Date:   Fri, 17 Feb 2023 08:37:54 +0000
From:   Tudor Ambarus <tudor.ambarus@...aro.org>
To:     "Darrick J. Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>
Cc:     Jun Nie <jun.nie@...aro.org>, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lee Jones <joneslee@...gle.com>
Subject: Re: [PATCH] ext4: fix another off-by-one fsmap error on 1k block
 filesystems



On 2/17/23 08:10, Tudor Ambarus wrote:
> Hi, Darrick,
> 
> On 2/16/23 18:55, Darrick J. Wong wrote:
>> From: Darrick J. Wong <djwong@...nel.org>
>>
>> Apparently syzbot figured out that issuing this FSMAP call:
>>
>> struct fsmap_head cmd = {
>>     .fmh_count    = ...;
>>     .fmh_keys    = {
>>         { .fmr_device = /* ext4 dev */, .fmr_physical = 0, },
>>         { .fmr_device = /* ext4 dev */, .fmr_physical = 0, },
>>     },
>> ...
>> };
>> ret = ioctl(fd, FS_IOC_GETFSMAP, &cmd);
>>
>> Produces this crash if the underlying filesystem is a 1k-block ext4
>> filesystem:
>>
>> kernel BUG at fs/ext4/ext4.h:3331!
>> invalid opcode: 0000 [#1] PREEMPT SMP
>> CPU: 3 PID: 3227965 Comm: xfs_io Tainted: G        W  O       
>> 6.2.0-rc8-achx
>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 
>> 04/01/2014
>> RIP: 0010:ext4_mb_load_buddy_gfp+0x47c/0x570 [ext4]
>> RSP: 0018:ffffc90007c03998 EFLAGS: 00010246
>> RAX: ffff888004978000 RBX: ffffc90007c03a20 RCX: ffff888041618000
>> RDX: 0000000000000000 RSI: 00000000000005a4 RDI: ffffffffa0c99b11
>> RBP: ffff888012330000 R08: ffffffffa0c2b7d0 R09: 0000000000000400
>> R10: ffffc90007c03950 R11: 0000000000000000 R12: 0000000000000001
>> R13: 00000000ffffffff R14: 0000000000000c40 R15: ffff88802678c398
>> FS:  00007fdf2020c880(0000) GS:ffff88807e100000(0000) 
>> knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007ffd318a5fe8 CR3: 000000007f80f001 CR4: 00000000001706e0
>> Call Trace:
>>   <TASK>
>>   ext4_mballoc_query_range+0x4b/0x210 [ext4 
>> dfa189daddffe8fecd3cdfd00564e0f265a8ab80]
>>   ext4_getfsmap_datadev+0x713/0x890 [ext4 
>> dfa189daddffe8fecd3cdfd00564e0f265a8ab80]
>>   ext4_getfsmap+0x2b7/0x330 [ext4 
>> dfa189daddffe8fecd3cdfd00564e0f265a8ab80]
>>   ext4_ioc_getfsmap+0x153/0x2b0 [ext4 
>> dfa189daddffe8fecd3cdfd00564e0f265a8ab80]
>>   __ext4_ioctl+0x2a7/0x17e0 [ext4 
>> dfa189daddffe8fecd3cdfd00564e0f265a8ab80]
>>   __x64_sys_ioctl+0x82/0xa0
>>   do_syscall_64+0x2b/0x80
>>   entry_SYSCALL_64_after_hwframe+0x46/0xb0
>> RIP: 0033:0x7fdf20558aff
>> RSP: 002b:00007ffd318a9e30 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
>> RAX: ffffffffffffffda RBX: 00000000000200c0 RCX: 00007fdf20558aff
>> RDX: 00007fdf1feb2010 RSI: 00000000c0c0583b RDI: 0000000000000003
>> RBP: 00005625c0634be0 R08: 00005625c0634c40 R09: 0000000000000001
>> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fdf1feb2010
>> R13: 00005625be70d994 R14: 0000000000000800 R15: 0000000000000000
>>
>> For GETFSMAP calls, the caller selects a physical block device by
>> writing its block number into fsmap_head.fmh_keys[01].fmr_device.
>> To query mappings for a subrange of the device, the starting byte of the
>> range is written to fsmap_head.fmh_keys[0].fmr_physical and the last
>> byte of the range goes in fsmap_head.fmh_keys[1].fmr_physical.
>>
>> IOWs, to query what mappings overlap with bytes 3-14 of /dev/sda, you'd
>> set the inputs as follows:
>>
>>     fmh_keys[0] = { .fmr_device = major(8, 0), .fmr_physical = 3},
>>     fmh_keys[1] = { .fmr_device = major(8, 0), .fmr_physical = 14},
>>
>> Which would return you whatever is mapped in the 12 bytes starting at
>> physical offset 3.
>>
>> The crash is due to insufficient range validation of keys[1] in
>> ext4_getfsmap_datadev.  On 1k-block filesystems, block 0 is not part of
>> the filesystem, which means that s_first_data_block is nonzero.
>> ext4_get_group_no_and_offset subtracts this quantity from the blocknr
>> argument before cracking it into a group number and a block number
>> within a group.  IOWs, block group 0 spans blocks 1-8192 (1-based)
>> instead of 0-8191 (0-based) like what happens with larger blocksizes.
>>
>> The net result of this encoding is that blocknr < s_first_data_block is
>> not a valid input to this function.  The end_fsb variable is set from
>> the keys that are copied from userspace, which means that in the above
>> example, its value is zero.  That leads to an underflow here:
>>
>>     blocknr = blocknr - le32_to_cpu(es->s_first_data_block);
>>
>> The division then operates on -1:
>>
>>     offset = do_div(blocknr, EXT4_BLOCKS_PER_GROUP(sb)) >>
>>         EXT4_SB(sb)->s_cluster_bits;
>>
>> Leaving an impossibly large group number (2^32-1) in blocknr.
>> ext4_getfsmap_check_keys checked that keys[0].fmr_physical and
>> keys[1].fmr_physical are in increasing order, but
>> ext4_getfsmap_datadev adjusts keys[0].fmr_physical to be at least
>> s_first_data_block.  This implies that we have to check it again after
>> the adjustment, which is the piece that I forgot.
>>
>> Fixes: 4a4956249dac ("ext4: fix off-by-one fsmap error on 1k block 
>> filesystems")
>> Link: 
>> https://syzkaller.appspot.com/bug?id=79d5768e9bfe362911ac1a5057a36fc6b5c30002
>> Signed-off-by: Darrick J. Wong <djwong@...nel.org>
>> ---
>>   fs/ext4/fsmap.c |    2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/ext4/fsmap.c b/fs/ext4/fsmap.c
>> index 4493ef0c715e..cdf9bfe10137 100644
>> --- a/fs/ext4/fsmap.c
>> +++ b/fs/ext4/fsmap.c
>> @@ -486,6 +486,8 @@ static int ext4_getfsmap_datadev(struct 
>> super_block *sb,
>>           keys[0].fmr_physical = bofs;
>>       if (keys[1].fmr_physical >= eofs)
>>           keys[1].fmr_physical = eofs - 1;
>> +    if (keys[1].fmr_physical < keys[0].fmr_physical)
>> +        return 0;
> 
> This is an indirect implication, we can be more straightforward. Also we
> should stop the execution when high_key->fmr_physical == bofs. So maybe:
> 
> --- a/fs/ext4/fsmap.c
> +++ b/fs/ext4/fsmap.c
> @@ -479,6 +479,8 @@ static int ext4_getfsmap_datadev(struct super_block 
> *sb,
>          int error = 0;
> 
>          bofs = le32_to_cpu(sbi->s_es->s_first_data_block);
> +       if (keys[1].fmr_physical <= bofs)
> +               return 0;
>          eofs = ext4_blocks_count(sbi->s_es);
>          if (keys[0].fmr_physical >= eofs)
>                  return 0;

