On Tue, Nov 09, 2021 at 03:57:12PM +0100, Christian Brauner wrote:
From: Christian Brauner christian.brauner@ubuntu.com
When calling setattr_prepare() to determine the validity of the attributes the ia_{g,u}id fields contain the value that will be written to inode->i_{g,u}id. When the {g,u}id attribute of the file isn't altered and the caller's fs{g,u}id matches the current {g,u}id attribute the attribute change is allowed.
The value in ia_{g,u}id does already account for idmapped mounts and will have taken the relevant idmapping into account. So in order to verify that the {g,u}id attribute isn't changed we simple need to compare the ia_{g,u}id value against the inode's i_{g,u}id value.
This only has any meaning for idmapped mounts as idmapping helpers are idempotent without them. And for idmapped mounts this really only has a meaning when circular idmappings are used, i.e. mappings where e.g. id 1000 is mapped to id 1001 and id 1001 is mapped to id 1000. Such ciruclar mappings can e.g. be useful when sharing the same home directory between multiple users at the same time.
As an example consider a directory with two files: /source/file1 owned by {g,u}id 1000 and /source/file2 owned by {g,u}id 1001. Assume we create an idmapped mount at /target with an idmapping that maps files owned by {g,u}id 1000 to being owned by {g,u}id 1001 and files owned by {g,u}id 1001 to being owned by {g,u}id 1000. In effect, the idmapped mount at /target switches the ownership of /source/file1 and source/file2, i.e. /target/file1 will be owned by {g,u}id 1001 and /target/file2 will be owned by {g,u}id 1000.
This means that a user with fs{g,u}id 1000 must be allowed to setattr /target/file2 from {g,u}id 1000 to {g,u}id 1000. Similar, a user with fs{g,u}id 1001 must be allowed to setattr /target/file1 from {g,u}id 1001 to {g,u}id 1001. Conversely, a user with fs{g,u}id 1000 must fail to setattr /target/file1 from {g,u}id 1001 to {g,u}id 1000. And a user with fs{g,u}id 1001 must fail to setattr /target/file2 from {g,u}id 1000 to {g,u}id 1000. Both cases must fail with EPERM for non-capable callers.
Before this patch we could end up denying legitimate attribute changes and allowing invalid attribute changes when circular mappings are used. To even get into this situation the caller must've been privileged both to create that mapping and to create that idmapped mount.
This hasn't been seen in the wild anywhere but came up when expanding the testsuite during work on a series of hardening patches. All idmapped fstests pass without any regressions and we add new tests to verify the behavior of circular mappings.
Fixes: 2f221d6f7b88 ("attr: handle idmapped mounts") Cc: Seth Forshee seth.forshee@digitalocean.com Cc: Christoph Hellwig hch@lst.de Cc: Al Viro viro@zeniv.linux.org.uk Cc: stable@vger.kernel.org CC: linux-fsdevel@vger.kernel.org Signed-off-by: Christian Brauner christian.brauner@ubuntu.com
Christoph, can you take a look, by any chance? I'd like to get this to Linus this week.
fs/attr.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/fs/attr.c b/fs/attr.c index 473d21b3a86d..66899b6e9bd8 100644 --- a/fs/attr.c +++ b/fs/attr.c @@ -35,7 +35,7 @@ static bool chown_ok(struct user_namespace *mnt_userns, kuid_t uid) { kuid_t kuid = i_uid_into_mnt(mnt_userns, inode);
- if (uid_eq(current_fsuid(), kuid) && uid_eq(uid, kuid))
- if (uid_eq(current_fsuid(), kuid) && uid_eq(uid, inode->i_uid)) return true; if (capable_wrt_inode_uidgid(mnt_userns, inode, CAP_CHOWN)) return true;
@@ -62,7 +62,7 @@ static bool chgrp_ok(struct user_namespace *mnt_userns, { kgid_t kgid = i_gid_into_mnt(mnt_userns, inode); if (uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) &&
(in_group_p(gid) || gid_eq(gid, kgid)))
(in_group_p(gid) || gid_eq(gid, inode->i_gid)))base-commit: 8bb7eca972ad531c9b149c0a51ab43a417385813
2.30.2