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map)
340{
341	struct uid_gid_map *map, *map_up;
342	u32 idx, nr_mappings;
343
344	if (!is_valid_mnt_idmap(idmap))
345		return 0;
346
347	/*
348	 * Idmappings are shown relative to the caller's idmapping.
349	 * This is both the most intuitive and most useful solution.
350	 */
351	if (uid_map) {
352		map = &idmap->uid_map;
353		map_up = &current_user_ns()->uid_map;
354	} else {
355		map = &idmap->gid_map;
356		map_up = &current_user_ns()->gid_map;
357	}
358
359	for (idx = 0, nr_mappings = 0; idx < map->nr_extents; idx++) {
360		uid_t lower;
361		struct uid_gid_extent *extent;
362
363		if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
364			extent = &map->extent[idx];
365		else
366			extent = &map->forward[idx];
367
368		/*
369		 * Verify that the whole range of the mapping can be
370		 * resolved in the caller's idmapping. If it cannot be
371		 * resolved skip the mapping.
372		 */
373		lower = map_id_range_up(map_up, extent->lower_first, extent->count);
374		if (lower == (uid_t) -1)
375			continue;
376
377		seq_printf(seq, "%u %u %u", extent->first, lower, extent->count);
378
379		seq->count++; /* mappings are separated by \0 */
380		if (seq_has_overflowed(seq))
381			return -EAGAIN;
382
383		nr_mappings++;
384	}
385
386	return nr_mappings;
387}