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Merge patch series "open_tree_attr: do not allow id-mapping changes without OPEN_TREE_CLONE"

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Aleksa Sarai <cyphar@cyphar.com> says:

As described in commit 7a54947e72 ('Merge patch series "fs: allow
changing idmappings"'), open_tree_attr(2) was necessary in order to
allow for a detached mount to be created and have its idmappings changed
without the risk of any racing threads operating on it. For this reason,
mount_setattr(2) still does not allow for id-mappings to be changed.

However, there was a bug in commit 2462651ffa ("fs: allow changing
idmappings") which allowed users to bypass this restriction by calling
open_tree_attr(2) *without* OPEN_TREE_CLONE.

can_idmap_mount() prevented this bug from allowing an attached
mountpoint's id-mapping from being modified (thanks to an is_anon_ns()
check), but this still allows for detached (but visible) mounts to have
their be id-mapping changed. This risks the same UAF and locking issues
as described in the merge commit, and was likely unintentional.

For what it's worth, I found this while working on the open_tree_attr(2)
man page, and was trying to figure out what open_tree_attr(2)'s
behaviour was in the (slightly fruity) ~OPEN_TREE_CLONE case.

* patches from https://lore.kernel.org/20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com:
  selftests/mount_setattr: add smoke tests for open_tree_attr(2) bug
  open_tree_attr: do not allow id-mapping changes without OPEN_TREE_CLONE

Link: https://lore.kernel.org/20250808-open_tree_attr-bugfix-idmap-v1-0-0ec7bc05646c@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Christian Brauner
2025-08-08 15:45:26 +02:00
parent 6b65028e2b 81e4b9cf36
commit bef2981bad
2 changed files with 66 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

3
fs/namespace.c
View File

@@ -5176,7 +5176,8 @@ SYSCALL_DEFINE5(open_tree_attr, int, dfd, const char __user *, filename,
int ret;
struct mount_kattr kattr = {};
kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
if (flags & OPEN_TREE_CLONE)
kattr.kflags = MOUNT_KATTR_IDMAP_REPLACE;
if (flags & AT_RECURSIVE)
kattr.kflags |= MOUNT_KATTR_RECURSE;

