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Date: Fri, 4 Feb 2022 09:53:38 +0300
From: "Anton V. Boyarshinov" <boyarsh@...linux.org>
To: viro@...iv.linux.org.uk,
linux-fsdevel@...r.kernel.org
Cc: "Anton V. Boyarshinov" <boyarsh@...linux.org>,
ebiederm@...ssion.com,
legion@...nel.org,
ldv@...linux.org,
linux-kernel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: [PATCH] Add ability to disallow idmapped mounts
Idmapped mounts may have security implications [1] and have
no knobs to be disallowed at runtime or compile time.
This patch adds a sysctl and a config option to set its default value.
[1] https://lore.kernel.org/all/m18s7481xc.fsf@fess.ebiederm.org/
Based on work from Alexey Gladkov <legion@...nel.org>.
Signed-off-by: Anton V. Boyarshinov <boyarsh@...linux.org>
---
Documentation/admin-guide/sysctl/fs.rst | 12 ++++++++++++
fs/Kconfig | 8 ++++++++
fs/namespace.c | 21 ++++++++++++++++++++-
3 files changed, 40 insertions(+), 1 deletion(-)
diff --git a/Documentation/admin-guide/sysctl/fs.rst b/Documentation/admin-guide/sysctl/fs.rst
index 2a501c9ddc55..f758c4ae5f66 100644
--- a/Documentation/admin-guide/sysctl/fs.rst
+++ b/Documentation/admin-guide/sysctl/fs.rst
@@ -105,6 +105,18 @@ you have some awesome number of simultaneous system users,
you might want to raise the limit.
+idmap_mounts
+------------
+
+Idmapped mounts may have security implications.
+This knob controls whether creation of idmapped mounts is allowed.
+When set to "1", creation of idmapped mounts is allowed.
+When set to "0", creation of idmapped mounts is not allowed.
+
+The default value is
+* 0, if ``IDMAP_MOUNTS_DEFAULT_OFF`` is enabled in the kernel configuration;
+* 1, otherwise.
+
file-max & file-nr
------------------
diff --git a/fs/Kconfig b/fs/Kconfig
index 7a2b11c0b803..d2203ba0183d 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -385,4 +385,12 @@ source "fs/unicode/Kconfig"
config IO_WQ
bool
+config IDMAP_MOUNTS_DEFAULT_OFF
+ bool "Disallow idmappad mounts by default"
+ help
+ Idmapped mounts may have security implications.
+ Enable this to disallow idmapped mounts by setting
+ the default value of /proc/sys/fs/idmap_mounts to 0.
+
+
endmenu
diff --git a/fs/namespace.c b/fs/namespace.c
index 40b994a29e90..66501ad75537 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -39,6 +39,10 @@
/* Maximum number of mounts in a mount namespace */
static unsigned int sysctl_mount_max __read_mostly = 100000;
+/* Whether idmapped mounts are allowed. */
+static int sysctl_idmap_mounts __read_mostly =
+ IS_ENABLED(CONFIG_IDMAP_MOUNTS_DEFAULT_OFF) ? 0 : 1;
+
static unsigned int m_hash_mask __read_mostly;
static unsigned int m_hash_shift __read_mostly;
static unsigned int mp_hash_mask __read_mostly;
@@ -3965,7 +3969,13 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
if (!is_anon_ns(mnt->mnt_ns))
return -EINVAL;
- return 0;
+ /* So far, there are concerns about the safety of idmaps. */
+ if (!sysctl_idmap_mounts) {
+ pr_warn_once("VFS: idmapped mounts are not allowed.\n");
+ return -EPERM;
+ } else {
+ return 0;
+ }
}
static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
@@ -4631,6 +4641,15 @@ static struct ctl_table fs_namespace_sysctls[] = {
.proc_handler = proc_dointvec_minmax,
.extra1 = SYSCTL_ONE,
},
+ {
+ .procname = "idmap_mounts",
+ .data = &sysctl_idmap_mounts,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_ONE,
+ },
{ }
};
--
2.33.0
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