[PATCH] lsm: handle the NULL buffer case in lsm_fill_user_ctx()

[Casey Schaufler]

On 3/15/2024 8:02 AM, Serge E. Hallyn wrote:
> On Wed, Mar 13, 2024 at 10:22:03PM -0400, Paul Moore wrote:
>> Passing a NULL buffer into the lsm_get_self_attr() syscall is a valid
>> way to quickly determine the minimum size of the buffer needed to for
>> the syscall to return all of the LSM attributes to the caller.
>> Unfortunately we/I broke that behavior in commit d7cf3412a9f6
>> ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()")
>> such that it returned an error to the caller; this patch restores the
>> original desired behavior of using the NULL buffer as a quick way to
>> correctly size the attribute buffer.
>>
>> Cc: stable at vger.kernel.org
>> Fixes: d7cf3412a9f6 ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()")
>> Signed-off-by: Paul Moore <paul at paul-moore.com>
>> ---
>>  security/security.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/security/security.c b/security/security.c
>> index 5b2e0a15377d..7e118858b545 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -780,7 +780,9 @@ static int lsm_superblock_alloc(struct super_block *sb)
>>   * @id: LSM id
>>   * @flags: LSM defined flags
>>   *
>> - * Fill all of the fields in a userspace lsm_ctx structure.
>> + * Fill all of the fields in a userspace lsm_ctx structure.  If @uctx is NULL
>> + * simply calculate the required size to output via @utc_len and return
>> + * success.
>>   *
>>   * Returns 0 on success, -E2BIG if userspace buffer is not large enough,
>>   * -EFAULT on a copyout error, -ENOMEM if memory can't be allocated.
>> @@ -799,6 +801,10 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
>>  		goto out;
>>  	}
>>  
>> +	/* no buffer - return success/0 and set @uctx_len to the req size */
>> +	if (!uctx)
>> +		goto out;
> If the user just passes in *uctx_len=0, then they will get -E2BIG
> but still will get the length in *uctx_len.

Yes.

> To use it this new way, they have to first set *uctx_len to a
> value larger than nctx_len could possibly be, else they'll...
> still get -E2BIG.

Not sure I understand the problem. A return of 0 or E2BIG gets the
caller the size. 

> So I'm not sure this patch has value.
>
>>  	nctx = kzalloc(nctx_len, GFP_KERNEL);
>>  	if (nctx == NULL) {
>>  		rc = -ENOMEM;
>> -- 
>> 2.44.0
>>