Re: [bpf PATCH 1/3] bpf: sock_ops ctx access may stomp registers in corner case

[Daniel Borkmann <daniel@iogearbox.net>]

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@gmail.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) \
>>>