linux-kernel - Re: [PATCH v2] cred: Propagate security_prepare

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Date:   Thu, 26 May 2022 16:43:44 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Frederick Lawler <fred@...udflare.com>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-cachefs@...hat.com, linux-cifs@...r.kernel.org,
        samba-technical@...ts.samba.org, linux-mm@...ck.org,
        linux-nfs@...r.kernel.org, linux-unionfs@...r.kernel.org,
        linux-security-module@...r.kernel.org, netdev@...r.kernel.org,
        keyrings@...r.kernel.org, selinux@...r.kernel.org,
        serge@...lyn.com, amir73il@...il.com, kernel-team@...udflare.com
Subject: Re: [PATCH v2] cred: Propagate security_prepare_creds() error code

Frederick Lawler <fred@...udflare.com> writes:

> While experimenting with the security_prepare_creds() LSM hook, we
> noticed that our EPERM error code was not propagated up the callstack.
> Instead ENOMEM is always returned.  As a result, some tools may send a
> confusing error message to the user:
>
> $ unshare -rU
> unshare: unshare failed: Cannot allocate memory
>
> A user would think that the system didn't have enough memory, when
> instead the action was denied.
>
> This problem occurs because prepare_creds() and prepare_kernel_cred()
> return NULL when security_prepare_creds() returns an error code. Later,
> functions calling prepare_creds() and prepare_kernel_cred() return
> ENOMEM because they assume that a NULL meant there was no memory
> allocated.
>
> Fix this by propagating an error code from security_prepare_creds() up
> the callstack.
>
> Signed-off-by: Frederick Lawler <fred@...udflare.com>

The fs/aio.c part looks ok to me.  We should probably also update the
man page for io_submit, though, to document the conditions under which
EPERM can be returned.

Acked-by: Jeff Moyer <jmoyer@...hat.com>