On Mon, Aug 24, 2020 at 7:30 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote:
>
> In security_read_policy(), the policy length is computed using
> security_policydb_len(), which does a separate transaction, and then
> another transaction is done to write the policydb into a buffer of this
> length.
>
> The bug is that the policy might be re-loaded in between the two
> transactions and so the length can be wrong. In case the new length is
> lower than the old length, the length is corrected at the end of the
> function. In case the new length is higher than the old one, an error is
> returned.
>
> Since we can't call vmalloc_user() under read_lock(), fix it by checking
> if the allocated buffer is sufficiently large after doing the allocation
> and taking the read lock and if not, retry the whole operation with the
> new size.
>
> Fixes: cee74f47a6ba ("SELinux: allow userspace to read policy back out of the kernel")
> Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> ---
> security/selinux/ss/services.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index a48fc1b337ba9..2c9072f095985 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -3849,14 +3849,22 @@ int security_read_policy(struct selinux_state *state,
>
> *len = security_policydb_len(state);
>
> +again:
> *data = vmalloc_user(*len);
> if (!*data)
> return -ENOMEM;
>
> + read_lock(&state->ss->policy_rwlock);
> + if (*len < state->ss->policy->policydb.len) {
> + *len = state->ss->policy->policydb.len;
> + read_unlock(&state->ss->policy_rwlock);
> + vfree(*data);
> + goto again;
> + }
> +
> fp.data = *data;
> fp.len = *len;
>
> - read_lock(&state->ss->policy_rwlock);
> rc = policydb_write(&state->ss->policy->policydb, &fp);
> read_unlock(&state->ss->policy_rwlock);
>
security_read_policy() is called with fsi->mutex held by selinuxfs, so
policy reload cannot occur in between the length computation and the
writing of the policydb. Right? It's another case where we could
pass down the mutex as in my rcu patches for a lockdep assertion.
