[PATCH 2/2] fs/proc/kcore.c: Add bounce buffer for ktext data

[Jiri Olsa]

We hit hardened usercopy feature check for kernel text
access by reading kcore file:

  usercopy: kernel memory exposure attempt detected from ffffffff8179a01f (<kernel text>) (4065 bytes)
  kernel BUG at mm/usercopy.c:75!

Bypassing this check for kcore by adding bounce buffer
for ktext data.

Reported-by: Steve Best <sbest@xxxxxxxxxx>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 fs/proc/kcore.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index bd3ac9dca252..5c89a07e3d7f 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -509,7 +509,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 			if (kern_addr_valid(start)) {
 				unsigned long n;
 
-				n = copy_to_user(buffer, (char *)start, tsz);
+				/*
+				 * Using bounce buffer to bypass the
+				 * hardened user copy kernel text checks.
+				 */
+				memcpy(buf, (char *) start, tsz);
+				n = copy_to_user(buffer, buf, tsz);
 				/*
 				 * We cannot distinguish between fault on source
 				 * and fault on destination. When this happens
-- 
2.7.4