[PATCH v19 22/23] LSM: Add /proc attr entry for full LSM context

[John Johansen]

On 7/30/20 1:44 PM, Casey Schaufler wrote:
> On 7/30/2020 3:03 AM, John Johansen wrote:
>> On 7/24/20 1:32 PM, Casey Schaufler wrote:
>>> Add an entry /proc/.../attr/context which displays the full
>>> process security "context" in compound format:
>>>         lsm1\0value\0lsm2\0value\0...
>>> This entry is not writable.
>>>
>>> A security module may decide that its policy does not allow
>>> this information to be displayed. In this case none of the
>>> information will be displayed.
>>>
>>> Reviewed-by: Kees Cook <keescook at chromium.org>
>>> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
>>> Cc: linux-api at vger.kernel.org
>>> ---
>>>  Documentation/security/lsm.rst       | 28 +++++++++++
>>>  fs/proc/base.c                       |  1 +
>>>  include/linux/lsm_hooks.h            |  6 +++
>>>  security/apparmor/include/procattr.h |  2 +-
>>>  security/apparmor/lsm.c              |  8 +++-
>>>  security/apparmor/procattr.c         | 22 +++++----
>>>  security/security.c                  | 70 ++++++++++++++++++++++++++++
>>>  security/selinux/hooks.c             |  2 +-
>>>  security/smack/smack_lsm.c           |  2 +-
>>>  9 files changed, 126 insertions(+), 15 deletions(-)
> 
> <snip>
> 
>>>  
>>>  /**
>>> diff --git a/security/security.c b/security/security.c
>>> index d35e578fa45b..bce6be720401 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -754,6 +754,48 @@ static void __init lsm_early_task(struct task_struct *task)
>>>  		panic("%s: Early task alloc failed.\n", __func__);
>>>  }
>>>  
>>> +/**
>>> + * append_ctx - append a lsm/context pair to a compound context
>>> + * @ctx: the existing compound context
>>> + * @ctxlen: size of the old context, including terminating nul byte
>>> + * @lsm: new lsm name, nul terminated
>>> + * @new: new context, possibly nul terminated
>>> + * @newlen: maximum size of @new
>>> + *
>>> + * replace @ctx with a new compound context, appending @newlsm and @new
>>> + * to @ctx. On exit the new data replaces the old, which is freed.
>>> + * @ctxlen is set to the new size, which includes a trailing nul byte.
>>> + *
>>> + * Returns 0 on success, -ENOMEM if no memory is available.
>>> + */
>>> +static int append_ctx(char **ctx, int *ctxlen, const char *lsm, char *new,
>>> +		      int newlen)
>>> +{
>>> +	char *final;
>>> +	size_t llen;
>>> +
>>> +	llen = strlen(lsm) + 1;
>>> +	/*
>>> +	 * A security module may or may not provide a trailing nul on
>>> +	 * when returning a security context. There is no definition
>>> +	 * of which it should be, and there are modules that do it
>>> +	 * each way.
>>> +	 */
>>> +	newlen = strnlen(new, newlen) + 1;
>>> +
>>> +	final = kzalloc(*ctxlen + llen + newlen, GFP_KERNEL);
>>> +	if (final == NULL)
>>> +		return -ENOMEM;
>>> +	if (*ctxlen)
>>> +		memcpy(final, *ctx, *ctxlen);
>>> +	memcpy(final + *ctxlen, lsm, llen);
>>> +	memcpy(final + *ctxlen + llen, new, newlen);
>> if @new doesn't have a newline appended at its end this will read 1 byte
>> passed the end of the @new buffer. Nor will the result have a trailing
>> \0 as expected unless we get lucky.
> 
> @new will never have a newline at the end. The trailing nul comes
> from the allocation being done with kzalloc(). This function has to
> be considered in the context of its caller.
> 

ugh, sorry not trailing newline, I meant trailing \0. The problem isn't
the kzalloc, the target has the space. It is the source @new. It is
dangerous to assume that the @new buffer has a null byte after its
declared length. Which is potentially what we are doing if @new
doesn't have an embedded null byte. In that case strlen(new, newlen)
will then return newlen and we add 1 to it.

which means in the memcpy we are copying an extra byte beyond what
was declared to exist in @new.

>>
>>
>>> +	kfree(*ctx);
>>> +	*ctx = final;
>>> +	*ctxlen = *ctxlen + llen + newlen;
>>> +	return 0;
>>> +}
>>> +
>>>  /*
>>>   * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
>>>   * can be accessed with:
>>> @@ -2124,6 +2166,10 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
>>>  				char **value)
>>>  {
>>>  	struct security_hook_list *hp;
>>> +	char *final = NULL;
>>> +	char *cp;
>>> +	int rc = 0;
>>> +	int finallen = 0;
>> these are only used by context so they could be moved under its if, this
>> is really just a style comment and I'll leave it up to you
> 
> Old coding habits die hard. Unless there's value to gain, I'll leave it
> as is.
> 
>>
>>>  	int display = lsm_task_display(current);
>>>  	int slot = 0;
>>>  
>>> @@ -2151,6 +2197,30 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
>>>  		return -ENOMEM;
>>>  	}
>>>  
>>> +	if (!strcmp(name, "context")) {
>>> +		hlist_for_each_entry(hp, &security_hook_heads.getprocattr,
>>> +				     list) {
>>> +			rc = hp->hook.getprocattr(p, "context", &cp);
>>> +			if (rc == -EINVAL)
>>> +				continue;
>>> +			if (rc < 0) {
>>> +				kfree(final);
>>> +				return rc;
>>> +			}
>>> +			rc = append_ctx(&final, &finallen, hp->lsmid->lsm,
>>> +					cp, rc);
>>> +			kfree(cp);
>>> +			if (rc < 0) {
>>> +				kfree(final);
>>> +				return rc;
>>> +			}
>>> +		}
>>> +		if (final == NULL)
>>> +			return -EINVAL;
>>> +		*value = final;
>>> +		return finallen;
>>> +	}
>>> +
>>>  	hlist_for_each_entry(hp, &security_hook_heads.getprocattr, list) {
>>>  		if (lsm != NULL && strcmp(lsm, hp->lsmid->lsm))
>>>  			continue;
>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>> index c13c207c5da1..43d5c09b9a9e 100644
>>> --- a/security/selinux/hooks.c
>>> +++ b/security/selinux/hooks.c
>>> @@ -6288,7 +6288,7 @@ static int selinux_getprocattr(struct task_struct *p,
>>>  			goto bad;
>>>  	}
>>>  
>>> -	if (!strcmp(name, "current"))
>>> +	if (!strcmp(name, "current") || !strcmp(name, "context"))
>>>  		sid = __tsec->sid;
>>>  	else if (!strcmp(name, "prev"))
>>>  		sid = __tsec->osid;
>>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>>> index 6f0cdb40addc..d7bb6442f192 100644
>>> --- a/security/smack/smack_lsm.c
>>> +++ b/security/smack/smack_lsm.c
>>> @@ -3463,7 +3463,7 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
>>>  	char *cp;
>>>  	int slen;
>>>  
>>> -	if (strcmp(name, "current") != 0)
>>> +	if (strcmp(name, "current") != 0 && strcmp(name, "context") != 0)
>>>  		return -EINVAL;
>>>  
>>>  	cp = kstrdup(skp->smk_known, GFP_KERNEL);
>>>