Linux-ext4 Archive on
 help / color / Atom feed
From: Dan Carpenter <>
To: "Theodore Ts'o" <>,
	Suraj Jitindar Singh <>
Cc: Andreas Dilger <>,,,
	Walter Harms <>
Subject: [PATCH] ext4: potential crash on allocation error in ext4_alloc_flex_bg_array()
Date: Fri, 28 Feb 2020 12:22:56 +0300
Message-ID: <20200228092142.7irbc44yaz3by7nb@kili.mountain> (raw)

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 <>
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)),
 		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]);
 			ext4_msg(sb, KERN_ERR,
 				 "not enough memory for %d flex groups", size);

             reply index

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-28  9:22 Dan Carpenter [this message]
2020-02-28 18:45 ` Jitindar SIngh, Suraj

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200228092142.7irbc44yaz3by7nb@kili.mountain \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-ext4 Archive on

Archives are clonable:
	git clone --mirror linux-ext4/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-ext4 linux-ext4/ \
	public-inbox-index linux-ext4

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone