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@xxxxxxxxxxxx>
>>> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>>> Cc: linux-api@xxxxxxxxxxxxxxx
>>> ---
>>> 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);
>>>
