[PATCH v4 3/7] fs: use copy_from_user_nolog() to copy mount() data

[Peter Collingbourne]

With uaccess logging the contract is that the kernel must not report
accessing more data than necessary, as this can lead to false positive
reports in downstream consumers. This generally works out of the box
when instrumenting copy_{from,to}_user(), but with the data argument
to mount() we use copy_from_user() to copy PAGE_SIZE bytes (or as
much as we can, if the PAGE_SIZE sized access failed) and figure out
later how much we actually need.

To prevent this from leading to a false positive report, use
copy_from_user_nolog(), which will prevent the access from being logged.
Recall that it is valid for the kernel to report accessing less
data than it actually accessed, as uaccess logging is a best-effort
mechanism for reporting uaccesses.

Link: https://linux-review.googlesource.com/id/I5629b92a725c817acd9a861288338dd605cafee6
Signed-off-by: Peter Collingbourne <pcc at google.com>
---
 fs/namespace.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 659a8f39c61a..8f5f2aaca64e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -31,6 +31,7 @@
 #include <uapi/linux/mount.h>
 #include <linux/fs_context.h>
 #include <linux/shmem_fs.h>
+#include <linux/uaccess-buffer.h>
 
 #include "pnode.h"
 #include "internal.h"
@@ -3197,7 +3198,12 @@ static void *copy_mount_options(const void __user * data)
 	if (!copy)
 		return ERR_PTR(-ENOMEM);
 
-	left = copy_from_user(copy, data, PAGE_SIZE);
+	/*
+	 * Use copy_from_user_nolog to avoid reporting overly large accesses in
+	 * the uaccess buffer, as this can lead to false positive reports in
+	 * downstream consumers.
+	 */
+	left = copy_from_user_nolog(copy, data, PAGE_SIZE);
 
 	/*
 	 * Not all architectures have an exact copy_from_user(). Resort to
-- 
2.34.1.173.g76aa8bc2d0-goog