linux-ext4 - [PATCH] ext4: avoid oversized shift in ext4_fill_flex

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Date:	Mon, 26 Dec 2011 01:36:44 -0500
From:	Xi Wang <xi.wang@...il.com>
To:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>
Cc:	linux-ext4@...r.kernel.org, Xi Wang <xi.wang@...il.com>
Subject: [PATCH] ext4: avoid oversized shift in ext4_fill_flex_info()

Commit 503358ae fixed a division by zero, but groups_per_flex still
overflows due to an oversized shift, given a large s_log_groups_per_flex
like 36.  (1 << 36) is undefined in C; the result may vary depending
on the architecture, e.g., 16 on x86, thus bypassing the sanity check
(groups_per_flex < 2).

Signed-off-by: Xi Wang <xi.wang@...il.com>
---
 fs/ext4/super.c |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 3e1329e..6deaf41 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2010,14 +2010,15 @@ static int ext4_fill_flex_info(struct super_block *sb)
 	size_t size;
 	int i;
 
-	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
-	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
-
-	if (groups_per_flex < 2) {
+	if (sbi->s_es->s_log_groups_per_flex == 0 ||
+	    sbi->s_es->s_log_groups_per_flex >= 32) {
 		sbi->s_log_groups_per_flex = 0;
 		return 1;
 	}
 
+	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
+	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
+
 	/* We allocate both existing and potentially added groups */
 	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
 			((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<
-- 
1.7.5.4

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