Hi Eric,
On 6/13/22 12:04 PM, Eric W. Biederman wrote:
Frederick Lawler <fred@xxxxxxxxxxxxxx> 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
