KASAN: use-after-free Write in page_counter_uncharge

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a1d21081 Merge git://git.kernel.org/pub/scm/linux/kernel/g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17ceb0ce900000
kernel config: https://syzkaller.appspot.com/x/.config?x=21f0d1d2df6d5fc
dashboard link: https://syzkaller.appspot.com/bug?extid=b305848212deec86eabe
compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b30584...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in instrument_atomic_write include/linux/instrumented.h:71 [inline]
BUG: KASAN: use-after-free in atomic64_sub_return include/asm-generic/atomic-instrumented.h:970 [inline]
BUG: KASAN: use-after-free in atomic_long_sub_return include/asm-generic/atomic-long.h:113 [inline]
BUG: KASAN: use-after-free in page_counter_cancel mm/page_counter.c:54 [inline]
BUG: KASAN: use-after-free in page_counter_uncharge+0x3d/0xc0 mm/page_counter.c:155
Write of size 8 at addr ffff8880371c0148 by task syz-executor.0/9304

CPU: 0 PID: 9304 Comm: syz-executor.0 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1f0/0x31e lib/dump_stack.c:118
print_address_description+0x66/0x620 mm/kasan/report.c:383
__kasan_report mm/kasan/report.c:513 [inline]
kasan_report+0x132/0x1d0 mm/kasan/report.c:530
check_memory_region_inline mm/kasan/generic.c:183 [inline]
check_memory_region+0x2b5/0x2f0 mm/kasan/generic.c:192
instrument_atomic_write include/linux/instrumented.h:71 [inline]
atomic64_sub_return include/asm-generic/atomic-instrumented.h:970 [inline]
atomic_long_sub_return include/asm-generic/atomic-long.h:113 [inline]
page_counter_cancel mm/page_counter.c:54 [inline]
page_counter_uncharge+0x3d/0xc0 mm/page_counter.c:155
uncharge_batch+0x6c/0x350 mm/memcontrol.c:6764
uncharge_page+0x115/0x430 mm/memcontrol.c:6796
uncharge_list mm/memcontrol.c:6835 [inline]
mem_cgroup_uncharge_list+0x70/0xe0 mm/memcontrol.c:6877
release_pages+0x13a2/0x1550 mm/swap.c:911
tlb_batch_pages_flush mm/mmu_gather.c:49 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:242 [inline]
tlb_flush_mmu+0x780/0x910 mm/mmu_gather.c:249
tlb_finish_mmu+0xcb/0x200 mm/mmu_gather.c:328
exit_mmap+0x296/0x550 mm/mmap.c:3185
__mmput+0x113/0x370 kernel/fork.c:1076
exit_mm+0x4cd/0x550 kernel/exit.c:483
do_exit+0x576/0x1f20 kernel/exit.c:793
do_group_exit+0x161/0x2d0 kernel/exit.c:903
get_signal+0x139b/0x1d30 kernel/signal.c:2743
arch_do_signal+0x33/0x610 arch/x86/kernel/signal.c:811
exit_to_user_mode_loop kernel/entry/common.c:135 [inline]
exit_to_user_mode_prepare+0x8d/0x1b0 kernel/entry/common.c:166
syscall_exit_to_user_mode+0x5e/0x1a0 kernel/entry/common.c:241
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d239
Code: Bad RIP value.
RSP: 002b:00007f3d8a21acf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: 0000000000000001 RBX: 000000000118cf48 RCX: 000000000045d239
RDX: 00000000000f4240 RSI: 0000000000000081 RDI: 000000000118cf4c
RBP: 000000000118cf40 R08: 0000000000000009 R09: 0000000000000000
R10: ffffffffffffffff R11: 0000000000000246 R12: 000000000118cf4c
R13: 00007fff2711787f R14: 00007f3d8a21b9c0 R15: 000000000118cf4c

