[PATCH 2/2] lsm: fix size queries for getselfattr with NULL buffer

[Bradley Morgan <include@grrlz.net>]

The lsm_get_self_attr() syscall allows callers to pass in a NULL context
buffer to find out the size of the output needed. That path still
compared the computed entry size against the caller provided size first,
so a NULL buffer with size 0 incorrectly returned -E2BIG rather than
reporting the required size.

Only enforce the available buffer length after checking for the NULL
buffer. Cover the zero length sizing query in the self test.

Fixes: d7cf3412a9f6 ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()")
Cc: stable@vger.kernel.org
Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 security/security.c                                  | 8 ++++----
 tools/testing/selftests/lsm/lsm_get_self_attr_test.c | 5 ++---
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/security/security.c b/security/security.c
index 71aea8fdf014..fa0d7e036249 100644
--- a/security/security.c
+++ b/security/security.c
@@ -406,15 +406,15 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
 	int rc = 0;
 
 	nctx_len = ALIGN(struct_size(nctx, ctx, val_len), sizeof(void *));
+	/* no buffer - return success/0 and set @uctx_len to the req size */
+	if (!uctx)
+		goto out;
+
 	if (nctx_len > *uctx_len) {
 		rc = -E2BIG;
 		goto out;
 	}
 
-	/* no buffer - return success/0 and set @uctx_len to the req size */
-	if (!uctx)
-		goto out;
-
 	nctx = kzalloc(nctx_len, GFP_KERNEL);
 	if (nctx == NULL) {
 		rc = -ENOMEM;
diff --git a/tools/testing/selftests/lsm/lsm_get_self_attr_test.c b/tools/testing/selftests/lsm/lsm_get_self_attr_test.c
index 60caf8528f81..2f5ababc2b95 100644
--- a/tools/testing/selftests/lsm/lsm_get_self_attr_test.c
+++ b/tools/testing/selftests/lsm/lsm_get_self_attr_test.c
@@ -39,15 +39,14 @@ TEST(size_null_lsm_get_self_attr)
 
 TEST(ctx_null_lsm_get_self_attr)
 {
-	const long page_size = sysconf(_SC_PAGESIZE);
-	__u32 size = page_size;
+	__u32 size = 0;
 	int rc;
 
 	rc = lsm_get_self_attr(LSM_ATTR_CURRENT, NULL, &size, 0);
 
 	if (attr_lsm_count()) {
 		ASSERT_NE(-1, rc);
-		ASSERT_NE(1, size);
+		ASSERT_NE(0, size);
 	} else {
 		ASSERT_EQ(-1, rc);
 	}
-- 
2.53.0