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 <20190613201457.hv5z3pbi46z2cfwn@kafai-mbp.dhcp.thefacebook.com>
In-Reply-To: <20190613201457.hv5z3pbi46z2cfwn@kafai-mbp.dhcp.thefacebook.com>
From:   Arthur Fabre <afabre@cloudflare.com>
Date:   Fri, 14 Jun 2019 10:47:05 +0100
Message-ID: <CAOn4ftuJXXgaDOUUjFaO0ytyQCWt=-NUoxofbfHZjd6g-M_nZQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next] bpf: sk_storage: Fix out of bounds memory access
To:     Martin Lau <kafai@fb.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Thu, Jun 13, 2019 at 9:15 PM Martin Lau <kafai@fb.com> wrote:
>
> On Thu, Jun 13, 2019 at 10:15:38AM -0700, Andrii Nakryiko wrote:
> > On Thu, Jun 13, 2019 at 8:16 AM Arthur Fabre <afabre@cloudflare.com> wrote:
> > >
> > > bpf_sk_storage maps use multiple spin locks to reduce contention.
> > > The number of locks to use is determined by the number of possible CPUs.
> > > With only 1 possible CPU, bucket_log == 0, and 2^0 = 1 locks are used.
> Thanks for report.
>
> > >
> > > When updating elements, the correct lock is determined with hash_ptr().
> > > Calling hash_ptr() with 0 bits is undefined behavior, as it does:
> > >
> > > x >> (64 - bits)
> > >
> > > Using the value results in an out of bounds memory access.
> > > In my case, this manifested itself as a page fault when raw_spin_lock_bh()
> > > is called later, when running the self tests:
> > >
> > > ./tools/testing/selftests/bpf/test_verifier 773 775
> > >
> > > [   16.366342] BUG: unable to handle page fault for address: ffff8fe7a66f93f8
> > > [   16.367139] #PF: supervisor write access in kernel mode
> > > [   16.367751] #PF: error_code(0x0002) - not-present page
> > > [   16.368323] PGD 35a01067 P4D 35a01067 PUD 0
> > > [   16.368796] Oops: 0002 [#1] SMP PTI
> > > [   16.369175] CPU: 0 PID: 189 Comm: test_verifier Not tainted 5.2.0-rc2+ #10
> > > [   16.369960] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> > > [   16.371021] RIP: 0010:_raw_spin_lock_bh (/home/afabre/linux/./include/trace/events/initcall.h:48)
> > > [ 16.371571] Code: 02 00 00 31 c0 ba ff 00 00 00 3e 0f b1 17 75 01 c3 e9 82 12 5f ff 66 90 65 81 05 ad 14 6f 41 00 02 00 00 31 c0 ba 01 00 00 00 <3e> 0f b1 17 75 01 c3 89 c6 e9 f0 02 5f ff b8 00 02 00 00 3e 0f c1
> > > All code
> > > ========
> > >    0:   02 00                   add    (%rax),%al
> > >    2:   00 31                   add    %dh,(%rcx)
> > >    4:   c0 ba ff 00 00 00 3e    sarb   $0x3e,0xff(%rdx)
> > >    b:   0f b1 17                cmpxchg %edx,(%rdi)
> > >    e:   75 01                   jne    0x11
> > >   10:   c3                      retq
> > >   11:   e9 82 12 5f ff          jmpq   0xffffffffff5f1298
> > >   16:   66 90                   xchg   %ax,%ax
> > >   18:   65 81 05 ad 14 6f 41    addl   $0x200,%gs:0x416f14ad(%rip)        # 0x416f14d0
> > >   1f:   00 02 00 00
> > >   23:   31 c0                   xor    %eax,%eax
> > >   25:   ba 01 00 00 00          mov    $0x1,%edx
> > >   2a:   3e 0f b1 17             cmpxchg %edx,%ds:*(%rdi)                <-- trapping instruction
> > >   2e:   75 01                   jne    0x31
> > >   30:   c3                      retq
> > >   31:   89 c6                   mov    %eax,%esi
> > >   33:   e9 f0 02 5f ff          jmpq   0xffffffffff5f0328
> > >   38:   b8 00 02 00 00          mov    $0x200,%eax
> > >   3d:   3e                      ds
> > >   3e:   0f                      .byte 0xf
> > >   3f:   c1                      .byte 0xc1
> > >
> > > Code starting with the faulting instruction
> > > ===========================================
> > >    0:   3e 0f b1 17             cmpxchg %edx,%ds:(%rdi)
> > >    4:   75 01                   jne    0x7
> > >    6:   c3                      retq
> > >    7:   89 c6                   mov    %eax,%esi
> > >    9:   e9 f0 02 5f ff          jmpq   0xffffffffff5f02fe
