Re: UBSAN: Undefined behaviour in mm/page_alloc.c

[Dmitry Vyukov]

On Tue, Nov 13, 2018 at 3:29 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 13 Nov 2018 10:43:05 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
>> From: Michal Hocko <mhocko@xxxxxxxx>
>> Date: Fri, 9 Nov 2018 09:35:29 +0100
>> Subject: [PATCH] mm, page_alloc: check for max order in hot path
>>
>> Konstantin has noticed that kvmalloc might trigger the following warning
>> [Thu Nov  1 08:43:56 2018] WARNING: CPU: 0 PID: 6676 at mm/vmstat.c:986 __fragmentation_index+0x54/0x60
>
> um, wait...
>
>> [...]
>> [Thu Nov  1 08:43:56 2018] Call Trace:
>> [Thu Nov  1 08:43:56 2018]  fragmentation_index+0x76/0x90
>> [Thu Nov  1 08:43:56 2018]  compaction_suitable+0x4f/0xf0
>> [Thu Nov  1 08:43:56 2018]  shrink_node+0x295/0x310
>> [Thu Nov  1 08:43:56 2018]  node_reclaim+0x205/0x250
>> [Thu Nov  1 08:43:56 2018]  get_page_from_freelist+0x649/0xad0
>> [Thu Nov  1 08:43:56 2018]  ? get_page_from_freelist+0x2d4/0xad0
>> [Thu Nov  1 08:43:56 2018]  ? release_sock+0x19/0x90
>> [Thu Nov  1 08:43:56 2018]  ? do_ipv6_setsockopt.isra.5+0x10da/0x1290
>> [Thu Nov  1 08:43:56 2018]  __alloc_pages_nodemask+0x12a/0x2a0
>> [Thu Nov  1 08:43:56 2018]  kmalloc_large_node+0x47/0x90
>> [Thu Nov  1 08:43:56 2018]  __kmalloc_node+0x22b/0x2e0
>> [Thu Nov  1 08:43:56 2018]  kvmalloc_node+0x3e/0x70
>> [Thu Nov  1 08:43:56 2018]  xt_alloc_table_info+0x3a/0x80 [x_tables]
>> [Thu Nov  1 08:43:56 2018]  do_ip6t_set_ctl+0xcd/0x1c0 [ip6_tables]
>> [Thu Nov  1 08:43:56 2018]  nf_setsockopt+0x44/0x60
>> [Thu Nov  1 08:43:56 2018]  SyS_setsockopt+0x6f/0xc0
>> [Thu Nov  1 08:43:56 2018]  do_syscall_64+0x67/0x120
>> [Thu Nov  1 08:43:56 2018]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
>
> If kvalloc_node() is going to call kmalloc() without checking for a
> huge allocation request then surely it should set __GFP_NOWARN.


kmalloc won't warn about large allocations after "mm: don't warn about
large allocations for slab":
https://lkml.org/lkml/2018/9/27/1156
It will just return NULL. That was already the case for slub.


> And it
> shouldn't bother at all if size > KMALLOC_MAX_SIZE, surely?  So
> something like
>
> --- a/mm/util.c~a
> +++ a/mm/util.c
> @@ -393,11 +393,16 @@ void *kvmalloc_node(size_t size, gfp_t f
>         void *ret;
>
>         /*
> -        * vmalloc uses GFP_KERNEL for some internal allocations (e.g page tables)
> -        * so the given set of flags has to be compatible.
> +        * vmalloc uses GFP_KERNEL for some internal allocations (e.g page
> +        * tables) so the given set of flags has to be compatible.
>          */
> -       if ((flags & GFP_KERNEL) != GFP_KERNEL)
> +       if ((flags & GFP_KERNEL) != GFP_KERNEL) {
> +               if (size > KMALLOC_MAX_SIZE)
> +                       return NULL;
> +               if (size > PAGE_SIZE)
> +                       flags |= __GFP_NOWARN;
>                 return kmalloc_node(size, flags, node);
> +       }
>
>         /*
>          * We want to attempt a large physically contiguous block first because
>
>
>> the problem is that we only check for an out of bound order in the slow
>> path and the node reclaim might happen from the fast path already. This
>> is fixable by making sure that kvmalloc doesn't ever use kmalloc for
>> requests that are larger than KMALLOC_MAX_SIZE but this also shows that
>> the code is rather fragile. A recent UBSAN report just underlines that
>> by the following report
>>
>>  UBSAN: Undefined behaviour in mm/page_alloc.c:3117:19
>>  shift exponent 51 is too large for 32-bit type 'int'
>>  CPU: 0 PID: 6520 Comm: syz-executor1 Not tainted 4.19.0-rc2 #1
>>  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>  Call Trace:
>>   __dump_stack lib/dump_stack.c:77 [inline]
>>   dump_stack+0xd2/0x148 lib/dump_stack.c:113
>>   ubsan_epilogue+0x12/0x94 lib/ubsan.c:159
>>   __ubsan_handle_shift_out_of_bounds+0x2b6/0x30b lib/ubsan.c:425
>>   __zone_watermark_ok+0x2c7/0x400 mm/page_alloc.c:3117
>>   zone_watermark_fast mm/page_alloc.c:3216 [inline]
>>   get_page_from_freelist+0xc49/0x44c0 mm/page_alloc.c:3300
>>   __alloc_pages_nodemask+0x21e/0x640 mm/page_alloc.c:4370
>>   alloc_pages_current+0xcc/0x210 mm/mempolicy.c:2093
>>   alloc_pages include/linux/gfp.h:509 [inline]
>>   __get_free_pages+0x12/0x60 mm/page_alloc.c:4414
>>   dma_mem_alloc+0x36/0x50 arch/x86/include/asm/floppy.h:156
>>   raw_cmd_copyin drivers/block/floppy.c:3159 [inline]
>>   raw_cmd_ioctl drivers/block/floppy.c:3206 [inline]
>>   fd_locked_ioctl+0xa00/0x2c10 drivers/block/floppy.c:3544
>>   fd_ioctl+0x40/0x60 drivers/block/floppy.c:3571
>>   __blkdev_driver_ioctl block/ioctl.c:303 [inline]
>>   blkdev_ioctl+0xb3c/0x1a30 block/ioctl.c:601
>>   block_ioctl+0x105/0x150 fs/block_dev.c:1883
>>   vfs_ioctl fs/ioctl.c:46 [inline]
>>   do_vfs_ioctl+0x1c0/0x1150 fs/ioctl.c:687
>>   ksys_ioctl+0x9e/0xb0 fs/ioctl.c:702
>>   __do_sys_ioctl fs/ioctl.c:709 [inline]
>>   __se_sys_ioctl fs/ioctl.c:707 [inline]
>>   __x64_sys_ioctl+0x7e/0xc0 fs/ioctl.c:707
>>   do_syscall_64+0xc4/0x510 arch/x86/entry/common.c:290
>>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> And we could fix this in the floppy driver.
>
>> Note that this is not a kvmalloc path. It is just that the fast path
>> really depends on having sanitzed order as well. Therefore move the
>> order check to the fast path.
>
> But do we really need to do this?  Are there any other known potential
> callsites?
>
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