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Subject[PATCH 5.5 092/150] ext4: fix potential race between s_group_info online resizing and access
Date
From: Suraj Jitindar Singh <surajjs@amazon.com>

commit df3da4ea5a0fc5d115c90d5aa6caa4dd433750a7 upstream.

During an online resize an array of pointers to s_group_info gets replaced
so it can get enlarged. If there is a concurrent access to the array in
ext4_get_group_info() and this memory has been reused then this can lead to
an invalid memory access.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=206443
Link: https://lore.kernel.org/r/20200221053458.730016-3-tytso@mit.edu
Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Balbir Singh <sblbir@amazon.com>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/ext4/ext4.h | 8 ++++----
fs/ext4/mballoc.c | 52 +++++++++++++++++++++++++++++++++++-----------------
2 files changed, 39 insertions(+), 21 deletions(-)

--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1463,7 +1463,7 @@ struct ext4_sb_info {
#endif

/* for buddy allocator */
- struct ext4_group_info ***s_group_info;
+ struct ext4_group_info ** __rcu *s_group_info;
struct inode *s_buddy_cache;
spinlock_t s_md_lock;
unsigned short *s_mb_offsets;
@@ -2934,13 +2934,13 @@ static inline
struct ext4_group_info *ext4_get_group_info(struct super_block *sb,
ext4_group_t group)
{
- struct ext4_group_info ***grp_info;
+ struct ext4_group_info **grp_info;
long indexv, indexh;
BUG_ON(group >= EXT4_SB(sb)->s_groups_count);
- grp_info = EXT4_SB(sb)->s_group_info;
indexv = group >> (EXT4_DESC_PER_BLOCK_BITS(sb));
indexh = group & ((EXT4_DESC_PER_BLOCK(sb)) - 1);
- return grp_info[indexv][indexh];
+ grp_info = sbi_array_rcu_deref(EXT4_SB(sb), s_group_info, indexv);
+ return grp_info[indexh];
}

/*
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2356,7 +2356,7 @@ int ext4_mb_alloc_groupinfo(struct super
{
struct ext4_sb_info *sbi = EXT4_SB(sb);
unsigned size;
- struct ext4_group_info ***new_groupinfo;
+ struct ext4_group_info ***old_groupinfo, ***new_groupinfo;

size = (ngroups + EXT4_DESC_PER_BLOCK(sb) - 1) >>
EXT4_DESC_PER_BLOCK_BITS(sb);
@@ -2369,13 +2369,16 @@ int ext4_mb_alloc_groupinfo(struct super
ext4_msg(sb, KERN_ERR, "can't allocate buddy meta group");
return -ENOMEM;
}
- if (sbi->s_group_info) {
- memcpy(new_groupinfo, sbi->s_group_info,
+ rcu_read_lock();
+ old_groupinfo = rcu_dereference(sbi->s_group_info);
+ if (old_groupinfo)
+ memcpy(new_groupinfo, old_groupinfo,
sbi->s_group_info_size * sizeof(*sbi->s_group_info));
- kvfree(sbi->s_group_info);
- }
- sbi->s_group_info = new_groupinfo;
+ rcu_read_unlock();
+ rcu_assign_pointer(sbi->s_group_info, new_groupinfo);
sbi->s_group_info_size = size / sizeof(*sbi->s_group_info);
+ if (old_groupinfo)
+ ext4_kvfree_array_rcu(old_groupinfo);
ext4_debug("allocated s_groupinfo array for %d meta_bg's\n",
sbi->s_group_info_size);
return 0;
@@ -2387,6 +2390,7 @@ int ext4_mb_add_groupinfo(struct super_b
{
int i;
int metalen = 0;
+ int idx = group >> EXT4_DESC_PER_BLOCK_BITS(sb);
struct ext4_sb_info *sbi = EXT4_SB(sb);
struct ext4_group_info **meta_group_info;
struct kmem_cache *cachep = get_groupinfo_cache(sb->s_blocksize_bits);
@@ -2405,12 +2409,12 @@ int ext4_mb_add_groupinfo(struct super_b
"for a buddy group");
goto exit_meta_group_info;
}
- sbi->s_group_info[group >> EXT4_DESC_PER_BLOCK_BITS(sb)] =
- meta_group_info;
+ rcu_read_lock();
+ rcu_dereference(sbi->s_group_info)[idx] = meta_group_info;
+ rcu_read_unlock();
}

