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SubjectRe: tip -ENOBOOT - bisected to locking/refcounts, x86/asm: Implement fast refcount overflow protection
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On Wed, 2017-08-30 at 21:10 -0700, Kees Cook wrote:
> On Wed, Aug 30, 2017 at 9:01 PM, Kees Cook <keescook@chromium.org> wrote:
> > On Wed, Aug 30, 2017 at 8:12 PM, Mike Galbraith <efault@gmx.de> wrote:
> >> On Wed, 2017-08-30 at 19:27 -0700, Kees Cook wrote:
> >>
> >>> Interesting! Can you try with 633547973ffc3 ("net: convert
> >>> sk_buff.users from atomic_t to refcount_t") reverted? I'll see if
> >>> running haveged will help me trigger this on my system...
> >>
> >> With that (plus 230cd1279d001 fix to it) reverted, vbox boots.
> >
> > Wonderful! Thank you so much for helping track this down.
> >
> > So, it seems that sk_buff.users will need some more special attention
> > before we can convert it to refcount.
> >
> > x86-refcount will saturate with refcount_dec_and_test() if the result
> > is negative. But that would mean at least starting at 0. FULL should
> > have WARNed in this case, so I remain slightly confused why it was
> > missed by FULL.
>
> Actually, if this is a race condition it's possible that FULL is slow
> enough to miss it...
>
> I bet something briefly takes the refcount negative, and with
> unchecked atomics, it come back up positive again during the race.
> FULL may miss the race, and x86-refcount will catch it and saturate...

(gdb) list *in6_dev_get+0x1e
0xffffffff8166d3de is in in6_dev_get (./arch/x86/include/asm/refcount.h:52).
47 : "cc", "cx");
48 }
49
50 static __always_inline void refcount_inc(refcount_t *r)
51 {
52 asm volatile(LOCK_PREFIX "incl %0\n\t"
53 REFCOUNT_CHECK_LT_ZERO
54 : [counter] "+m" (r->refs.counter)
55 : : "cc", "cx");
56

gdb) list *in6_dev_get+0x10
0xffffffff8166d3d0 is in in6_dev_get (./include/net/addrconf.h:318).
313 {
314 struct inet6_dev *idev;
315
316 rcu_read_lock();
317 idev = rcu_dereference(dev->ip6_ptr);
318 if (idev)
319 refcount_inc(&idev->refcnt);
320 rcu_read_unlock();
321 return idev;
322

That's from kernel with no revert, but your silent saturation patch
still applied, AND built with gcc-6.3.1.  Kernel traps, but it boots
and works, as does kernel built with gcc-7.0.1.  Remove your silent
saturation patch, kernel doesn't notice a thing, just works.

With gcc-4.8.5, trap means you're as good as dead, with the other two,
trap means the intended.  Compiler, constraints, dark elves.. pick one.

Full first splat from bootable gcc-6.3.1 built kernel.

[ 1.293962] NET: Registered protocol family 10
[ 1.294635] refcount_t silent saturation at in6_dev_get+0x25/0x104 in swapper/0[1], uid/euid: 0/0
[ 1.295616] ------------[ cut here ]------------
[ 1.296120] WARNING: CPU: 0 PID: 1 at kernel/panic.c:612 refcount_error_report+0x94/0x9e
[ 1.296950] Modules linked in:
[ 1.297276] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.13.0.g152d54a-tip-default #53
[ 1.299179] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
[ 1.300743] task: ffff88013ab84040 task.stack: ffffc9000062c000
[ 1.301825] RIP: 0010:refcount_error_report+0x94/0x9e
[ 1.302804] RSP: 0018:ffffc9000062fc10 EFLAGS: 00010282
[ 1.303791] RAX: 0000000000000055 RBX: ffffffff81a34274 RCX: ffffffff81c605e8
[ 1.304991] RDX: 0000000000000001 RSI: 0000000000000096 RDI: 0000000000000246
[ 1.306189] RBP: ffffc9000062fd58 R08: 0000000000000000 R09: 0000000000000175
[ 1.307392] R10: 0000000000000000 R11: 0000000000000001 R12: ffff88013ab84040
[ 1.308583] R13: 0000000000000000 R14: 0000000000000004 R15: ffffffff81a256c8
[ 1.309768] FS: 0000000000000000(0000) GS:ffff88013fc00000(0000) knlGS:0000000000000000
[ 1.311052] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1.312100] CR2: 00007f4631fe8df0 CR3: 0000000137d09003 CR4: 00000000001606f0
[ 1.313301] Call Trace:
[ 1.314012] ex_handler_refcount+0x63/0x70
[ 1.314893] fixup_exception+0x32/0x40
[ 1.315737] do_trap+0x8c/0x170
[ 1.316519] do_error_trap+0x70/0xd0
[ 1.317340] ? in6_dev_get+0x23/0x104
[ 1.318172] ? netlink_broadcast_filtered+0x2bd/0x430
[ 1.319156] ? kmem_cache_alloc_trace+0xce/0x5d0
[ 1.320098] ? set_debug_rodata+0x11/0x11
[ 1.320964] invalid_op+0x1e/0x30
[ 1.322520] RIP: 0010:in6_dev_get+0x25/0x104
[ 1.323631] RSP: 0018:ffffc9000062fe00 EFLAGS: 00010202
[ 1.324614] RAX: ffff880137de2400 RBX: ffff880137df4600 RCX: ffff880137de24f0
[ 1.325793] RDX: ffff88013a5e4000 RSI: 00000000fffffe00 RDI: ffff88013a5e4000
[ 1.326964] RBP: 00000000000000d1 R08: 0000000000000000 R09: ffff880137de7600
[ 1.328150] R10: 0000000000000000 R11: ffff8801398a4df8 R12: 0000000000000000
[ 1.329374] R13: ffffffff82137872 R14: 014200ca00000000 R15: 0000000000000000
[ 1.330547] ? set_debug_rodata+0x11/0x11
[ 1.331392] ip6_route_init_special_entries+0x2a/0x89
[ 1.332369] addrconf_init+0x9e/0x203
[ 1.333173] inet6_init+0x1af/0x365
[ 1.333956] ? af_unix_init+0x4e/0x4e
[ 1.334753] do_one_initcall+0x4e/0x190
[ 1.335555] ? set_debug_rodata+0x11/0x11
[ 1.336369] kernel_init_freeable+0x189/0x20e
[ 1.337230] ? rest_init+0xd0/0xd0
[ 1.337999] kernel_init+0xa/0xf7
[ 1.338744] ret_from_fork+0x25/0x30
[ 1.339500] Code: 48 8b 95 80 00 00 00 41 55 49 8d 8c 24 f0 0a 00 00 45 8b 84 24 10 09 00 00 41 89 c1 48 89 de 48 c7 c7 60 7a a3 81 e8 07 de 05 00 <0f> ff 58 5b 5d 41 5c 41 5d c3 0f 1f 44 00 00 55 48 89 e5 41 56
[ 1.342243] ---[ end trace b5d40c0fccce776c ]---

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