Subject: Re: bpf: use-after-free in array_map_alloc

From: Vlastimil Babka <[email-masked]>
Date: Mon, 23 May 2016 14:01:43 +0200

[+CC Christoph, linux-mm]

On 04/17/2016 07:29 PM, Alexei Starovoitov wrote:
> On Sun, Apr 17, 2016 at 12:58:21PM -0400, Sasha Levin wrote:
>> Hi all,
>>
>> I've hit the following while fuzzing with syzkaller inside a KVM tools guest
>> running the latest -next kernel:
> 
> thanks for the report. Adding Tejun...

Looks like this report died, and meanwhile there's a CVE for it,
including a reproducer:

http://seclists.org/oss-sec/2016/q2/332

So maybe we should fix it now? :)

> if I read the report correctly it's not about bpf, but rather points to
> the issue inside percpu logic.
> First __alloc_percpu_gfp() is called, then the memory is freed with
> free_percpu() which triggers async pcpu_balance_work and then
> pcpu_extend_area_map is hitting use-after-free.
> I guess bpf percpu array map is stressing this logic the most.

I've been staring at it for a while (not knowing the code at all) and
the first thing that struck me is that pcpu_extend_area_map() is done
outside of pcpu_lock. So what prevents the chunk from being freed during
the extend?

> Any simpler steps to reproduce ?
> 
>> [ 2590.845375] ==================================================================
>>
>> [ 2590.845445] BUG: KASAN: use-after-free in pcpu_extend_area_map+0x8a/0x130 at addr ffff88035452a3cc
>>
>> [ 2590.845457] Read of size 4 by task syz-executor/31307
>>
>> [ 2590.845464] =============================================================================
>>
>> [ 2590.845476] BUG kmalloc-128 (Tainted: G        W      ): kasan: bad access detected
>>
>> [ 2590.845479] -----------------------------------------------------------------------------
>>
>> [ 2590.845479]
>>
>> [ 2590.845485] Disabling lock debugging due to kernel taint
>>
>> [ 2590.845496] INFO: Allocated in 0xbbbbbbbbbbbbbbbb age=18446609615465671625 cpu=0 pid=0
>>
>> [ 2590.845504] 	pcpu_mem_zalloc+0x7e/0xc0
>>
>> [ 2590.845521] 	___slab_alloc+0x7af/0x870
>>
>> [ 2590.845528] 	__slab_alloc.isra.22+0xf4/0x130
>>
>> [ 2590.845535] 	__kmalloc+0x1fe/0x340
>>
>> [ 2590.845543] 	pcpu_mem_zalloc+0x7e/0xc0
>>
>> [ 2590.845551] 	pcpu_create_chunk+0x79/0x600
>>
>> [ 2590.845558] 	pcpu_alloc+0x5d4/0xe10
>>
>> [ 2590.845567] 	__alloc_percpu_gfp+0x27/0x30
>>
>> [ 2590.845582] 	array_map_alloc+0x595/0x710
>>
>> [ 2590.845590] 	SyS_bpf+0x336/0xba0
>>
>> [ 2590.845605] 	do_syscall_64+0x2a6/0x4a0
>>
>> [ 2590.845639] 	return_from_SYSCALL_64+0x0/0x6a
>>
>> [ 2590.845647] INFO: Freed in 0x10022ebb3 age=18446628393062689737 cpu=0 pid=0
>>
>> [ 2590.845653] 	kvfree+0x45/0x50
>>
>> [ 2590.845660] 	__slab_free+0x6a/0x2f0
>>
>> [ 2590.845665] 	kfree+0x22c/0x270
>>
>> [ 2590.845671] 	kvfree+0x45/0x50
>>
>> [ 2590.845680] 	pcpu_balance_workfn+0x11a1/0x1280
>>
>> [ 2590.845693] 	process_one_work+0x973/0x10b0
>>
>> [ 2590.845700] 	worker_thread+0xcfd/0x1160
>>
>> [ 2590.845708] 	kthread+0x2e7/0x300
>>
>> [ 2590.845716] 	ret_from_fork+0x22/0x40
>>
>> [ 2590.845724] INFO: Slab 0xffffea000d514a00 objects=35 used=33 fp=0xffff88035452b740 flags=0x2fffff80004080
>>
>> [ 2590.845730] INFO: Object 0xffff88035452a3a0 @offset=9120 fp=0xbbbbbbbbbbbbbbbb
>>
>> [ 2590.845730]
>>
>> [ 2590.845743] Redzone ffff88035452a398: 00 00 00 00 00 00 00 00                          ........
>>
>> [ 2590.845751] Object ffff88035452a3a0: bb bb bb bb bb bb bb bb 00 00 00 00 00 00 00 00  ................
>>
>> [ 2590.845758] Object ffff88035452a3b0: b0 7b 17 3f 03 88 ff ff 00 00 08 00 00 00 08 00  .{.?............
>>
>> [ 2590.845765] Object ffff88035452a3c0: 00 00 e0 f9 ff e8 ff ff 01 00 00 00 10 00 00 00  ................
>>
>> [ 2590.845775] Object ffff88035452a3d0: 18 83 2c 3f 03 88 ff ff e0 ff ff ff 0f 00 00 00  ..,?............
>>
>> [ 2590.845783] Object ffff88035452a3e0: e0 a3 52 54 03 88 ff ff e0 a3 52 54 03 88 ff ff  ..RT......RT....
>>
>> [ 2590.845790] Object ffff88035452a3f0: 90 96 6b 9f ff ff ff ff e8 a7 13 3f 03 88 ff ff  ..k........?....
>>
>> [ 2590.845797] Object ffff88035452a400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>
>> [ 2590.845804] Object ffff88035452a410: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>
>> [ 2590.845811] Redzone ffff88035452a420: 00 00 00 00 00 00 00 00                          ........
>>
>> [ 2590.845818] Padding ffff88035452a558: b5 eb 22 00 01 00 00 00                          ..".....
>>
>> [ 2590.845833] CPU: 0 PID: 31307 Comm: syz-executor Tainted: G    B   W       4.6.0-rc3-next-20160412-sasha-00023-g0b02d6d-dirty #2998
>>
>> [ 2590.845851]  0000000000000000 00000000a66f8039 ffff880354f8fa60 ffffffffa0fc9d01
>>
>> [ 2590.845861]  ffffffff00000000 fffffbfff57ad2a0 0000000041b58ab3 ffffffffab65eee0
>>
>> [ 2590.845870]  ffffffffa0fc9b88 00000000a66f8039 ffff88035e9d4000 ffffffffab67cede
>>
>> [ 2590.845872] Call Trace:
>>
>> [ 2590.845903] dump_stack (lib/dump_stack.c:53)
>> [ 2590.845939] print_trailer (mm/slub.c:668)
>> [ 2590.845948] object_err (mm/slub.c:675)
>> [ 2590.845958] kasan_report_error (mm/kasan/report.c:180 mm/kasan/report.c:276)
>> [ 2590.846007] __asan_report_load4_noabort (mm/kasan/report.c:318)
>> [ 2590.846028] pcpu_extend_area_map (mm/percpu.c:445)