oops, it was too morning for me, this is wrong, as the high key can come 
with zero value. So if we want to keep the check in
ext4_getfsmap_datadev we can use your patch, but slightly adapted to
if (keys[1].fmr_physical <= keys[0].fmr_physical)
	return 0;

However I think I prefer the version from below if it makes sense.
Cheers,
ta

> 
> 
> But I'm not thrilled about this. Can we stop the execution earlier?
> How about something like:
> 
> diff --git a/fs/ext4/fsmap.c b/fs/ext4/fsmap.c
> index 4493ef0c715e..8c4b1fb5c10a 100644
> --- a/fs/ext4/fsmap.c
> +++ b/fs/ext4/fsmap.c
> @@ -586,10 +586,12 @@ static bool ext4_getfsmap_check_keys(struct 
> ext4_fsmap *low_key,
>   {
>          if (low_key->fmr_device > high_key->fmr_device)
>                  return false;
> -       if (low_key->fmr_device < high_key->fmr_device)
> +       if (low_key->fmr_device != 0 &&
> +           low_key->fmr_device < high_key->fmr_device)
>                  return true;
> 
> -       if (low_key->fmr_physical > high_key->fmr_physical)
> +       if (high_key->fmr_physical == 0 ||
> +           low_key->fmr_physical > high_key->fmr_physical)
>                  return false;
>          if (low_key->fmr_physical < high_key->fmr_physical)
>                  return true;
> 
> You'll notice that my proposal will stop the execution with -EINVAL, as
> the key range is invalid IMO. But we can adapt code to return zero if
> you like, what I'm interested in is whether this key check is valid for
> ext4_getfsmap_logdev() or not. How about the
> if (keys[1].fmr_physical == bofs) check, is it valid for
> ext4_getfsmap_logdev()?
> 
> I'd like that with your guidance to fix this bug, extend the xfs_io
> fsmap support to query ext4 fs range and to add an xfstest for keys
> validation, to check if stable kernels will be fixed. This would help me
> with the fs ramp-up, as I'd like to fix some other fs bugs too. If you
> want to handle this by yourself, it's fine too, but please let me know
> so that we don't duplicate effort.
> 
> Cheers,
> ta

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