- meta_group_info =
- sbi->s_group_info[group >> EXT4_DESC_PER_BLOCK_BITS(sb)];
+ meta_group_info = sbi_array_rcu_deref(sbi, s_group_info, idx);
i = group & (EXT4_DESC_PER_BLOCK(sb) - 1);

meta_group_info[i] = kmem_cache_zalloc(cachep, GFP_NOFS);
@@ -2458,8 +2462,13 @@ int ext4_mb_add_groupinfo(struct super_b
exit_group_info:
/* If a meta_group_info table has been allocated, release it now */
if (group % EXT4_DESC_PER_BLOCK(sb) == 0) {
- kfree(sbi->s_group_info[group >> EXT4_DESC_PER_BLOCK_BITS(sb)]);
- sbi->s_group_info[group >> EXT4_DESC_PER_BLOCK_BITS(sb)] = NULL;
+ struct ext4_group_info ***group_info;
+
+ rcu_read_lock();
+ group_info = rcu_dereference(sbi->s_group_info);
+ kfree(group_info[idx]);
+ group_info[idx] = NULL;
+ rcu_read_unlock();
}
exit_meta_group_info:
return -ENOMEM;
@@ -2472,6 +2481,7 @@ static int ext4_mb_init_backend(struct s
struct ext4_sb_info *sbi = EXT4_SB(sb);
int err;
struct ext4_group_desc *desc;
+ struct ext4_group_info ***group_info;
struct kmem_cache *cachep;

err = ext4_mb_alloc_groupinfo(sb, ngroups);
@@ -2507,11 +2517,16 @@ err_freebuddy:
while (i-- > 0)
kmem_cache_free(cachep, ext4_get_group_info(sb, i));
i = sbi->s_group_info_size;
+ rcu_read_lock();
+ group_info = rcu_dereference(sbi->s_group_info);
while (i-- > 0)
- kfree(sbi->s_group_info[i]);
+ kfree(group_info[i]);
+ rcu_read_unlock();
iput(sbi->s_buddy_cache);
err_freesgi:
- kvfree(sbi->s_group_info);
+ rcu_read_lock();
+ kvfree(rcu_dereference(sbi->s_group_info));
+ rcu_read_unlock();
return -ENOMEM;
}

@@ -2700,7 +2715,7 @@ int ext4_mb_release(struct super_block *
ext4_group_t ngroups = ext4_get_groups_count(sb);
ext4_group_t i;
int num_meta_group_infos;
- struct ext4_group_info *grinfo;
+ struct ext4_group_info *grinfo, ***group_info;
struct ext4_sb_info *sbi = EXT4_SB(sb);
struct kmem_cache *cachep = get_groupinfo_cache(sb->s_blocksize_bits);

@@ -2719,9 +2734,12 @@ int ext4_mb_release(struct super_block *
num_meta_group_infos = (ngroups +
EXT4_DESC_PER_BLOCK(sb) - 1) >>
EXT4_DESC_PER_BLOCK_BITS(sb);
+ rcu_read_lock();
+ group_info = rcu_dereference(sbi->s_group_info);
for (i = 0; i < num_meta_group_infos; i++)
- kfree(sbi->s_group_info[i]);
- kvfree(sbi->s_group_info);
+ kfree(group_info[i]);
+ kvfree(group_info);
+ rcu_read_unlock();
}
kfree(sbi->s_mb_offsets);
kfree(sbi->s_mb_maxs);

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