[PATCH 4.9 28/88] ext4: potential crash on allocation error in ext4_alloc_flex_bg_array()

[Greg Kroah-Hartman]

From: Dan Carpenter <dan.carpenter@xxxxxxxxxx>

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 <surajjs@xxxxxxxxxx>
Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
Link: https://lore.kernel.org/r/20200228092142.7irbc44yaz3by7nb@xxxxxxxxxxxxx
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2120,7 +2120,7 @@ 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;
@@ -2141,8 +2141,8 @@ int ext4_alloc_flex_bg_array(struct supe
 					      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);