statfs: fix potential Spectre v1

user_params.request is indirectly controlled by user-space, hence leading to a potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

fs/statfs.c:908 __do_sys_fsinfo() warn: potential spectre issue 'fsinfo_buffer_sizes' [r]

Fix this by sanitizing user_params.request before using it to index fsinfo_buffer_sizes

Notice that given that speculation windows are large, the policy is to kill the speculation on the first load and not worry if it can be completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

---
 fs/statfs.c | 3 +++
 1 file changed, 3 insertions(+)

index f714f05..d74a60a 100644
@@ -11,6 +11,7 @@
 #include <linux/compat.h>
 #include <linux/fsinfo.h>
 #include <linux/fs_parser.h>
+#include <linux/nospec.h>
 #include "internal.h"
 
 static int flags_by_mnt(int mnt_flags)

@@ -886,6 +887,8 @@ SYSCALL_DEFINE5(fsinfo,
 			return -EINVAL;
 		if (user_params.request >= FSINFO_ATTR__NR)
 			return -EOPNOTSUPP;
+		user_params.request = array_index_nospec(user_params.request,
+							 FSINFO_ATTR__NR);
 		params.at_flags = user_params.at_flags;
 		params.request = user_params.request;
 		params.Nth = user_params.Nth;

2.7.4