ext4: potential crash on allocation error in ext4_alloc_flex_bg_array()

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@amazon.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20200228092142.7irbc44yaz3by7nb@kili.mountain
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ff1b764..0c7c4ad 100644 (file)
--- 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);