Re: Dcache oops

[Oleg Drokin]

On Jun 3, 2016, at 4:07 PM, Al Viro wrote:

> On Fri, Jun 03, 2016 at 02:35:41PM -0400, Oleg Drokin wrote:
> 
>>>> [ 2642.364383] BUG: unable to handle kernel paging request at ffff880113f82000
>>>> [ 2642.365014] IP: [<ffffffff817f87d4>] bad_gs+0xd1d/0x1ba9
>>> 
>>> *ow*
>>> Could you dump your vmlinux (and System.map) somewhere on anonftp?
>>> This 'bad_gs' is there simply because it's one of the few labels in
>>> .fixup - to say anything useful we'll need to find out where we'd
>>> really come from.
>> 
>> I see.
>> vmlinux with debug symbols: http://knox.linuxhacker.ru/tmp/dcache/vmlinux.gz
>> System.map: http://knox.linuxhacker.ru/tmp/dcache/System.map.gz
> 
> OK...
> ffffffff817f87cd:       48 8d 0a                lea    (%rdx),%rcx
> ffffffff817f87d0:       48 83 e1 f8             and    $0xfffffffffffffff8,%rcx
> ffffffff817f87d4:       4c 8b 01                mov    (%rcx),%r8
> ffffffff817f87d7:       8d 0a                   lea    (%rdx),%ecx
> ffffffff817f87d9:       83 e1 07                and    $0x7,%ecx
> ffffffff817f87dc:       c1 e1 03                shl    $0x3,%ecx
> ffffffff817f87df:       49 d3 e8                shr    %cl,%r8
> ffffffff817f87e2:       e9 9b b3 a4 ff          jmpq   ffffffff81243b82 <__d_lookup+0x132>
> 
> Aha...  It's load_unaligned_zeropad() from dentry_string_cmp(), hitting
> a genuinely unmapped address.  That sends it into fixup, where it tries to
> load an aligned word containing the address in question, in hope that
> fault was on attempt to cross into the next page.  No such luck, address
> was aligned in the first place (it's in %rdx - 0xffff880113f82000), so
> we still oops.
> 
> The unexpected part is that unmapped address did *NOT* come from a dentry;
> it's .name of qstr we were looking for.  And your call chain was
> __d_lookup() <- d_lookup() <- lookup_open(), so in lookup_open() it was
> nd->last.name...
> 
> Can the same thing be reproduced (with NFS fix) on v4.6, ede4090, 7f427d3,
> 4e8440b?

Well, that was faster than I expected. 4e8440b triggers right away, so I guess
there's no point in trying the later ones?
BTW, just to confirm you are noticing - this is a DEBUG_PAGEALLOC build,
so all freed memory is unmapped which is likely causing this oops - as a sign
of use after free.

[   54.990119] BUG: unable to handle kernel paging request at ffff8800d2b7f000
[   54.990423] IP: [<ffffffff817f91b6>] bad_gs+0xcff/0x1b89
[   54.990598] PGD 2dca067 PUD 11f900067 PMD 11f86a067 PTE 80000000d2b7f060
[   54.990942] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[   54.991044] Modules linked in: loop rpcsec_gss_krb5 joydev pcspkr i2c_piix4 acpi_cpufreq tpm_tis tpm nfsd drm_kms_helper ttm drm serio_raw virtio_blk
[   54.992301] CPU: 7 PID: 5550 Comm: file_concat.sh Not tainted 4.6.0-4e8440b+ #1
[   54.993019] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   54.993407] task: ffff8800d6ec6200 ti: ffff880110f44000 task.ti: ffff880110f44000
[   54.994112] RIP: 0010:[<ffffffff817f91b6>]  [<ffffffff817f91b6>] bad_gs+0xcff/0x1b89
[   54.994870] RSP: 0018:ffff880110f47ba0  EFLAGS: 00010286
[   54.995288] RAX: ffff8800d2760f30 RBX: ffff8800d2760f00 RCX: ffff8800d2b7f000
[   54.995736] RDX: ffff8800d2b7f000 RSI: 0000000000000000 RDI: 0000000000000001
[   54.996154] RBP: ffff880110f47be8 R08: 0000000000000000 R09: ffff8800d2760ed0
[   54.996628] R10: 0000000000000059 R11: ffff8800d6ec6d78 R12: ffff8800d2694ed0
[   54.997044] R13: ffff880110f47df0 R14: ffff8800d2760f50 R15: 00000000b761b37e
[   54.997450] FS:  00007fe8804d2700(0000) GS:ffff88011f5c0000(0000) knlGS:0000000000000000
[   54.998164] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   54.998624] CR2: ffff8800d2b7f000 CR3: 000000011931f000 CR4: 00000000000006e0
[   54.999225] Stack:
[   54.999596]  ffffffff81243965 0000000000000096 ffff8800d2b7f000 ffffffff00000001
[   55.000632]  000000000000042c ffff880110f47df0 ffff8800d2694ed0 ffff880110f47de0
[   55.001497]  ffff880110f47df0 ffff880110f47c18 ffffffff81243b94 ffffffff81234b3e
[   55.002353] Call Trace:
[   55.002709]  [<ffffffff81243965>] ? __d_lookup+0x5/0x1b0
[   55.003104]  [<ffffffff81243b94>] d_lookup+0x84/0xb0
[   55.003487]  [<ffffffff81234b3e>] ? lookup_open+0xfe/0x7a0
[   55.003878]  [<ffffffff81234b3e>] lookup_open+0xfe/0x7a0
[   55.004272]  [<ffffffff8123717e>] path_openat+0x4de/0xfc0
[   55.004665]  [<ffffffff8123925e>] do_filp_open+0x7e/0xe0
[   55.005061]  [<ffffffff812330a0>] ? lock_rename+0x100/0x100
[   55.005452]  [<ffffffff817f5367>] ? _raw_spin_unlock+0x27/0x40
[   55.005846]  [<ffffffff8124868c>] ? __alloc_fd+0xbc/0x170
[   55.006246]  [<ffffffff81226896>] do_sys_open+0x116/0x1f0
[   55.006635]  [<ffffffff8122698e>] SyS_open+0x1e/0x20
[   55.007036]  [<ffffffff817f5b36>] entry_SYSCALL_64_fastpath+0x1e/0xad
[   55.007434] Code: e1 03 49 d3 e8 e9 8f a3 a4 ff 48 8d 0a 48 83 e1 f8 4c 8b 01 8d 0a 83 e1 07 c1 e1 03 49 d3 e8 e9 4e a4 a4 ff 48 8d 0a 48 83 e1 f8 <4c> 8b 01 8d 0a 83 e1 07 c1 e1 03 49 d3 e8 e9 c9 a8 a4 ff b9 f2 
[   55.011146] RIP  [<ffffffff817f91b6>] bad_gs+0xcff/0x1b89
[   55.011584]  RSP <ffff880110f47ba0>
[   55.011943] CR2: ffff8800d2b7f000