77
tools/testing/selftests/mount_setattr/mount_setattr_test.c
View File

@@ -107,6 +107,26 @@
#endif
#endif
#ifndef __NR_open_tree_attr
#if defined __alpha__
#define __NR_open_tree_attr 577
#elif defined _MIPS_SIM
#if _MIPS_SIM == _MIPS_SIM_ABI32 /* o32 */
#define __NR_open_tree_attr (467 + 4000)
#endif
#if _MIPS_SIM == _MIPS_SIM_NABI32 /* n32 */
#define __NR_open_tree_attr (467 + 6000)
#endif
#if _MIPS_SIM == _MIPS_SIM_ABI64 /* n64 */
#define __NR_open_tree_attr (467 + 5000)
#endif
#elif defined __ia64__
#define __NR_open_tree_attr (467 + 1024)
#else
#define __NR_open_tree_attr 467
#endif
#endif
#ifndef MOUNT_ATTR_IDMAP
#define MOUNT_ATTR_IDMAP 0x00100000
#endif
@@ -121,6 +141,12 @@ static inline int sys_mount_setattr(int dfd, const char *path, unsigned int flag
return syscall(__NR_mount_setattr, dfd, path, flags, attr, size);
}
static inline int sys_open_tree_attr(int dfd, const char *path, unsigned int flags,
struct mount_attr *attr, size_t size)
{
return syscall(__NR_open_tree_attr, dfd, path, flags, attr, size);
}
static ssize_t write_nointr(int fd, const void *buf, size_t count)
{
ssize_t ret;
@@ -1222,6 +1248,12 @@ TEST_F(mount_setattr_idmapped, attached_mount_inside_current_mount_namespace)
attr.userns_fd = get_userns_fd(0, 10000, 10000);
ASSERT_GE(attr.userns_fd, 0);
ASSERT_NE(sys_mount_setattr(open_tree_fd, "", AT_EMPTY_PATH, &attr, sizeof(attr)), 0);
/*
* Make sure that open_tree_attr() without OPEN_TREE_CLONE is not a way
* to bypass this mount_setattr() restriction.
*/
ASSERT_LT(sys_open_tree_attr(open_tree_fd, "", AT_EMPTY_PATH, &attr, sizeof(attr)), 0);
ASSERT_EQ(close(attr.userns_fd), 0);
ASSERT_EQ(close(open_tree_fd), 0);
}
@@ -1255,6 +1287,12 @@ TEST_F(mount_setattr_idmapped, attached_mount_outside_current_mount_namespace)
ASSERT_GE(attr.userns_fd, 0);
ASSERT_NE(sys_mount_setattr(open_tree_fd, "", AT_EMPTY_PATH, &attr,
sizeof(attr)), 0);
/*
* Make sure that open_tree_attr() without OPEN_TREE_CLONE is not a way
* to bypass this mount_setattr() restriction.
*/
ASSERT_LT(sys_open_tree_attr(open_tree_fd, "", AT_EMPTY_PATH, &attr, sizeof(attr)), 0);
ASSERT_EQ(close(attr.userns_fd), 0);
ASSERT_EQ(close(open_tree_fd), 0);
}
@@ -1321,6 +1359,19 @@ TEST_F(mount_setattr_idmapped, detached_mount_outside_current_mount_namespace)
ASSERT_EQ(close(open_tree_fd), 0);
}
static bool expected_uid_gid(int dfd, const char *path, int flags,
uid_t expected_uid, gid_t expected_gid)
{
int ret;
struct stat st;
ret = fstatat(dfd, path, &st, flags);
if (ret < 0)
return false;
return st.st_uid == expected_uid && st.st_gid == expected_gid;
}
/**
* Validate that currently changing the idmapping of an idmapped mount fails.
*/
@@ -1331,6 +1382,8 @@ TEST_F(mount_setattr_idmapped, change_idmapping)
.attr_set = MOUNT_ATTR_IDMAP,
};
ASSERT_TRUE(expected_uid_gid(-EBADF, "/mnt/D", 0, 0, 0));
if (!mount_setattr_supported())
SKIP(return, "mount_setattr syscall not supported");
@@ -1348,27 +1401,25 @@ TEST_F(mount_setattr_idmapped, change_idmapping)
AT_EMPTY_PATH, &attr, sizeof(attr)), 0);
ASSERT_EQ(close(attr.userns_fd), 0);
EXPECT_FALSE(expected_uid_gid(open_tree_fd, ".", 0, 0, 0));
EXPECT_TRUE(expected_uid_gid(open_tree_fd, ".", 0, 10000, 10000));
/* Change idmapping on a detached mount that is already idmapped. */
attr.userns_fd = get_userns_fd(0, 20000, 10000);
ASSERT_GE(attr.userns_fd, 0);
ASSERT_NE(sys_mount_setattr(open_tree_fd, "", AT_EMPTY_PATH, &attr, sizeof(attr)), 0);
/*
* Make sure that open_tree_attr() without OPEN_TREE_CLONE is not a way
* to bypass this mount_setattr() restriction.
*/
EXPECT_LT(sys_open_tree_attr(open_tree_fd, "", AT_EMPTY_PATH, &attr, sizeof(attr)), 0);
EXPECT_FALSE(expected_uid_gid(open_tree_fd, ".", 0, 20000, 20000));
EXPECT_TRUE(expected_uid_gid(open_tree_fd, ".", 0, 10000, 10000));
ASSERT_EQ(close(attr.userns_fd), 0);
ASSERT_EQ(close(open_tree_fd), 0);
}
static bool expected_uid_gid(int dfd, const char *path, int flags,
uid_t expected_uid, gid_t expected_gid)
{
int ret;
struct stat st;
ret = fstatat(dfd, path, &st, flags);
if (ret < 0)
return false;
return st.st_uid == expected_uid && st.st_gid == expected_gid;
}
TEST_F(mount_setattr_idmapped, idmap_mount_tree_invalid)
{
int open_tree_fd = -EBADF;