Re: [mm] 6471384af2: kernel_BUG_at_mm/slub.c

Alexander Potapenko
2019-07-31T15:30:27Z

On Wed, Jul 31, 2019 at 12:39 AM Laura Abbott <labbott@redhat.com> wrote:
>
> On 7/29/19 5:43 AM, kernel test robot wrote:
> > FYI, we noticed the following commit (built with gcc-7):
> >
> > commit: 6471384af2a6530696fc0203bafe4de41a23c9ef ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options")
> > https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux.git master
> >
> > in testcase: trinity
> > with following parameters:
> >
> > runtime: 300s
> >
> > test-description: Trinity is a linux system call fuzz tester.
> > test-url: http://codemonkey.org.uk/projects/trinity/
> >
> >
> > on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 8G
> >
> > caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> >
> >
> > +------------------------------------------+------------+------------+
> > | | ba5c5e4a5d | 6471384af2 |
> > +------------------------------------------+------------+------------+
> > | boot_successes | 8 | 0 |
> > | boot_failures | 2 | 15 |
> > | invoked_oom-killer:gfp_mask=0x | 1 | |
> > | Mem-Info | 1 | |
> > | kernel_BUG_at_security/keys/keyring.c | 1 | |
> > | invalid_opcode:#[##] | 1 | 15 |
> > | RIP:__key_link_begin | 1 | |
> > | Kernel_panic-not_syncing:Fatal_exception | 1 | 15 |
> > | kernel_BUG_at_mm/slub.c | 0 | 15 |
> > | RIP:kfree | 0 | 15 |
> > +------------------------------------------+------------+------------+
> >
> >
> > If you fix the issue, kindly add following tag
> > Reported-by: kernel test robot <rong.a.chen@intel.com>
> >
> >
> > [ 4.478342] kernel BUG at mm/slub.c:306!
> > [ 4.482437] invalid opcode: 0000 [#1] PREEMPT PTI
> > [ 4.485750] CPU: 0 PID: 0 Comm: swapper Not tainted 5.2.0-05754-g6471384a #4
> > [ 4.490635] RIP: 0010:kfree+0x58a/0x5c0
> > [ 4.493679] Code: 48 83 05 78 37 51 02 01 0f 0b 48 83 05 7e 37 51 02 01 48 83 05 7e 37 51 02 01 48 83 05 7e 37 51 02 01 48 83 05 d6 37 51 02 01 <0f> 0b 48 83 05 d4 37 51 02 01 48 83 05 d4 37 51 02 01 48 83 05 d4
> > [ 4.506827] RSP: 0000:ffffffff82603d90 EFLAGS: 00010002
> > [ 4.510475] RAX: ffff8c3976c04320 RBX: ffff8c3976c04300 RCX: 0000000000000000
> > [ 4.515420] RDX: ffff8c3976c04300 RSI: 0000000000000000 RDI: ffff8c3976c04320
> > [ 4.520331] RBP: ffffffff82603db8 R08: 0000000000000000 R09: 0000000000000000
> > [ 4.525288] R10: ffff8c3976c04320 R11: ffffffff8289e1e0 R12: ffffd52cc8db0100
> > [ 4.530180] R13: ffff8c3976c01a00 R14: ffffffff810f10d4 R15: ffff8c3976c04300
> > [ 4.535079] FS: 0000000000000000(0000) GS:ffffffff8266b000(0000) knlGS:0000000000000000
> > [ 4.540628] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 4.544593] CR2: ffff8c397ffff000 CR3: 0000000125020000 CR4: 00000000000406b0
> > [ 4.549558] Call Trace:
> > [ 4.551266] apply_wqattrs_prepare+0x154/0x280
> > [ 4.554357] apply_workqueue_attrs_locked+0x4e/0xe0
> > [ 4.557728] apply_workqueue_attrs+0x36/0x60
> > [ 4.560654] alloc_workqueue+0x25a/0x6d0
> > [ 4.563381] ? kmem_cache_alloc_trace+0x1e3/0x500
> > [ 4.566628] ? __mutex_unlock_slowpath+0x44/0x3f0
> > [ 4.569875] workqueue_init_early+0x246/0x348
> > [ 4.573025] start_kernel+0x3c7/0x7ec
> > [ 4.575558] x86_64_start_reservations+0x40/0x49
> > [ 4.578738] x86_64_start_kernel+0xda/0xe4
> > [ 4.581600] secondary_startup_64+0xb6/0xc0
> > [ 4.584473] Modules linked in:
> > [ 4.586620] ---[ end trace f67eb9af4d8d492b ]---
>
> I think this is catching an edge case with the freelist walking code
> in slab_free_freelist_hook. If we're not doing a bulk free,
> getting the free pointer from the object is going to be bogus so
> there's a chance it could trigger the bug in set_freepointer even
> if we don't actually care about it since it's going to get
> overwritten when we actually free. It's probably more robust
> to make sure we're terminating it with NULL. Lightly tested:
I see, my understanding was that it's safe to write the value we've
previously read with get_freepointer().
But chances are that the random data from this pointer simply matches
the object pointer.

> diff --git a/mm/slub.c b/mm/slub.c
> index e6c030e47364..8834563cdb4b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1432,7 +1432,9 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
> void *old_tail = *tail ? *tail : *head;
> int rsize;
>
> - if (slab_want_init_on_free(s))
> + if (slab_want_init_on_free(s)) {
> + void *p = NULL;
> +
> do {
> object = next;
> next = get_freepointer(s, object);
> @@ -1445,8 +1447,10 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
> : 0;
> memset((char *)object + s->inuse, 0,
> s->size - s->inuse - rsize);
> - set_freepointer(s, object, next);
> + set_freepointer(s, object, p);
> + p = object;
> } while (object != old_tail);
> + }
>
> /*
> * Compiler cannot detect this function can be removed if slab_free_hook()
>
This one looks good, care to send a patch? Otherwise I can do that for you.

--
Alexander Potapenko
Software Engineer

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