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ext4: allow ext4_get_group_info() to fail

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[ Upstream commit 5354b2af34064a4579be8bc0e2f15a7b70f14b5f ]

Previously, ext4_get_group_info() would treat an invalid group number
as BUG(), since in theory it should never happen.  However, if a
malicious attaker (or fuzzer) modifies the superblock via the block
device while it is the file system is mounted, it is possible for
s_first_data_block to get set to a very large number.  In that case,
when calculating the block group of some block number (such as the
starting block of a preallocation region), could result in an
underflow and very large block group number.  Then the BUG_ON check in
ext4_get_group_info() would fire, resutling in a denial of service
attack that can be triggered by root or someone with write access to
the block device.

For a quality of implementation perspective, it's best that even if
the system administrator does something that they shouldn't, that it
will not trigger a BUG.  So instead of BUG'ing, ext4_get_group_info()
will call ext4_error and return NULL.  We also add fallback code in
all of the callers of ext4_get_group_info() that it might NULL.

Also, since ext4_get_group_info() was already borderline to be an
inline function, un-inline it.  The results in a next reduction of the
compiled text size of ext4 by roughly 2k.

Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20230430154311.579720-2-tytso@mit.edu
Reported-by: syzbot+e2efa3efc15a1c9e95c3@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=69b28112e098b070f639efb356393af3ffec4220
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Theodore Ts'o
2023-04-29 00:06:28 -04:00
committed by Greg Kroah-Hartman
parent 371d8b8ea0
commit 100c0ad6c0
5 changed files with 82 additions and 29 deletions
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15
fs/ext4/ext4.h
View File

@@ -2551,6 +2551,8 @@ extern void ext4_check_blocks_bitmap(struct super_block *);
extern struct ext4_group_desc * ext4_get_group_desc(struct super_block * sb,
ext4_group_t block_group,
struct buffer_head ** bh);
extern struct ext4_group_info *ext4_get_group_info(struct super_block *sb,
ext4_group_t group);
extern int ext4_should_retry_alloc(struct super_block *sb, int *retries);
extern struct buffer_head *ext4_read_block_bitmap_nowait(struct super_block *sb,
@@ -3198,19 +3200,6 @@ static inline void ext4_isize_set(struct ext4_inode *raw_inode, loff_t i_size)
raw_inode->i_size_high = cpu_to_le32(i_size >> 32);
}
static inline
struct ext4_group_info *ext4_get_group_info(struct super_block *sb,
ext4_group_t group)
{
struct ext4_group_info **grp_info;
long indexv, indexh;
BUG_ON(group >= EXT4_SB(sb)->s_groups_count);
indexv = group >> (EXT4_DESC_PER_BLOCK_BITS(sb));
indexh = group & ((EXT4_DESC_PER_BLOCK(sb)) - 1);
grp_info = sbi_array_rcu_deref(EXT4_SB(sb), s_group_info, indexv);
return grp_info[indexh];
}
/*
* Reading s_groups_count requires using smp_rmb() afterwards. See
* the locking protocol documented in the comments of ext4_group_add()