lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Mon, 24 Nov 2008 21:17:53 +0300
From:	Alex Zhuravlev <Alex.Zhuravlev@....COM>
To:	"Aneesh Kumar K.V" <>
Cc:	Theodore Tso <tytso@....EDU>,,,
Subject: Re: [PATCH -V2 3/5] ext4: Fix the race between read_block_bitmap and

Aneesh Kumar K.V wrote:
> With commit c806e68f we do a init_bitmap every time we do a
> read_block_bitmap.

can you explain why do we need to init it every time?

thanks, Alex

> To quote the update commit message that i have
>     ext4: Fix race between read_block_bitmap() and mark_diskspace_used()
>     We need to make sure we update the block bitmap and clear
>     EXT4_BG_BLOCK_UNINIT flag with sb_bgl_lock held.  We look at
>     EXT4_BG_BLOCK_UNINIT and reinit the block bitmap each time in
>     ext4_read_block_bitmap (introduced by commit c806e68f), and this can
>     race with block allocations in ext4_mb_mark_diskspace_used().
>     ext4_read_block_bitmap does:
>     spin_lock(sb_bgl_lock(EXT4_SB(sb), block_group));
>     if (desc->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
>     	ext4_init_block_bitmap(sb, bh, block_group, desc);
>     Now on the block allocation side we do
>     mb_set_bits(sb_bgl_lock(sbi, ac->ac_b_ex.fe_group), bitmap_bh->b_data,
>     			ac->ac_b_ex.fe_start, ac->ac_b_ex.fe_len);
>     ....
>     spin_lock(sb_bgl_lock(sbi, ac->ac_b_ex.fe_group));
>     if (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT)) {
>     	gdp->bg_flags &= cpu_to_le16(~EXT4_BG_BLOCK_UNINIT);
>     ie on allocation we update the bitmap then we take the sb_bgl_lock
>     and clear the EXT4_BG_BLOCK_UNINIT flag. What can happen is a
>     parallel ext4_read_block_bitmap can zero out the bitmap in between
>     the above mb_set_bits and spin_lock(sb_bg_lock..)
>     The race results in below user visible errors
>     EXT4-fs error (device sdb1): ext4_mb_release_inode_pa: free 100, pa_free 105
>     EXT4-fs error (device sdb1): mb_free_blocks: double-free of inode 0's block 50(bit 100 in group 0)
> -aneesh

To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to
More majordomo info at

Powered by blists - more mailing lists