Allocated by task 9270:
kasan_save_stack mm/kasan/common.c:48 [inline]
kasan_set_track mm/kasan/common.c:56 [inline]
__kasan_kmalloc+0x100/0x130 mm/kasan/common.c:461
__do_kmalloc mm/slab.c:3655 [inline]
__kmalloc+0x218/0x320 mm/slab.c:3664
mem_cgroup_alloc mm/memcontrol.c:5210 [inline]
mem_cgroup_css_alloc+0x86/0x1590 mm/memcontrol.c:5278
css_create kernel/cgroup/cgroup.c:5128 [inline]
cgroup_apply_control_enable+0x581/0x1200 kernel/cgroup/cgroup.c:3059
cgroup_apply_control+0x37/0x6c0 kernel/cgroup/cgroup.c:3141
cgroup_subtree_control_write+0x95b/0x10c0 kernel/cgroup/cgroup.c:3299
cgroup_file_write+0x21e/0x5c0 kernel/cgroup/cgroup.c:3697
kernfs_fop_write+0x3f4/0x4f0 fs/kernfs/file.c:315
vfs_write+0x2d3/0xd10 fs/read_write.c:576
ksys_write+0x11b/0x220 fs/read_write.c:631
do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 8108:
kasan_save_stack mm/kasan/common.c:48 [inline]
kasan_set_track+0x3d/0x70 mm/kasan/common.c:56
kasan_set_free_info+0x17/0x30 mm/kasan/generic.c:355
__kasan_slab_free+0xdd/0x110 mm/kasan/common.c:422
__cache_free mm/slab.c:3418 [inline]
kfree+0x10a/0x220 mm/slab.c:3756
mem_cgroup_free+0x97f/0x9b0 mm/memcontrol.c:5196
css_free_rwork_fn+0xe4/0x970 kernel/cgroup/cgroup.c:4941
process_one_work+0x789/0xfc0 kernel/workqueue.c:2269
worker_thread+0xaa4/0x1460 kernel/workqueue.c:2415
kthread+0x37e/0x3a0 drivers/block/aoe/aoecmd.c:1234
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Last call_rcu():
kasan_save_stack+0x27/0x50 mm/kasan/common.c:48
kasan_record_aux_stack+0x7b/0xb0 mm/kasan/generic.c:346
__call_rcu kernel/rcu/tree.c:2894 [inline]
call_rcu+0x139/0x840 kernel/rcu/tree.c:2968
queue_rcu_work+0x74/0x90 kernel/workqueue.c:1747
process_one_work+0x789/0xfc0 kernel/workqueue.c:2269
worker_thread+0xaa4/0x1460 kernel/workqueue.c:2415
kthread+0x37e/0x3a0 drivers/block/aoe/aoecmd.c:1234
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Second to last call_rcu():
kasan_save_stack+0x27/0x50 mm/kasan/common.c:48
kasan_record_aux_stack+0x7b/0xb0 mm/kasan/generic.c:346
__call_rcu kernel/rcu/tree.c:2894 [inline]
call_rcu+0x139/0x840 kernel/rcu/tree.c:2968
__percpu_ref_switch_to_atomic lib/percpu-refcount.c:192 [inline]
__percpu_ref_switch_mode+0x2c1/0x4f0 lib/percpu-refcount.c:237
percpu_ref_kill_and_confirm+0x8f/0x130 lib/percpu-refcount.c:350
cgroup_apply_control_disable kernel/cgroup/cgroup.c:3108 [inline]
cgroup_finalize_control+0x7c5/0xd60 kernel/cgroup/cgroup.c:3171
cgroup_subtree_control_write+0x968/0x10c0 kernel/cgroup/cgroup.c:3300
cgroup_file_write+0x21e/0x5c0 kernel/cgroup/cgroup.c:3697
kernfs_fop_write+0x3f4/0x4f0 fs/kernfs/file.c:315
vfs_write+0x2d3/0xd10 fs/read_write.c:576
ksys_write+0x11b/0x220 fs/read_write.c:631
do_syscall_64+0x31/0x70 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x44/0xa9

The buggy address belongs to the object at ffff8880371c0000
which belongs to the cache kmalloc-4k of size 4096
The buggy address is located 328 bytes inside of
4096-byte region [ffff8880371c0000, ffff8880371c1000)
The buggy address belongs to the page:
page:00000000cea4402b refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x371c0
head:00000000cea4402b order:1 compound_mapcount:0
flags: 0xfffe0000010200(slab|head)
raw: 00fffe0000010200 ffffea0001337088 ffffea0000dc7088 ffff8880aa440900
raw: 0000000000000000 ffff8880371c0000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880371c0000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880371c0080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8880371c0100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8880371c0180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8880371c0200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


---
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syzbot engineers can be reached at syzk...@googlegroups.com.

