Re: [PATCH] percpu: fix data race in pcpu_alloc_noprof() and extend spinlock protection area

[Jeongjun Park]

2024년 9월 9일 (월) 오전 3:53, Dennis Zhou <dennis@xxxxxxxxxx>님이 작성:
>
> Hello,
>
> On Sun, Sep 08, 2024 at 10:52:10PM +0900, Jeongjun Park wrote:
> > I got the following KCSAN report during syzbot testing:
> >
> > ==================================================================
> > BUG: KCSAN: data-race in pcpu_alloc_noprof / pcpu_free_area
> >
> > read-write to 0xffffffff883f872c of 4 bytes by task 3378 on cpu 0:
> >  pcpu_update_empty_pages mm/percpu.c:602 [inline]
> >  pcpu_block_update_hint_free mm/percpu.c:1044 [inline]
> >  pcpu_free_area+0x4dc/0x570 mm/percpu.c:1302
> >  free_percpu+0x1c6/0xb30 mm/percpu.c:2277
> >  xt_percpu_counter_free+0x63/0x80 net/netfilter/x_tables.c:1951
> >  cleanup_entry+0x195/0x1c0 net/ipv6/netfilter/ip6_tables.c:671
> >  __do_replace+0x470/0x580 net/ipv6/netfilter/ip6_tables.c:1099
> >  do_replace net/ipv6/netfilter/ip6_tables.c:1158 [inline]
> >  do_ip6t_set_ctl+0x820/0x8c0 net/ipv6/netfilter/ip6_tables.c:1644
> >  nf_setsockopt+0x195/0x1b0 net/netfilter/nf_sockopt.c:101
> >  ipv6_setsockopt+0x126/0x140 net/ipv6/ipv6_sockglue.c:998
> >  tcp_setsockopt+0x93/0xb0 net/ipv4/tcp.c:3768
> >  sock_common_setsockopt+0x64/0x80 net/core/sock.c:3735
> >  do_sock_setsockopt net/socket.c:2324 [inline]
> >  __sys_setsockopt+0x1d8/0x250 net/socket.c:2347
> >  __do_sys_setsockopt net/socket.c:2356 [inline]
> >  __se_sys_setsockopt net/socket.c:2353 [inline]
> >  __x64_sys_setsockopt+0x66/0x80 net/socket.c:2353
> >  x64_sys_call+0x278d/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:55
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0x54/0x120 arch/x86/entry/common.c:83
> >  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >
> > read to 0xffffffff883f872c of 4 bytes by task 3374 on cpu 1:
> >  pcpu_alloc_noprof+0x9a5/0x10c0 mm/percpu.c:1894
> >  xt_percpu_counter_alloc+0x79/0x110 net/netfilter/x_tables.c:1931
> >  find_check_entry net/ipv4/netfilter/ip_tables.c:526 [inline]
> >  translate_table+0x921/0xf70 net/ipv4/netfilter/ip_tables.c:716
> >  do_replace net/ipv4/netfilter/ip_tables.c:1137 [inline]
> >  do_ipt_set_ctl+0x7bd/0x8b0 net/ipv4/netfilter/ip_tables.c:1635
> >  nf_setsockopt+0x195/0x1b0 net/netfilter/nf_sockopt.c:101
> >  ip_setsockopt+0xea/0x100 net/ipv4/ip_sockglue.c:1424
> >  tcp_setsockopt+0x93/0xb0 net/ipv4/tcp.c:3768
> >  sock_common_setsockopt+0x64/0x80 net/core/sock.c:3735
> >  do_sock_setsockopt net/socket.c:2324 [inline]
> >  __sys_setsockopt+0x1d8/0x250 net/socket.c:2347
> >  __do_sys_setsockopt net/socket.c:2356 [inline]
> >  __se_sys_setsockopt net/socket.c:2353 [inline]
> >  __x64_sys_setsockopt+0x66/0x80 net/socket.c:2353
> >  x64_sys_call+0x278d/0x2d60 arch/x86/include/generated/asm/syscalls_64.h:55
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0x54/0x120 arch/x86/entry/common.c:83
> >  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >
> > value changed: 0x00000005 -> 0x00000006
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 1 UID: 0 PID: 3374 Comm: syz-executor.3 Not tainted 6.11.0-rc6-syzkaller-00326-gd1f2d51b711a-dirty #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
> > ==================================================================
> >
> > The global variable pcpu_nr_empty_pop_pages can be protected by pcpu_lock,
> > but since pcpu_alloc_noprof reads outside the spinlock protection section,
> > a data race may occur and the branch of the conditional statement may change.
> > Therefore, the reading of pcpu_nr_empty_pop_pages should be modified to be
> > performed within the spinlock protection section.
> >
> > However, the for_each_clear_bitrange_from loop requires and uses a spinlock,
> > but it repeatedly locks and unlocks the spinlock unnecessarily.
> >
> > Therefore, I think it is appropriate to remove the repeated spin_lock and
> > spin_unlock in for_each_clear_bitrange_from and perform the operation of
> > reading pcpu_nr_empty_pop_pages and then perform spin_unlock to postpone
> > the point in time when the spin_unlock is performed.
> >
> > Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> > Fixes: e04d320838f5 ("percpu: indent the population block in pcpu_alloc()")
> > Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
> > ---
> >  mm/percpu.c |  5 ++---
> >  1 files changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 20d91af8c033..5c958a54da51 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -1864,7 +1864,6 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
> >
> >  area_found:
> >       pcpu_stats_area_alloc(chunk, size);
> > -     spin_unlock_irqrestore(&pcpu_lock, flags);
> >
> >       /* populate if not all pages are already there */
> >       if (!is_atomic) {
> > @@ -1878,14 +1877,12 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
> >
> >                       ret = pcpu_populate_chunk(chunk, rs, re, pcpu_gfp);
> >
> > -                     spin_lock_irqsave(&pcpu_lock, flags);
> >                       if (ret) {
> >                               pcpu_free_area(chunk, off);
> >                               err = "failed to populate";
> >                               goto fail_unlock;
> >                       }
> >                       pcpu_chunk_populated(chunk, rs, re);
> > -                     spin_unlock_irqrestore(&pcpu_lock, flags);
> >               }
>
> We don't want to do this because pcpu_populate_chunk() calls
> alloc_pages_node() which can block.
> >
> >               mutex_unlock(&pcpu_alloc_mutex);
> > @@ -1894,6 +1891,8 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
> >       if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
> >               pcpu_schedule_balance_work();
> >
> > +     spin_unlock_irqrestore(&pcpu_lock, flags);
> > +
> >       /* clear the areas and return address relative to base address */
> >       for_each_possible_cpu(cpu)
> >               memset((void *)pcpu_chunk_addr(chunk, cpu, 0) + off, 0, size);
> > --
>
> I sent out [1] which is a more appropriate fix. I'll merge it later
> today.
>
> Thanks,
> Dennis
>
> [1] https://lore.kernel.org/lkml/20240906031151.80719-1-dennis@xxxxxxxxxx/
>

Oh, you already patched that bug a few days ago.
Sorry for taking up your time.

Regards,
Jeongjun Park