[PATCH 3.16 154/328] fs/quota: Fix spectre gadget in do_quotactl

[Ben Hutchings]

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jeremy Cline <jcline@xxxxxxxxxx>

commit 7b6924d94a60c6b8c1279ca003e8744e6cd9e8b1 upstream.

'type' is user-controlled, so sanitize it after the bounds check to
avoid using it in speculative execution. This covers the following
potential gadgets detected with the help of smatch:

* fs/ext4/super.c:5741 ext4_quota_read() warn: potential spectre issue
  'sb_dqopt(sb)->files' [r]
* fs/ext4/super.c:5778 ext4_quota_write() warn: potential spectre issue
  'sb_dqopt(sb)->files' [r]
* fs/f2fs/super.c:1552 f2fs_quota_read() warn: potential spectre issue
  'sb_dqopt(sb)->files' [r]
* fs/f2fs/super.c:1608 f2fs_quota_write() warn: potential spectre issue
  'sb_dqopt(sb)->files' [r]
* fs/quota/dquot.c:412 mark_info_dirty() warn: potential spectre issue
  'sb_dqopt(sb)->info' [w]
* fs/quota/dquot.c:933 dqinit_needed() warn: potential spectre issue
  'dquots' [r]
* fs/quota/dquot.c:2112 dquot_commit_info() warn: potential spectre
  issue 'dqopt->ops' [r]
* fs/quota/dquot.c:2362 vfs_load_quota_inode() warn: potential spectre
  issue 'dqopt->files' [w] (local cap)
* fs/quota/dquot.c:2369 vfs_load_quota_inode() warn: potential spectre
  issue 'dqopt->ops' [w] (local cap)
* fs/quota/dquot.c:2370 vfs_load_quota_inode() warn: potential spectre
  issue 'dqopt->info' [w] (local cap)
* fs/quota/quota.c:110 quota_getfmt() warn: potential spectre issue
  'sb_dqopt(sb)->info' [r]
* fs/quota/quota_v2.c:84 v2_check_quota_file() warn: potential spectre
  issue 'quota_magics' [w]
* fs/quota/quota_v2.c:85 v2_check_quota_file() warn: potential spectre
  issue 'quota_versions' [w]
* fs/quota/quota_v2.c:96 v2_read_file_info() warn: potential spectre
  issue 'dqopt->info' [r]
* fs/quota/quota_v2.c:172 v2_write_file_info() warn: potential spectre
  issue 'dqopt->info' [r]

Additionally, a quick inspection indicates there are array accesses with
'type' in quota_on() and quota_off() functions which are also addressed
by this.

Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Jeremy Cline <jcline@xxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
[bwh: Backported to 3.16: The maximum valid quota type is command-dependent.
 Introduce a local variable rather than repeating the expression.]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/quota/quota.c
+++ b/fs/quota/quota.c
@@ -17,6 +17,7 @@
 #include <linux/quotaops.h>
 #include <linux/types.h>
 #include <linux/writeback.h>
+#include <linux/nospec.h>
 
 static int check_quotactl_permission(struct super_block *sb, int type, int cmd,
 				     qid_t id)
@@ -405,10 +406,12 @@ static int quota_rmxquota(struct super_b
 static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id,
 		       void __user *addr, struct path *path)
 {
+	unsigned int max_quotas = XQM_COMMAND(cmd) ? XQM_MAXQUOTAS : MAXQUOTAS;
 	int ret;
 
-	if (type >= (XQM_COMMAND(cmd) ? XQM_MAXQUOTAS : MAXQUOTAS))
+	if (type >= max_quotas)
 		return -EINVAL;
+	type = array_index_nospec(type, max_quotas);
 	if (!sb->s_qcop)
 		return -ENOSYS;