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[Andrew Morton]

On Tue, 18 Aug 2020 07:50:28 -0700 syzbot <syzbot+b30584...@syzkaller.appspotmail.com> wrote:

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: a1d21081 Merge git://git.kernel.org/pub/scm/linux/kernel/g..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17ceb0ce900000
> kernel config: https://syzkaller.appspot.com/x/.config?x=21f0d1d2df6d5fc
> dashboard link: https://syzkaller.appspot.com/bug?extid=b305848212deec86eabe
> compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)

Presumably this is the same as
http://lkml.kernel.org/r/00000000000011...@google.com.

[Hillf Danton]

Tue, 18 Aug 2020 07:50:28 -0700

Add extra grab to memcg on uncharging in order to make
uncharge_batch() a step away from UAF.

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6774,6 +6774,9 @@ static void uncharge_batch(const struct
__this_cpu_add(ug->memcg->vmstats_percpu->nr_page_events, ug->nr_pages);
memcg_check_events(ug->memcg, ug->dummy_page);
local_irq_restore(flags);
+
+ /* balance with css_get() in uncharge_page() */
+ css_put(&ug->memcg->css);
}

static void uncharge_page(struct page *page, struct uncharge_gather *ug)
@@ -6797,6 +6800,8 @@ static void uncharge_page(struct page *p
uncharge_gather_clear(ug);
}
ug->memcg = page->mem_cgroup;
+ /* make uncharge_batch(ug); safe */
+ css_get(&ug->memcg->css);
}

nr_pages = compound_nr(page);

[Michal Hocko]

On Tue 18-08-20 16:18:56, Andrew Morton wrote:
> On Tue, 18 Aug 2020 07:50:28 -0700 syzbot <syzbot+b30584...@syzkaller.appspotmail.com> wrote:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: a1d21081 Merge git://git.kernel.org/pub/scm/linux/kernel/g..
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=17ceb0ce900000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=21f0d1d2df6d5fc
> > dashboard link: https://syzkaller.appspot.com/bug?extid=b305848212deec86eabe
> > compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
>
> Presumably this is the same as
> http://lkml.kernel.org/r/00000000000011...@google.com.

Very likely.

This looks like a reference count unbalance when memcg is released
early. My first guess would be 1a3e1f40962c ("mm: memcontrol: decouple
reference counting from page accounting").

Unless I am missing something nothing really prevents the memcg for the
current batch to go away. uncharge_page collects all the charges for the
same memcg but it later drops the reference for the current page. Later
on when the memcg changes or when the final clean up is done in uncharge_list
uncharge_batch needs to access memcg but this might be after the last
page dropped the reference and memcg went away. The whole process of
tear down is quite complex and takes some time with all the RCU/WQ
involvement so this is quite unlikely to hit.

That being said the below should cure the reference count but I am not
sure this is a complete fix. If this looks reasonable I will post the
full patch. Johannes?

---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b807952b4d43..11b6dd1c4f64 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6753,6 +6753,7 @@ struct uncharge_gather {

static inline void uncharge_gather_clear(struct uncharge_gather *ug)
{
+ css_put(&ug->memcg->css);
memset(ug, 0, sizeof(*ug));
}

@@ -6797,6 +6798,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)

uncharge_gather_clear(ug);
}
ug->memcg = page->mem_cgroup;

+ css_get(&ug->memcg->css);
}

nr_pages = compound_nr(page);

--
Michal Hocko
SUSE Labs

[Michal Hocko]

On Wed 19-08-20 08:34:22, Michal Hocko wrote:
[...]

