KEYS: always initialize keyring_index_key::desc_len

syzbot hit the 'BUG_ON(index_key->desc_len == 0);' in __key_link_begin()
called from construct_alloc_key() during sys_request_key(), because the
length of the key description was never calculated.

The problem is that we rely on ->desc_len being initialized by
search_process_keyrings(), specifically by search_nested_keyrings().
But, if the process isn't subscribed to any keyrings that never happens.

Fix it by always initializing keyring_index_key::desc_len as soon as the
description is set, like we already do in some places.

The following program reproduces the BUG_ON() when it's run as root and
no session keyring has been installed.  If it doesn't work, try removing
pam_keyinit.so from /etc/pam.d/login and rebooting.

    #include <stdlib.h>
    #include <unistd.h>
    #include <keyutils.h>

    int main(void)
    {
            int id = add_key("keyring", "syz", NULL, 0, KEY_SPEC_USER_KEYRING);

            keyctl_setperm(id, KEY_OTH_WRITE);
            setreuid(5000, 5000);
            request_key("user", "desc", "", id);
    }

Reported-by: syzbot+ec24e95ea483de0a24da@syzkaller.appspotmail.com
Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: James Morris <james.morris@microsoft.com>

4 files changed, 4 insertions, 6 deletions

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index eadebb92986a..f81372f53dd7 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -661,9 +661,6 @@ static bool search_nested_keyrings(struct key *keyring,
         BUG_ON((ctx->flags & STATE_CHECKS) == 0 ||
                (ctx->flags & STATE_CHECKS) == STATE_CHECKS);
 
-        if (ctx->index_key.description)
-                ctx->index_key.desc_len = strlen(ctx->index_key.description);
-
         /* Check to see if this top-level keyring is what we are looking for
          * and whether it is valid or not.
          */
@@ -914,6 +911,7 @@ key_ref_t keyring_search(key_ref_t keyring,
         struct keyring_search_context ctx = {
                 .index_key.type         = type,
                 .index_key.description  = description,
+                .index_key.desc_len     = strlen(description),
                 .cred                   = current_cred(),
                 .match_data.cmp         = key_default_cmp,
                 .match_data.raw_data    = description,
diff --git a/security/keys/proc.c b/security/keys/proc.c
index d2b802072693..78ac305d715e 100644
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -165,8 +165,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
         int rc;
 
         struct keyring_search_context ctx = {
-                .index_key.type         = key->type,
-                .index_key.description  = key->description,
+                .index_key              = key->index_key,
                 .cred                   = m->file->f_cred,
                 .match_data.cmp         = lookup_user_key_possessed,
                 .match_data.raw_data    = key,
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 3f56a312dd35..7a0c6b666ff0 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -531,6 +531,7 @@ struct key *request_key_and_link(struct key_type *type,
         struct keyring_search_context ctx = {
                 .index_key.type         = type,
                 .index_key.description  = description,
+                .index_key.desc_len     = strlen(description),
                 .cred                   = current_cred(),
                 .match_data.cmp         = key_default_cmp,
                 .match_data.raw_data    = description,
diff --git a/security/keys/request_key_auth.c b/security/keys/request_key_auth.c
index afc304e8b61e..bda6201c6c45 100644
--- a/security/keys/request_key_auth.c
+++ b/security/keys/request_key_auth.c
@@ -247,7 +247,7 @@ struct key *key_get_instantiation_authkey(key_serial_t target_id)
         struct key *authkey;
         key_ref_t authkey_ref;
 
-        sprintf(description, "%x", target_id);
+        ctx.index_key.desc_len = sprintf(description, "%x", target_id);
 
         authkey_ref = search_process_keyrings(&ctx);