Re: 5.10.1: UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:1

[Randy Dunlap <rdunlap@infradead.org>]

On 12/18/20 2:20 AM, Toralf Förster wrote:
> On 12/18/20 7:54 AM, Randy Dunlap wrote:
>> Hi,
>>
>> [adding linux-mm]
>>
>> On 12/16/20 1:54 AM, Toralf Förster wrote:
>>> Hi,
>>>
>>> I got this recently at this hardened Gentoo Linux server:
>>>
>>> Linux mr-fox 5.10.1 #1 SMP Tue Dec 15 22:09:42 CET 2020 x86_64 Intel(R)
>>> Xeon(R) CPU E5-1650 v3 @ 3.50GHz GenuineIntel GNU/Linux
>>>
>>>
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206972]
>>> ================================================================================
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206977] UBSAN: shift-out-of-bounds
>>> in ./include/linux/log2.h:57:13
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206980] shift exponent 64 is too
>>> large for 64-bit type 'long unsigned int'
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206982] CPU: 11 PID: 21051 Comm:
>>> cc1 Tainted: G                T 5.10.1 #1
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206984] Hardware name: ASUSTeK
>>> COMPUTER INC. Z10PA-U8 Series/Z10PA-U8 Series, BIOS 3703 08/02/2018
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206985] Call Trace:
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206993]  dump_stack+0x57/0x6a
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206996]  ubsan_epilogue+0x5/0x40
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.206999]
>>> __ubsan_handle_shift_out_of_bounds.cold+0x61/0x10e
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207002]
>>> ondemand_readahead.cold+0x16/0x21
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207007]
>>> generic_file_buffered_read+0x452/0x890
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207011]  new_sync_read+0x156/0x200
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207014]  vfs_read+0xf8/0x190
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207016]  ksys_read+0x65/0xe0
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207018]  do_syscall_64+0x33/0x40
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207021]
>>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207024] RIP: 0033:0x7f01b2df198e
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207026] Code: c0 e9 b6 fe ff ff 50
>>> 48 8d 3d 66 c3 09 00 e8 59 e2 01 00 66 0f 1f 84 00 00 00 00 00 64 8b 04
>>> 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00 f0 ff ff 77 5a c3 66 0f 1f
>>> 84 00 00 00 00 00 48 83 ec 28
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207028] RSP: 002b:00007fff2167e998
>>> EFLAGS: 00000246 ORIG_RAX: 0000000000000000
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207030] RAX: ffffffffffffffda RBX:
>>> 0000000000000000 RCX: 00007f01b2df198e
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207032] RDX: 0000000000000000 RSI:
>>> 00000000054dcc50 RDI: 0000000000000004
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207033] RBP: 00000000054dcc50 R08:
>>> 00000000054dcc50 R09: 0000000000000000
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207034] R10: 0000000000000000 R11:
>>> 0000000000000246 R12: 00000000054dc3b0
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207035] R13: 0000000000008000 R14:
>>> 00000000054c9800 R15: 0000000000000000
>>> Dec 15 23:31:51 mr-fox kernel: [ 1974.207037]
>>> ================================================================================
>>>
>>>
>>> Known issue ?
>>
>> Not that I have heard about, but that's not conclusive.
>>
>> Looks to me like this is in mm/readahead.c:
>>
>> static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
>> {
>>     unsigned long newsize = roundup_pow_of_two(size);
>>
>>
>> What filesystem?  What workload?
> 
> / is a 32 GB ext4 filesystem.
> Data are at 3 BTRFS filesystems, 1x 500 GB and 2x 1.6TB.
> 
> 2 Tor relays run at 100% each and utilizes the 1 GBit/s by 50%-60% [1]
> 
> 7 build bots are running over the Gentoo software repostory [2]
> 1 AFL bot fuzzies the Tor sources.
> Those 8 jobs are contained by a cgroup of 9 CPUs and 120 GB RAM [3],
> each job is contained further by an own sub cgroup of 1.5 CPU and 20 GB
> RAM [4]
> 
> The host is monitored using sysstat, the load is about 11.8, CPU[all] at
> 80%, proc/s at 1800, cswchs/s at 20000 and so on.
> 
> 
> [1] https://metrics.torproject.org/rs.html#search/zwiebeltoralf
> [2] https://zwiebeltoralf.de/tinderbox.html
> [3] https://github.com/toralf/tinderbox/blob/master/bin/cgroup.sh
> [4] https://github.com/toralf/tinderbox/blob/master/bin/bwrap.sh#L15
> 
> -- 

Hi Toralf,

Is this something that happens more than once?
I think we would like to find out what is causing it.
I see a couple of problems.

(a)
  UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
  shift exponent 64 is too large for 64-bit type 'long unsigned int'

<linux/log2.h>:57: is like so:

    50	/**
    51	 * __roundup_pow_of_two() - round up to nearest power of two
    52	 * @n: value to round up
    53	 */
    54	static inline __attribute__((const))
    55	unsigned long __roundup_pow_of_two(unsigned long n)
    56	{
    57		return 1UL << fls_long(n - 1);
    58	}

It's OK/valid for fls_long() [fls64()] to return 64 for a bit
position -- it just means the high-order bit in a 64-bit value.
So this code should either always subtract 1 from fls_long() or
do that if fls_long() == 64.


(b) in mm/readahead.c:get_init_ra_size():

   305	/*
   306	 * Set the initial window size, round to next power of 2 and square
   307	 * for small size, x 4 for medium, and x 2 for large
   308	 * for 128k (32 page) max ra
   309	 * 1-8 page = 32k initial, > 8 page = 128k initial
   310	 */
   311	static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
   312	{
   313		unsigned long newsize = roundup_pow_of_two(size);

It looks like 'size' is either extremely large or it might be negative if
it were a signed long instead of unsigned, so maybe it's 0x80000000_00000000
or 0xffffffff_ffffffff or something similar. I think that we should add a
WARN_ON_ONCE() there to try to catch whatever it is.


Is this something that you could test if I send some patches?

Unless other people have some other ideas, that is...

thanks.
-- 
~Randy