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Date: Mon, 13 Jun 2022 15:52:38 -0500 From: Frederick Lawler <fred@...udflare.com> To: "Eric W. Biederman" <ebiederm@...ssion.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, Jeff Moyer <jmoyer@...hat.com>, Paul Moore <paul@...l-moore.com> Subject: Re: [PATCH v3] cred: Propagate security_prepare_creds() error code Hi Eric, On 6/13/22 12:04 PM, Eric W. Biederman wrote: > 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. > > Why would it make sense for security_prepare_creds to return an error > code other than ENOMEM? > > That seems a bit of a violation of what that function is supposed to do > The API allows LSM authors to decide what error code is returned from the cred_prepare hook. security_task_alloc() is a similar hook, and has its return code propagated. I'm proposing we follow security_task_allocs() pattern, and add visibility for failure cases in prepare_creds(). > I have probably missed a very interesting discussion where that was > mentioned but I don't see link to the discussion or anything explaining > why we want to do that in this change. > AFAIK, this is the start of the discussion. > Eric >
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