[Devel] [PATCH RHEL7 COMMIT] ext4: Fix high probable use-after-free

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-1062.12.1.vz7.131.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1062.12.1.vz7.131.8
------>
commit d7bb1a3fd3c649bf327021a33f964d9150a06acc
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Fri Mar 6 18:28:54 2020 +0300

    ext4: Fix high probable use-after-free
    
    Here we have even worse race than in mainstream.
    
    https://jira.sw.ru/browse/PSBM-101798
    
    Fixes: 86521524314e "ext4: replace ext4_kvmalloc() with kvmalloc()"
    Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 fs/ext4/super.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 7b77cabae690c..3adeeb00f118f 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2196,7 +2196,7 @@ int ext4_alloc_group_desc_bh_array(struct super_block *sb, ext4_group_t ngroup)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	unsigned long num_desc = DIV_ROUND_UP(ngroup,  EXT4_DESC_PER_BLOCK(sb));
-	struct buffer_head **n_group_desc;
+	struct buffer_head **o_group_desc, **n_group_desc;
 
 	if (num_desc <= sbi->s_gdb_count)
 		return 0;
@@ -2208,11 +2208,13 @@ int ext4_alloc_group_desc_bh_array(struct super_block *sb, ext4_group_t ngroup)
 		return -ENOMEM;
 	}
 
-	memcpy(n_group_desc, sbi->s_group_desc,
+	o_group_desc = sbi->s_group_desc;
+	memcpy(n_group_desc, o_group_desc,
 	       sbi->s_gdb_count * sizeof(struct buffer_head *));
-	kvfree(sbi->s_group_desc);
+	WRITE_ONCE(sbi->s_group_desc, n_group_desc);
 
-	sbi->s_group_desc = n_group_desc;
+	/* FIXME: rcu is needed here. See ms commit 1d0c3924a92e */
+	kvfree(o_group_desc);
 	return 0;
 }