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Subject[PATCH 4.14 034/126] ext4: potential crash on allocation error in ext4_alloc_flex_bg_array()
From: Dan Carpenter <>

commit 37b0b6b8b99c0e1c1f11abbe7cf49b6d03795b3f upstream.

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")
Reviewed-by: Suraj Jitindar Singh <>
Signed-off-by: Dan Carpenter <>
Signed-off-by: Theodore Ts'o <>
Signed-off-by: Greg Kroah-Hartman <>

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

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2238,7 +2238,7 @@ int ext4_alloc_flex_bg_array(struct supe
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;
@@ -2259,8 +2259,8 @@ int ext4_alloc_flex_bg_array(struct supe
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);

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