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

[Casey Schaufler <casey@schaufler-ca.com>]

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@chromium.org>
>> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
>> Cc: linux-api@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.

>
>
>> +	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);
>>