> > >    e:   b8 00 02 00 00          mov    $0x200,%eax
> > >   13:   3e                      ds
> > >   14:   0f                      .byte 0xf
> > >   15:   c1                      .byte 0xc1
> > > [   16.373398] RSP: 0018:ffffa759809d3be0 EFLAGS: 00010246
> > > [   16.373954] RAX: 0000000000000000 RBX: ffff8fe7a66f93f0 RCX: 0000000000000040
> > > [   16.374645] RDX: 0000000000000001 RSI: ffff8fdaf9f0d180 RDI: ffff8fe7a66f93f8
> > > [   16.375338] RBP: ffff8fdaf9f0d180 R08: ffff8fdafba2c320 R09: ffff8fdaf9f0d0c0
> > > [   16.376028] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8fdafa346700
> > > [   16.376719] R13: ffff8fe7a66f93f8 R14: ffff8fdaf9f0d0c0 R15: 0000000000000001
> > > [   16.377413] FS:  00007fda724c0740(0000) GS:ffff8fdafba00000(0000) knlGS:0000000000000000
> > > [   16.378204] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > [   16.378763] CR2: ffff8fe7a66f93f8 CR3: 0000000139d1c006 CR4: 0000000000360ef0
> > > [   16.379453] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > [   16.380144] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > [   16.380864] Call Trace:
> > > [   16.381112] selem_link_map (/home/afabre/linux/./include/linux/compiler.h:221 /home/afabre/linux/net/core/bpf_sk_storage.c:243)
> > > [   16.381476] sk_storage_update (/home/afabre/linux/net/core/bpf_sk_storage.c:355 /home/afabre/linux/net/core/bpf_sk_storage.c:414)
> > > [   16.381888] bpf_sk_storage_get (/home/afabre/linux/net/core/bpf_sk_storage.c:760 /home/afabre/linux/net/core/bpf_sk_storage.c:741)
> > > [   16.382285] ___bpf_prog_run (/home/afabre/linux/kernel/bpf/core.c:1447)
> > > [   16.382679] ? __bpf_prog_run32 (/home/afabre/linux/kernel/bpf/core.c:1603)
> > > [   16.383074] ? alloc_file_pseudo (/home/afabre/linux/fs/file_table.c:232)
> > > [   16.383486] ? kvm_clock_get_cycles (/home/afabre/linux/arch/x86/kernel/kvmclock.c:98)
> > > [   16.383906] ? ktime_get (/home/afabre/linux/kernel/time/timekeeping.c:265 /home/afabre/linux/kernel/time/timekeeping.c:369 /home/afabre/linux/kernel/time/timekeeping.c:754)
> > > [   16.384243] ? bpf_test_run (/home/afabre/linux/net/bpf/test_run.c:47)
> > > [   16.384613] ? bpf_prog_test_run_skb (/home/afabre/linux/net/bpf/test_run.c:313)
> > > [   16.385065] ? security_capable (/home/afabre/linux/security/security.c:696 (discriminator 19))
> > > [   16.385460] ? __do_sys_bpf (/home/afabre/linux/kernel/bpf/syscall.c:2072 /home/afabre/linux/kernel/bpf/syscall.c:2848)
> > > [   16.385854] ? __handle_mm_fault (/home/afabre/linux/mm/memory.c:3507 /home/afabre/linux/mm/memory.c:3532 /home/afabre/linux/mm/memory.c:3666 /home/afabre/linux/mm/memory.c:3897 /home/afabre/linux/mm/memory.c:4021)
> > > [   16.386273] ? __dentry_kill (/home/afabre/linux/fs/dcache.c:595)
> > > [   16.386652] ? do_syscall_64 (/home/afabre/linux/arch/x86/entry/common.c:301)
> > > [   16.387031] ? entry_SYSCALL_64_after_hwframe (/home/afabre/linux/./include/trace/events/initcall.h:10 /home/afabre/linux/./include/trace/events/initcall.h:10)
> > > [   16.387541] Modules linked in:
> > > [   16.387846] CR2: ffff8fe7a66f93f8
> > > [   16.388175] ---[ end trace 891cf27b5b9c9cc6 ]---
> > > [   16.388628] RIP: 0010:_raw_spin_lock_bh (/home/afabre/linux/./include/trace/events/initcall.h:48)
> > > [ 16.389089] Code: 02 00 00 31 c0 ba ff 00 00 00 3e 0f b1 17 75 01 c3 e9 82 12 5f ff 66 90 65 81 05 ad 14 6f 41 00 02 00 00 31 c0 ba 01 00 00 00 <3e> 0f b1 17 75 01 c3 89 c6 e9 f0 02 5f ff b8 00 02 00 00 3e 0f c1
> > > All code
> > > ========
> > >    0:   02 00                   add    (%rax),%al
> > >    2:   00 31                   add    %dh,(%rcx)
> > >    4:   c0 ba ff 00 00 00 3e    sarb   $0x3e,0xff(%rdx)
> > >    b:   0f b1 17                cmpxchg %edx,(%rdi)
> > >    e:   75 01                   jne    0x11
> > >   10:   c3                      retq
> > >   11:   e9 82 12 5f ff          jmpq   0xffffffffff5f1298
