[PATCH v2] keys: reject descriptions that exceed the index length

[Daehyeon Ko <4ncienth@gmail.com>]

struct keyring_index_key::desc_len is a u16.  User-provided key
descriptions are limited to 4095 bytes, but a key type preparser can
generate a longer description when the caller passes NULL.

The X.509 parser forms a description from the certificate subject and
twice the raw serial length when the certificate has no Subject Key
Identifier.  A certificate with a two-byte subject and a 32766-byte serial
therefore produces a 65536-byte description.  Assigning strlen() to
desc_len wraps it to zero, after which __key_link_begin() hits:

    BUG_ON(index_key->desc_len == 0);

This was reproduced through keyctl on a v6.12.105 KASAN kernel with
panic=1 and oops=panic.  The process ran as uid and gid 1000 with no
capabilities, and the console recorded:

    CapInh: 0000000000000000
    CapPrm: 0000000000000000
    CapEff: 0000000000000000
    [   14.044055] ------------[ cut here ]------------
    [   14.044211] kernel BUG at security/keys/keyring.c:1308!
    [   14.044375] Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
    [   14.044558] CPU: 0 UID: 1000 PID: 138 Comm: keyctl Not tainted 6.12.105-dirty #1
    [   14.044788] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
    [   14.045025] RIP: 0010:__key_link_begin+0x1e5/0x250
    [   14.047630] Call Trace:
    [   14.047711]  <TASK>
    [   14.047785]  __key_create_or_update+0x490/0xcd0
    [   14.047954]  ? __pfx___key_create_or_update+0x10/0x10
    [   14.048139]  ? __pfx_lookup_user_key_possessed+0x10/0x10
    [   14.048323]  key_create_or_update+0x47/0x60
    [   14.048484]  __do_sys_add_key+0x219/0x430
    [   14.048632]  ? __pfx___do_sys_add_key+0x10/0x10
    [   14.048794]  ? switch_fpu_return+0x127/0x250
    [   14.048980]  ? srso_return_thunk+0x5/0x5f
    [   14.049163]  ? arch_exit_to_user_mode_prepare.constprop.0+0x6f/0xa0
    [   14.049417]  ? srso_return_thunk+0x5/0x5f
    [   14.049582]  do_syscall_64+0x5a/0x130
    [   14.049721]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
    [   14.051772]  </TASK>
    [   14.051975] ---[ end trace 0000000000000000 ]---
    [   14.054729] Kernel panic - not syncing: Fatal exception

The same boundary was independently reproduced on 3/3 fresh v6.12.105
KASAN boots with the source reproducer.

A one-byte-short control with a 32765-byte serial parsed successfully and
returned EDQUOT without a splat, showing that the wrap boundary is causal.

Measure generated descriptions before narrowing the length and reject
values that cannot be represented.  On 3/3 fresh patched mainline KASAN
boots, the boundary returned EINVAL and the control continued to return
EDQUOT, with no BUG, Oops, KASAN report, warning or panic.

A tested source reproducer exists and is available to maintainers
privately.

Fixes: f771fde82051 ("keys: Simplify key description management")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Changes in v2:
- Add the observed keyctl crash trace and before/after runtime results to the
  commit message.
- Clarify that the serial is used when the certificate has no Subject Key
  Identifier.
- No code changes.

 security/keys/key.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/security/keys/key.c b/security/keys/key.c
index b34a64d81d47ab..f2f472b45f4eee 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -14,6 +14,7 @@
 #include <linux/workqueue.h>
 #include <linux/random.h>
 #include <linux/err.h>
+#include <linux/limits.h>
 #include "internal.h"
 
 struct kmem_cache *key_jar;
@@ -820,6 +821,7 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
 	const struct cred *cred = current_cred();
 	struct key *keyring, *key = NULL;
 	key_ref_t key_ref;
+	size_t desc_len;
 	int ret;
 	struct key_restriction *restrict_link = NULL;
 
@@ -865,7 +867,12 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
 		if (!index_key.description)
 			goto error_free_prep;
 	}
-	index_key.desc_len = strlen(index_key.description);
+	desc_len = strlen(index_key.description);
+	if (desc_len > U16_MAX) {
+		key_ref = ERR_PTR(-EINVAL);
+		goto error_free_prep;
+	}
+	index_key.desc_len = desc_len;
 	key_set_index_key(&index_key);
 
 	ret = __key_link_lock(keyring, &index_key);

base-commit: 0a0d1d55dad570724bf8c7ea83409639cfb4be9b
-- 
2.54.0