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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 14 Jun 2019 15:31:51 -0700
Message-ID: <CAEf4BzZNO8Px2BRcs5WMxfrfRaekxF=_fz_p2A+eL94L0DrfQg@mail.gmail.com>
Subject: Re: [PATCH v2 bpf-next] bpf: sk_storage: Fix out of bounds memory access
To:     Arthur Fabre <afabre@cloudflare.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        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 Fri, Jun 14, 2019 at 2:45 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.
>
> 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 ]---
>

I think the bug is pretty clear without this detailed example, I'd
remove it from commit message.

> Force the minimum number of locks to two.
>
> Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
> Fixes: 6ac99e8f23d4 ("bpf: Introduce bpf sk local storage")
> ---

Thanks for fixing this!

Acked-by: Andrii Nakryiko <andriin@fb.com>


>  net/core/bpf_sk_storage.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
> index f40e3d35fd9c..94c7f77ecb6b 100644
> --- a/net/core/bpf_sk_storage.c
> +++ b/net/core/bpf_sk_storage.c
> @@ -634,7 +634,8 @@ static struct bpf_map *bpf_sk_storage_map_alloc(union bpf_attr *attr)
>                 return ERR_PTR(-ENOMEM);
>         bpf_map_init_from_attr(&smap->map, attr);
>
> -       smap->bucket_log = ilog2(roundup_pow_of_two(num_possible_cpus()));
> +       /* Use at least 2 buckets, select_bucket() is undefined behavior with 1 bucket */
> +       smap->bucket_log = max_t(u32, 1, ilog2(roundup_pow_of_two(num_possible_cpus())));
>         nbuckets = 1U << smap->bucket_log;
>         cost = sizeof(*smap->buckets) * nbuckets + sizeof(*smap);
>
> --
> 2.20.1
>