KEYS: always initialize keyring_index_key::desc_len [Linux 5.0]

KEYS: always initialize keyring_index_key::desc_len [Linux 5.0]

This Linux kernel change "KEYS: always initialize keyring_index_key::desc_len" is included in the Linux 5.0 release. This change is authored by Eric Biggers <ebiggers [at] google.com> on Fri Feb 22 15:36:18 2019 +0000. The commit for this change in Linux stable tree is ede0fa9 (patch).

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: [email protected]
Fixes: b2a4df200d57 ("KEYS: Expand the capacity of a keyring")
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: David Howells <[email protected]>
Cc: [email protected]
Signed-off-by: James Morris <[email protected]>

There are 10 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 security/keys/keyring.c          | 4 +---
 security/keys/proc.c             | 3 +--
 security/keys/request_key.c      | 1 +
 security/keys/request_key_auth.c | 2 +-
 4 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index eadebb92..f81372f 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 d2b8020..78ac305 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 3f56a31..7a0c6b6 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 afc304e..bda6201 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);

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