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Date:   Fri, 30 Dec 2022 13:42:45 +0200
From:   Tudor Ambarus <>
To:     Eric Biggers <>
Subject: Re: [PATCH v2] ext4: Fix possible use-after-free in ext4_find_extent

Hi, Eric,

On 30.12.2022 08:56, Eric Biggers wrote:
> On Fri, Dec 30, 2022 at 08:29:31AM +0200, Tudor Ambarus wrote:
>> syzbot reported a use-after-free Read in ext4_find_extent that is hit when
>> using a corrupted file system. The bug was reported on Android 5.15, but
>> using the same reproducer triggers the bug on v6.2-rc1 as well.
>> Fix the use-after-free by checking the extent header magic. An alternative
>> would be to check the values of EXT4_{FIRST,LAST}_{EXTENT,INDEX} used in
>> ext4_ext_binsearch() and ext4_ext_binsearch_idx(), so that we make sure
>> that pointers returned by EXT4_{FIRST,LAST}_{EXTENT,INDEX} don't exceed the
>> bounds of the extent tree node. But this alternative will not squash
>> the bug for the cases where eh->eh_entries fit into eh->eh_max. We could
>> also try to check the sanity of the path, but costs more than checking just
>> the header magic, so stick to the header magic sanity check.
>> Link:
>> Reported-by:
>> Cc:
>> Signed-off-by: Tudor Ambarus <>
>> ---
>> v2: drop wrong/uneeded le16_to_cpu() conversion for eh->eh_magic
>>   fs/ext4/extents.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index 9de1c9d1a13d..bedc8c098449 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -894,6 +894,12 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
>>   		gfp_flags |= __GFP_NOFAIL;
>>   	eh = ext_inode_hdr(inode);
>> +	if (eh->eh_magic != EXT4_EXT_MAGIC) {
>> +		EXT4_ERROR_INODE(inode, "Extent header has invalid magic.");
>> +		ret = -EFSCORRUPTED;
>> +		goto err;
>> +	}
>> +
> This is (incompletely) validating the extent header in the inode.  Isn't that
> supposed to happen when the inode is loaded?  See how __ext4_iget() calls
> ext4_ext_check_inode().  Why isn't that working here?

Seems that __ext4_iget() is not called on writes. You can find below the 
sequence of calls that leads to the bug. The debug was done on v6.2-rc1.
I assume the extents check is no longer done on writes since
commit 7a262f7c69163cd4811f2f838faef5c5b18439c9. The commit doesn't
specify the reason though.


#20 0xffffffff85000087 in entry_SYSCALL_64 ()
     at /home/atudor/work/aosp/arch/x86/entry/entry_64.S:120
#19 do_syscall_64 (regs=0xffffc90001fbff58, nr=82542)
     at /home/atudor/work/aosp/arch/x86/entry/common.c:80
#18 0xffffffff84f27caf in do_syscall_x64 (regs=0xffffc90001fbff58, nr=82542)
     at /home/atudor/work/aosp/arch/x86/entry/common.c:50
#17 __x64_sys_write (regs=<optimized out>) at 
#16 __se_sys_write (fd=4, buf=536871424, count=0)
     at /home/atudor/work/aosp/fs/read_write.c:646
#15 0xffffffff81c668fb in __do_sys_write (fd=4, buf=0x20000200 
"threaded", count=0)
     at /home/atudor/work/aosp/fs/read_write.c:649
#14 0xffffffff81c66758 in ksys_write (fd=fd@...ry=4, 
buf=buf@...ry=0x20000200 "threaded",
     count=392007683) at /home/atudor/work/aosp/fs/read_write.c:637
#13 vfs_write (file=file@...ry=0xffff8881352f9900, buf=<optimized out>,
     buf@...ry=0x20000200 "threaded", count=<optimized out>, 
     pos=<optimized out>, pos@...ry=0xffffc90001fbfe80)
     at /home/atudor/work/aosp/fs/read_write.c:584
#12 new_sync_write (filp=<optimized out>, buf=0x20000200 "threaded", 
     ppos=0xffffc90001fbfe80) at /home/atudor/work/aosp/fs/read_write.c:491
#11 0xffffffff81c65ba5 in call_write_iter (file=0xffff8881352f9900, 
kio=<optimized out>,
     iter=<optimized out>) at /home/atudor/work/aosp/include/linux/fs.h:2186
#10 0xffffffff81f11e78 in ext4_file_write_iter (iocb=<optimized out>,
     from=0xffffc90001fbfd70) at /home/atudor/work/aosp/fs/ext4/file.c:700
#9  0xffffffff81f148ff in ext4_buffered_write_iter (iocb=0xffffc90001fbfd20,
     from=from@...ry=0xffffc90001fbfd70) at 
#8  0xffffffff81a4aef4 in generic_perform_write 
     i=<optimized out>, i@...ry=0xffffc90001fbfd70)
     at /home/atudor/work/aosp/mm/filemap.c:3772
#7  0xffffffff81f682b9 in ext4_da_write_begin (file=<optimized out>,
     mapping=<optimized out>, pos=<optimized out>, len=<optimized out>,
     pagep=<optimized out>, fsdata=<optimized out>)
     at /home/atudor/work/aosp/fs/ext4/inode.c:3082
#6  0xffffffff81f66995 in ext4_block_write_begin 
     pos=pos@...ry=32768, len=len@...ry=4096,
     get_block=0xffffffff81f47b10 <ext4_da_get_block_prep>)
     at /home/atudor/work/aosp/fs/ext4/inode.c:1098
#5  ext4_da_get_block_prep (inode=0xffff888117bbe7a8, iblock=16, 
     create=<optimized out>) at /home/atudor/work/aosp/fs/ext4/inode.c:1870
#4  ext4_da_map_blocks (inode=0xffff888117bbe7a8, iblock=16, 
     map=<optimized out>) at /home/atudor/work/aosp/fs/ext4/inode.c:1806
#3  0xffffffff81f48526 in ext4_insert_delayed_block 
     lblk=<optimized out>) at /home/atudor/work/aosp/fs/ext4/inode.c:1696
#2  0xffffffff81ef9b96 in ext4_clu_mapped 
(inode=inode@...ry=0xffff888117bbe7a8, lclu=1)
     at /home/atudor/work/aosp/fs/ext4/extents.c:5809
#1  ext4_find_extent (inode=inode@...ry=0xffff888117bbe7a8, block=16,
     orig_path=orig_path@...ry=0x0 <fixed_percpu_data>, flags=flags@...ry=0)
     at /home/atudor/work/aosp/fs/ext4/extents.c:931
#0  ext4_ext_binsearch_idx (path=0xffff888136ad3800, block=16, 
inode=<optimized out>)
     at /home/atudor/work/aosp/fs/ext4/extents.c:768

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