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Date:   Fri, 28 Feb 2020 12:22:56 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     "Theodore Ts'o" <tytso@mit.edu>,
        Suraj Jitindar Singh <surajjs@amazon.com>
Cc:     Andreas Dilger <adilger.kernel@dilger.ca>,
        linux-ext4@vger.kernel.org, kernel-janitors@vger.kernel.org,
        Walter Harms <wharms@bfs.de>
Subject: [PATCH] ext4: potential crash on allocation error in
 ext4_alloc_flex_bg_array()
Message-ID: <20200228092142.7irbc44yaz3by7nb@kili.mountain>
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If sbi->s_flex_groups_allocated is zero and the first allocation fails
then this code will crash.  The problem is that "i--" will set "i" to
-1 but when we compare "i >= sbi->s_flex_groups_allocated" then the -1
is type promoted to unsigned and becomes UINT_MAX.  Since UINT_MAX
is more than zero, the condition is true so we call kvfree(new_groups[-1]).
The loop will carry on freeing invalid memory until it crashes.

Fixes: 7c990728b99e ("ext4: fix potential race between s_flex_groups online resizing and access")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
I changed this from a -- loop to a ++ loop because I knew it would make
Walter Harms happy.  He hates -- loops and I don't when his birthday so
I'm celebrating it today.  :)

 fs/ext4/super.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ff1b764b0c0e..0c7c4adb664e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2391,7 +2391,7 @@ int ext4_alloc_flex_bg_array(struct super_block *sb, ext4_group_t ngroup)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct flex_groups **old_groups, **new_groups;
-	int size, i;
+	int size, i, j;
 
 	if (!sbi->s_log_groups_per_flex)
 		return 0;
@@ -2412,8 +2412,8 @@ int ext4_alloc_flex_bg_array(struct super_block *sb, ext4_group_t ngroup)
 					 sizeof(struct flex_groups)),
 					 GFP_KERNEL);
 		if (!new_groups[i]) {
-			for (i--; i >= sbi->s_flex_groups_allocated; i--)
-				kvfree(new_groups[i]);
+			for (j = sbi->s_flex_groups_allocated; j < i; j++)
+				kvfree(new_groups[j]);
 			kvfree(new_groups);
 			ext4_msg(sb, KERN_ERR,
 				 "not enough memory for %d flex groups", size);
-- 
2.11.0