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b807952b4d43..11b6dd1c4f64 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6753,6 +6753,7 @@ struct uncharge_gather {
>
> static inline void uncharge_gather_clear(struct uncharge_gather *ug)
> {
> + css_put(&ug->memcg->css);
> memset(ug, 0, sizeof(*ug));
> }
>
> @@ -6797,6 +6798,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> uncharge_gather_clear(ug);
> }
> ug->memcg = page->mem_cgroup;
> + css_get(&ug->memcg->css);
> }
>
> nr_pages = compound_nr(page);

This is not a proper fix because uncharge_gather_clear is called also to
initialize the initial state so ug->memcg would be a garbage from the
stack. The proper fix with the full changelog should be. Let's add more
people involved in the original commit to the CC. The initial report is
http://lkml.kernel.org/r/00000000000014...@google.com resp.
http://lkml.kernel.org/r/00000000000011...@google.com

From 73a40589cab12122170fb9f90222982e81d41423 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mho...@suse.com>
Date: Thu, 20 Aug 2020 10:44:58 +0200
Subject: [PATCH] memcg: fix use-after-free in uncharge_batch

syzbot has reported an use-after-free in the uncharge_batch path

tlb_batch_pages_flush mm/mmu_gather.c:49 [inline]
tlb_flush_mmu_free mm/mmu_gather.c:242 [inline]
tlb_flush_mmu+0x780/0x910 mm/mmu_gather.c:249
tlb_finish_mmu+0xcb/0x200 mm/mmu_gather.c:328
exit_mmap+0x296/0x550 mm/mmap.c:3185
__mmput+0x113/0x370 kernel/fork.c:1076
exit_mm+0x4cd/0x550 kernel/exit.c:483
do_exit+0x576/0x1f20 kernel/exit.c:793
do_group_exit+0x161/0x2d0 kernel/exit.c:903
get_signal+0x139b/0x1d30 kernel/signal.c:2743
arch_do_signal+0x33/0x610 arch/x86/kernel/signal.c:811
exit_to_user_mode_loop kernel/entry/common.c:135 [inline]
exit_to_user_mode_prepare+0x8d/0x1b0 kernel/entry/common.c:166
syscall_exit_to_user_mode+0x5e/0x1a0 kernel/entry/common.c:241
entry_SYSCALL_64_after_hwframe+0x44/0xa9

1a3e1f40962c ("mm: memcontrol: decouple reference counting from page

accounting") has reworked the memcg lifetime to be bound the the struct
page rather than charges. It has also removed the css_put_many from
uncharge_batch and that is causing the above splat. uncharge_batch is
supposed to uncharge accumulated charges for all pages freed from the
same memcg. The queuing is done by uncharge_page which however drops the
memcg reference after it adds charges to the batch. If the current page
happens to be the last one holding the reference for its memcg then the
memcg is OK to go and the next page to be freed will trigger batched
uncharge which needs to access the memcg which is gone already.

Fix the issue by taking a reference for the memcg in the current batch.

Fixes: 1a3e1f40962c ("mm: memcontrol: decouple reference counting from page accounting")
Reported-by: syzbot+b30584...@syzkaller.appspotmail.com
Reported-by: syzbot+b5ea6f...@syzkaller.appspotmail.com
Signed-off-by: Michal Hocko <mho...@suse.com>
---
mm/memcontrol.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b807952b4d43..cfa6cbad21d5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6774,6 +6774,9 @@ static void uncharge_batch(const struct uncharge_gather *ug)

__this_cpu_add(ug->memcg->vmstats_percpu->nr_page_events, ug->nr_pages);
memcg_check_events(ug->memcg, ug->dummy_page);
local_irq_restore(flags);
+

+ /* drop reference from uncharge_page */
+ css_put(&ug->memcg->css);
}

static void uncharge_page(struct page *page, struct uncharge_gather *ug)

@@ -6797,6 +6800,9 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)

uncharge_gather_clear(ug);
}
ug->memcg = page->mem_cgroup;
+

+ /* pairs with css_put in uncharge_batch */

+ css_get(&ug->memcg->css);
}

nr_pages = compound_nr(page);
--

2.28.0

[Shakeel Butt]

Seems correct to me.

Reviewed-by: Shakeel Butt <shak...@google.com>

[Johannes Weiner]

Nice catch! The fix looks correct - ug now holds a reference count for
its ug->memcg pointer.

Acked-by: Johannes Weiner <han...@cmpxchg.org>