This line is:

	if (new_alloc <= chunk->map_alloc)

i.e. the first time the function touches some part of the chunk object.

>> [ 2590.846038] pcpu_alloc (mm/percpu.c:940)
>> [ 2590.846128] __alloc_percpu_gfp (mm/percpu.c:1068)
>> [ 2590.846140] array_map_alloc (kernel/bpf/arraymap.c:36 kernel/bpf/arraymap.c:99)
>> [ 2590.846150] SyS_bpf (kernel/bpf/syscall.c:35 kernel/bpf/syscall.c:183 kernel/bpf/syscall.c:830 kernel/bpf/syscall.c:787)
>> [ 2590.846203] do_syscall_64 (arch/x86/entry/common.c:350)
>> [ 2590.846214] entry_SYSCALL64_slow_path (arch/x86/entry/entry_64.S:251)
>> [ 2590.846217] Memory state around the buggy address:
>>
>> [ 2590.846224]  ffff88035452a280: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>
>> [ 2590.846230]  ffff88035452a300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>
>> [ 2590.846237] >ffff88035452a380: fc fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb
>>
>> [ 2590.846240]                                               ^
>>
>> [ 2590.846247]  ffff88035452a400: fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc
>>
>> [ 2590.846253]  ffff88035452a480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>
>> [ 2590.846256] ==================================================================
>>

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