> > >   16:   66 90                   xchg   %ax,%ax
> > >   18:   65 81 05 ad 14 6f 41    addl   $0x200,%gs:0x416f14ad(%rip)        # 0x416f14d0
> > >   1f:   00 02 00 00
> > >   23:   31 c0                   xor    %eax,%eax
> > >   25:   ba 01 00 00 00          mov    $0x1,%edx
> > >   2a:   3e 0f b1 17             cmpxchg %edx,%ds:*(%rdi)                <-- trapping instruction
> > >   2e:   75 01                   jne    0x31
> > >   30:   c3                      retq
> > >   31:   89 c6                   mov    %eax,%esi
> > >   33:   e9 f0 02 5f ff          jmpq   0xffffffffff5f0328
> > >   38:   b8 00 02 00 00          mov    $0x200,%eax
> > >   3d:   3e                      ds
> > >   3e:   0f                      .byte 0xf
> > >   3f:   c1                      .byte 0xc1
> > >
> > > Code starting with the faulting instruction
> > > ===========================================
> > >    0:   3e 0f b1 17             cmpxchg %edx,%ds:(%rdi)
> > >    4:   75 01                   jne    0x7
> > >    6:   c3                      retq
> > >    7:   89 c6                   mov    %eax,%esi
> > >    9:   e9 f0 02 5f ff          jmpq   0xffffffffff5f02fe
> > >    e:   b8 00 02 00 00          mov    $0x200,%eax
> > >   13:   3e                      ds
> > >   14:   0f                      .byte 0xf
> > >   15:   c1                      .byte 0xc1
> > > [   16.390899] RSP: 0018:ffffa759809d3be0 EFLAGS: 00010246
> > > [   16.391410] RAX: 0000000000000000 RBX: ffff8fe7a66f93f0 RCX: 0000000000000040
> > > [   16.392102] RDX: 0000000000000001 RSI: ffff8fdaf9f0d180 RDI: ffff8fe7a66f93f8
> > > [   16.392795] RBP: ffff8fdaf9f0d180 R08: ffff8fdafba2c320 R09: ffff8fdaf9f0d0c0
> > > [   16.393481] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8fdafa346700
> > > [   16.394169] R13: ffff8fe7a66f93f8 R14: ffff8fdaf9f0d0c0 R15: 0000000000000001
> > > [   16.394870] FS:  00007fda724c0740(0000) GS:ffff8fdafba00000(0000) knlGS:0000000000000000
> > > [   16.395641] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > [   16.396193] CR2: ffff8fe7a66f93f8 CR3: 0000000139d1c006 CR4: 0000000000360ef0
> > > [   16.396876] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > [   16.397557] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > [   16.398246] Kernel panic - not syncing: Fatal exception in interrupt
> > > [   16.399067] Kernel Offset: 0x3ce00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> > > [   16.400098] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---
> > >
> > > Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
> > > Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
> > > ---
> > >  net/core/bpf_sk_storage.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
> > > index f40e3d35fd9c..7ae0686c5418 100644
> > > --- a/net/core/bpf_sk_storage.c
> > > +++ b/net/core/bpf_sk_storage.c
> > > @@ -90,7 +90,13 @@ struct bpf_sk_storage {
> > >  static struct bucket *select_bucket(struct bpf_sk_storage_map *smap,
> > >                                     struct bpf_sk_storage_elem *selem)
> > >  {
> > > -       return &smap->buckets[hash_ptr(selem, smap->bucket_log)];
> > > +       /* hash_ptr is undefined behavior with 0 bits */
> > > +       int bucket = 0;
> > > +       if (smap->bucket_log != 0) {
> > > +               bucket = hash_ptr(selem, smap->bucket_log);
> > > +       }
> >
> > Would it be better instead to make sure that bucket_log is always at
> > least 1? Having bucket_log as zero can bite us in the future again.
> I also think it is better off to have a max_t(u32, 1, ...) done in
> bpf_sk_storage_map_alloc().  Having an extra bucket should be fine in
> this case.

Makes sense, done in v2.

> >
> > > +
> > > +       return &smap->buckets[bucket];
> > >  }
> > >
> > >  static int omem_charge(struct sock *sk, unsigned int size)
> > > --
> > > 2.20.1
> > >