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landlock: Don't lose track of restrictions on cred_transfer

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When a process' cred struct is replaced, this _almost_ always invokes
the cred_prepare LSM hook; but in one special case (when
KEYCTL_SESSION_TO_PARENT updates the parent's credentials), the
cred_transfer LSM hook is used instead.  Landlock only implements the
cred_prepare hook, not cred_transfer, so KEYCTL_SESSION_TO_PARENT causes
all information on Landlock restrictions to be lost.

This basically means that a process with the ability to use the fork()
and keyctl() syscalls can get rid of all Landlock restrictions on
itself.

Fix it by adding a cred_transfer hook that does the same thing as the
existing cred_prepare hook. (Implemented by having hook_cred_prepare()
call hook_cred_transfer() so that the two functions are less likely to
accidentally diverge in the future.)

Cc: stable@kernel.org
Fixes: 385975dca5 ("landlock: Set up the security framework and manage credentials")
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20240724-landlock-houdini-fix-v1-1-df89a4560ca3@google.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>
This commit is contained in:
Jann Horn 2024-07-24 14:49:01 +02:00 committed by Mickaël Salaün
parent 0c38364824
commit 39705a6c29
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GPG Key ID: E5E3D0E88C82F6D2
1 changed files with 9 additions and 2 deletions
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11
security/landlock/cred.c
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@ -14,8 +14,8 @@
#include "ruleset.h"
#include "setup.h"
static int hook_cred_prepare(struct cred *const new,
const struct cred *const old, const gfp_t gfp)
static void hook_cred_transfer(struct cred *const new,
const struct cred *const old)
{
struct landlock_ruleset *const old_dom = landlock_cred(old)->domain;
@ -23,6 +23,12 @@ static int hook_cred_prepare(struct cred *const new,
landlock_get_ruleset(old_dom);
landlock_cred(new)->domain = old_dom;
}
}
static int hook_cred_prepare(struct cred *const new,
const struct cred *const old, const gfp_t gfp)
{
hook_cred_transfer(new, old);
return 0;
}
@ -36,6 +42,7 @@ static void hook_cred_free(struct cred *const cred)
static struct security_hook_list landlock_hooks[] __ro_after_init = {
LSM_HOOK_INIT(cred_prepare, hook_cred_prepare),
LSM_HOOK_INIT(cred_transfer, hook_cred_transfer),
LSM_HOOK_INIT(cred_free, hook_cred_free),
};