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From: John Fastabend <john.fastabend@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	John Fastabend <john.fastabend@gmail.com>
Cc: ecree@solarflare.com, yhs@fb.com, daniel@iogearbox.net,
	netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [bpf-next PATCH 05/10] bpf: verifier, return value is an int in do_refine_retval_range
Date: Thu, 26 Mar 2020 08:52:15 -0700
Message-ID: <5e7ccfaf79433_65132acbbe7fc5c4eb@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20200326062301.fvomwkz5grg3b5qb@ast-mbp>

Alexei Starovoitov wrote:
> On Tue, Mar 24, 2020 at 10:39:16AM -0700, John Fastabend wrote:
> > Mark 32-bit subreg region with max value because do_refine_retval_range()
> > catches functions with int return type (We will assume here that int is
> > a 32-bit type). Marking 64-bit region could be dangerous if upper bits
> > are not zero which could be possible.
> > 
> > Two reasons to pull this out of original patch. First it makes the original
> > fix impossible to backport. And second I've not seen this as being problematic
> > in practice unlike the other case.
> > 
> > Fixes: 849fa50662fbc ("bpf/verifier: refine retval R0 state for bpf_get_stack helper")
> > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > ---
> >  kernel/bpf/verifier.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 6372fa4..3731109 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -4328,7 +4328,7 @@ static void do_refine_retval_range(struct bpf_reg_state *regs, int ret_type,
> >  	     func_id != BPF_FUNC_probe_read_str))
> >  		return;
> >  
> > -	ret_reg->smax_value = meta->msize_max_value;
> > +	ret_reg->s32_max_value = meta->msize_max_value;
> 
> I think this is not correct.
> These two special helpers are invoked via BPF_CALL_x() which has u64 return value.
> So despite having 'int' return in bpf_helper_defs.h the upper 32-bit will be correct.
> I think this patch should do:
> ret_reg->smax_value = meta->msize_max_value;
> ret_reg->s32_max_value = meta->msize_max_value;

OK, I missed the u64 in BPF_CALL_x(). Setting both smax and s32_max
looks correct.  My logic above is wrong so I'll fix that. Thanks.

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-24 17:37 [bpf-next PATCH 00/10] ALU32 bounds tracking support John Fastabend
2020-03-24 17:37 ` [bpf-next PATCH 01/10] bpf: verifier, do_refine_retval_range may clamp umin to 0 incorrectly John Fastabend
2020-03-24 17:38 ` [bpf-next PATCH 02/10] bpf: verifer, refactor adjust_scalar_min_max_vals John Fastabend
2020-03-26  6:10   ` Alexei Starovoitov
2020-03-24 17:38 ` [bpf-next PATCH 03/10] bpf: verifer, adjust_scalar_min_max_vals to always call update_reg_bounds() John Fastabend
2020-03-24 17:38 ` [bpf-next PATCH 04/10] bpf: verifier, do explicit ALU32 bounds tracking John Fastabend
2020-03-26  6:20   ` Alexei Starovoitov
2020-03-26 15:18     ` John Fastabend
2020-03-24 17:39 ` [bpf-next PATCH 05/10] bpf: verifier, return value is an int in do_refine_retval_range John Fastabend
2020-03-26  6:23   ` Alexei Starovoitov
2020-03-26 15:52     ` John Fastabend [this message]
2020-03-24 17:39 ` [bpf-next PATCH 06/10] bpf: test_progs, add test to catch retval refine error handling John Fastabend
2020-03-24 17:39 ` [bpf-next PATCH 07/10] bpf: test_verifier, bpf_get_stack return value add <0 John Fastabend
2020-03-26  6:33   ` Alexei Starovoitov
2020-03-26 15:48     ` John Fastabend
2020-03-24 17:40 ` [bpf-next PATCH 08/10] bpf: test_verifier, #70 error message updates for 32-bit right shift John Fastabend
2020-03-24 17:40 ` [bpf-next PATCH 09/10] bpf: test_verifier, #65 error message updates for trunc of boundary-cross John Fastabend
2020-03-24 17:40 ` [bpf-next PATCH 10/10] bpf: test_verifier, add alu32 bounds tracking tests John Fastabend
2020-03-26  6:34   ` Alexei Starovoitov

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