Re: [PATCH] fs: Fix UBSAN detected shift-out-bounds error for bad superblock

["liaochang (A)" <liaochang1@huawei.com>]

在 2022/11/14 19:04, Jan Kara 写道:
> On Mon 14-11-22 10:49:57, Liao Chang wrote:
>> UBSAN: shift-out-of-bounds in fs/minix/bitmap.c:103:3
>> shift exponent 8192 is too large for 32-bit type 'unsigned int'
>> CPU: 1 PID: 32273 Comm: syz-executor.0 Tainted: G        W
>> 6.1.0-rc4-dirty #11
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>> BIOS rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014
>> Call Trace:
>>  <TASK>
>>  dump_stack_lvl+0xcd/0x134
>>  ubsan_epilogue+0xb/0x50
>>  __ubsan_handle_shift_out_of_bounds.cold+0xb1/0x18d
>>  minix_count_free_blocks.cold+0x16/0x1b
>>  minix_statfs+0x22a/0x490
>>  statfs_by_dentry+0x133/0x210
>>  user_statfs+0xa9/0x160
>>  __do_sys_statfs+0x7a/0xf0
>>  do_syscall_64+0x35/0x80
>>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>
>> The superblock stores on disk contains the size of a data zone, which is
>> too large to used as the shift when kernel try to calculate the total
>> size of zones, so it needs to check the superblock when kernel mounts
>> MINIX-FS.
>>
>> Signed-off-by: Liao Chang <liaochang1@huawei.com>
> 
> Thanks for the patch. Just one nit:
> 
>> diff --git a/fs/minix/inode.c b/fs/minix/inode.c
>> index da8bdd1712a7..f1d1c2312817 100644
>> --- a/fs/minix/inode.c
>> +++ b/fs/minix/inode.c
>> @@ -166,6 +166,12 @@ static bool minix_check_superblock(struct super_block *sb)
>>  	    sb->s_maxbytes > (7 + 512 + 512*512) * BLOCK_SIZE)
>>  		return false;
>>  
>> +	/* the total size of zones must no exceed the limitation of U32_MAX. */
>> +	if (sbi->s_log_zone_size && (sbi->s_nzones - sbi->s_firstdatazone) &&
>> +	    (__builtin_clzl((__u32)(sbi->s_nzones - sbi->s_firstdatazone)) <=
> 
> Why this strange __builtin_clzl() function? We have a ffs() function in
> the kernel for this :)

Great suggestion, i should use a compiler neutral API to caclulate leading zero count,
what about count_leading_zeros()?

Thanks.

> 
> 								Honza
> 
>> +	     sbi->s_log_zone_size))
>> +		return false;
>> +
>>  	return true;
>>  }
>>  
>> -- 
>> 2.17.1
>>

-- 
BR,
Liao, Chang