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smb: client: reject userspace cifs.idmap descriptions

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cifs.idmap key descriptions carry authority-bearing fields (owner and
group SIDs and uid/gid values in "os:"/"gs:"/"oi:"/"gi:" form) that the
cifs.idmap upcall helper treats as kernel-originating inputs.  Unlike
its sibling cifs.spnego, the cifs.idmap key type has no vet_description
hook, so userspace can create keys of this type through
request_key(2)/add_key(2) and supply those fields without CIFS origin.
A request_key(2) call with a non-NULL callout then drives a root
usermodehelper upcall (/sbin/request-key -> cifs.idmap) that consumes
the unvetted description in root context.

Only accept cifs.idmap descriptions while CIFS is using its private
root_cred to request the key.  id_to_sid()/sid_to_id() already run
under override_creds(root_cred), so the kernel-originated path is
unaffected.

This mirrors commit 3da1fdf4ef ("smb: client: reject userspace
cifs.spnego descriptions"), which applied the same restriction to
cifs.spnego.

Fixes: 4d79dba0e0 ("cifs: Add idmap key and related data structures and functions (try #17 repost)")
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Cc: stable@vger.kernel.org
Assisted-by: CodeBuddy:Kimi-K3
Signed-off-by: Aohan Mei <henrymei@tencent.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
This commit is contained in:
Aohan Mei 2026-09-02 20:52:13 +08:00 committed by Paulo Alcantara
parent 5be5bdda58
commit d9d7eeb0ce
1 changed files with 15 additions and 0 deletions
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15
fs/smb/client/cifsacl.c
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@ -100,8 +100,23 @@ cifs_idmap_key_destroy(struct key *key)
kfree(key->payload.data[0]);
}
static int
cifs_idmap_key_vet_description(const char *description)
{
/*
* cifs.idmap descriptions are authority-bearing inputs to the
* cifs.idmap upcall helper. Only allow the kernel to create this
* type of key using the private root_cred installed in
* init_cifs_idmap; reject userspace request_key(2)/add_key(2).
*/
if (current_cred() != root_cred)
return -EPERM;
return 0;
}
static struct key_type cifs_idmap_key_type = {
.name = "cifs.idmap",
.vet_description = cifs_idmap_key_vet_description,
.instantiate = cifs_idmap_key_instantiate,
.destroy = cifs_idmap_key_destroy,
.describe = user_describe,