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userns: fix id map generating with holes under specific conditions #88

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merged 2 commits into from
Dec 6, 2023
Merged

userns: fix id map generating with holes under specific conditions #88

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525ad6b
userns: fix id map generating with holes under specific conditions
Snaipe Nov 22, 2023
70ea867
userns: add implicit 0-max to root when no sub[ug]ids are assigned
Snaipe Nov 23, 2023
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42 changes: 25 additions & 17 deletions userns.c
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Original file line number Diff line number Diff line change
Expand Up @@ -90,15 +90,17 @@ void id_map_parse(id_map map, char *opt)

$ cat /etc/subuid
barney:100000:65535
barney:200000:100000

Then, calling the function as such:

id_map_load_subids(map, "/etc/subuid", 1000, "barney");

will populate the contents of `map` with the following data:

inner=0 outer=1000 1
inner=0 outer=1000 1
inner=0 outer=100000 65535
inner=0 outer=200000 100000
*/
void id_map_load_subids(id_map map, const char *subid_path, const struct id *id)
{
Expand All @@ -121,6 +123,7 @@ void id_map_load_subids(id_map map, const char *subid_path, const struct id *id)
size assumptions tend to bite back, and pages are extremely cheap. */
char line[4096];

bool found = false;
while (fgets(line, sizeof (line), subids) != NULL) {
char entryname[ID_STR_MAX + 1];
entryname[ID_STR_MAX] = 0;
Expand All @@ -143,9 +146,18 @@ void id_map_load_subids(id_map map, const char *subid_path, const struct id *id)
}

range = id_map_append(map, range, 0, start, length);
found = true;
}

fclose(subids);

/* We're root. We don't care. Map the host range 1:1 if there are no
allotted subids */
if (!found && id->id == 0) {
/* UINT32_MAX - 1 is explicitly left out because the kernel rejects it
(see user_namespaces(7)). */
id_map_append(map, 0, 0, 0, UINT32_MAX - 2);
}
}

void id_map_generate(id_map allotted, id_map out, const char *subid_path, const struct id *id)
Expand All @@ -165,14 +177,19 @@ void id_map_generate(id_map allotted, id_map out, const char *subid_path, const
continue;
}

/* Ignore the [0-65535) range. */
if (range.outer <= ID_MAX) {
uint32_t shift = ID_MAX + 1 - range.outer;
if (shift > range.length) {
shift = range.length;
/* The current id already has been assigned; ignore it if it comes up
in the allotted map. */
if (range.outer <= id->id && id->id < range.outer + range.length) {
uint32_t len = id->id - range.outer;
if (len != 0) {
/* Add range up to (but excluding) id->id */
j = id_map_append(tmp, j, cur_id, range.outer, len);
cur_id += len;
}
range.outer += shift;
range.length -= shift;

/* Change the range to start one past id->id */
range.outer = id->id + 1;
range.length -= len + 1;
continue;
}

Expand All @@ -181,14 +198,6 @@ void id_map_generate(id_map allotted, id_map out, const char *subid_path, const
range.length = 0;
}

/* We're root. We don't care. Map the host range 1:1. */
if (cur_id == 1 && id->id == 0) {
/* UINT32_MAX - 1 is explicitly left out because the kernel rejects it
(see user_namespaces(7)). */
id_map_append(tmp, 0, 0, 0, UINT32_MAX - 2);
goto end;
}

/* Not enough subuids for a full mapping, but, well, it's not the end of
the world. Things might break, so let's at least tell the user. */

Expand All @@ -204,7 +213,6 @@ void id_map_generate(id_map allotted, id_map out, const char *subid_path, const
name, subid_path, cur_id, ID_MAX);
}

end:
memcpy(out, tmp, sizeof (tmp));
}

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