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Message-ID: <e15813be-68b6-b7ee-87ec-ab6da94aa249@iogearbox.net>
Date: Tue, 28 Jul 2020 23:09:32 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: John Fastabend <john.fastabend@...il.com>,
Martin KaFai Lau <kafai@...com>
Cc: ast@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [bpf PATCH 1/3] bpf: sock_ops ctx access may stomp registers in
corner case
On 7/28/20 10:55 PM, John Fastabend wrote:
> Martin KaFai Lau wrote:
>> On Tue, Jul 28, 2020 at 08:43:46AM -0700, John Fastabend wrote:
>>> I had a sockmap program that after doing some refactoring started spewing
>>> this splat at me:
>>>
>>> [18610.807284] BUG: unable to handle kernel NULL pointer dereference at 0000000000000001
>>> [...]
>>> [18610.807359] Call Trace:
>>> [18610.807370] ? 0xffffffffc114d0d5
>>> [18610.807382] __cgroup_bpf_run_filter_sock_ops+0x7d/0xb0
>>> [18610.807391] tcp_connect+0x895/0xd50
>>> [18610.807400] tcp_v4_connect+0x465/0x4e0
>>> [18610.807407] __inet_stream_connect+0xd6/0x3a0
>>> [18610.807412] ? __inet_stream_connect+0x5/0x3a0
>>> [18610.807417] inet_stream_connect+0x3b/0x60
>>> [18610.807425] __sys_connect+0xed/0x120
>>>
>
> [...]
>
>>> So three additional instructions if dst == src register, but I scanned
>>> my current code base and did not see this pattern anywhere so should
>>> not be a big deal. Further, it seems no one else has hit this or at
>>> least reported it so it must a fairly rare pattern.
>>>
>>> Fixes: 9b1f3d6e5af29 ("bpf: Refactor sock_ops_convert_ctx_access")
>> I think this issue dated at least back from
>> commit 34d367c59233 ("bpf: Make SOCK_OPS_GET_TCP struct independent")
>> There are a few refactoring since then, so fixing in much older
>> code may not worth it since it is rare?
>
> OK I just did a quick git annotate and pulled out the last patch
> there. I didn't go any farther back. The failure is rare and has
> the nice property that it crashes hard always. For example I found
> it by simply running some of our go tests after doing the refactor.
> I guess if it was in some path that doesn't get tested like an
> error case or something you might have an ugly surprise in production.
> I can imagine a case where tracking this down might be difficult.
>
> OTOH the backport wont be automatic past some of those reworks.
>
>>
>>> Signed-off-by: John Fastabend <john.fastabend@...il.com>
>>> ---
>>> net/core/filter.c | 26 ++++++++++++++++++++++++--
>>> 1 file changed, 24 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>> index 29e34551..c50cb80 100644
>>> --- a/net/core/filter.c
>>> +++ b/net/core/filter.c
>>> @@ -8314,15 +8314,31 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
>>> /* Helper macro for adding read access to tcp_sock or sock fields. */
>>> #define SOCK_OPS_GET_FIELD(BPF_FIELD, OBJ_FIELD, OBJ) \
>>> do { \
>>> + int fullsock_reg = si->dst_reg, reg = BPF_REG_9, jmp = 2; \
>>> BUILD_BUG_ON(sizeof_field(OBJ, OBJ_FIELD) > \
>>> sizeof_field(struct bpf_sock_ops, BPF_FIELD)); \
>>> + if (si->dst_reg == reg || si->src_reg == reg) \
>>> + reg--; \
>>> + if (si->dst_reg == reg || si->src_reg == reg) \
>>> + reg--; \
>>> + if (si->dst_reg == si->src_reg) { \
>>> + *insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, reg, \
>>> + offsetof(struct bpf_sock_ops_kern, \
>>> + temp)); \
>> Instead of sock_ops->temp, can BPF_REG_AX be used here as a temp?
>> e.g. bpf_convert_shinfo_access() has already used it as a temp also.
>
> Sure I will roll a v2 I agree that rax is a bit nicer. I guess for
> bpf-next we can roll the load over to use rax as well? Once the
> fix is in place I'll take a look it would be nice for consistency.
Keep in mind that REG_AX is also used from constant blinding side, so use with care.
Please make sure to add necessary selftest cases to test_verifier and run them under
constant blinding enabled for root so we have these rewrites exercised.
>> Also, it seems the "sk" access in sock_ops_convert_ctx_access() suffers
>> a similar issue.
>
> Good catch. I'll fix it up as well. Maybe with a second patch and test.
> Patches might be a bit verbose but makes it easier to track the bugs
> I think.
>
>>
>>> + fullsock_reg = reg; \
>>> + jmp+=2; \
>>> + } \
>>> *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF( \
>>> struct bpf_sock_ops_kern, \
>>> is_fullsock), \
>>> - si->dst_reg, si->src_reg, \
>>> + fullsock_reg, si->src_reg, \
>>> offsetof(struct bpf_sock_ops_kern, \
>>> is_fullsock)); \
>>> - *insn++ = BPF_JMP_IMM(BPF_JEQ, si->dst_reg, 0, 2); \
>>> + *insn++ = BPF_JMP_IMM(BPF_JEQ, fullsock_reg, 0, jmp); \
>>> + if (si->dst_reg == si->src_reg) \
>>> + *insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg, \
>>> + offsetof(struct bpf_sock_ops_kern, \
>>> + temp)); \
>>> *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF( \
>>> struct bpf_sock_ops_kern, sk),\
>>> si->dst_reg, si->src_reg, \
>>> @@ -8331,6 +8347,12 @@ static u32 sock_ops_convert_ctx_access(enum bpf_access_type type,
>>> OBJ_FIELD), \
>>> si->dst_reg, si->dst_reg, \
>>> offsetof(OBJ, OBJ_FIELD)); \
>>> + if (si->dst_reg == si->src_reg) { \
>>> + *insn++ = BPF_JMP_A(1); \
>>> + *insn++ = BPF_LDX_MEM(BPF_DW, reg, si->src_reg, \
>>> + offsetof(struct bpf_sock_ops_kern, \
>>> + temp)); \
>>> + } \
>>> } while (0)
>>>
>>> #define SOCK_OPS_GET_TCP_SOCK_FIELD(FIELD) \
>>>
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