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Date: Thu, 20 Aug 2020 23:17:45 -0700 From: Yonghong Song <yhs@...com> To: Udip Pant <udippant@...com>, Alexei Starovoitov <ast@...nel.org>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, Andrii Nakryiko <andriin@...com>, "David S . Miller" <davem@...emloft.net> CC: <netdev@...r.kernel.org>, <bpf@...r.kernel.org>, <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v2 bpf 1/2] bpf: verifier: check for packet data access based on target prog On 8/20/20 11:13 PM, Yonghong Song wrote: > > > On 8/20/20 5:28 PM, Udip Pant wrote: >> While using dynamic program extension (of type BPF_PROG_TYPE_EXT), we >> need to check the program type of the target program to grant the read / >> write access to the packet data. >> >> The BPF_PROG_TYPE_EXT type can be used to extend types such as XDP, SKB >> and others. Since the BPF_PROG_TYPE_EXT program type on itself is just a >> placeholder for those, we need this extended check for those target >> programs to actually work while using this option. >> >> Tested this with a freplace xdp program. Without this patch, the >> verifier fails with error 'cannot write into packet'. >> >> Signed-off-by: Udip Pant <udippant@...com> >> --- >> kernel/bpf/verifier.c | 6 +++++- >> 1 file changed, 5 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> index ef938f17b944..4d7604430994 100644 >> --- a/kernel/bpf/verifier.c >> +++ b/kernel/bpf/verifier.c >> @@ -2629,7 +2629,11 @@ static bool may_access_direct_pkt_data(struct >> bpf_verifier_env *env, >> const struct bpf_call_arg_meta *meta, >> enum bpf_access_type t) >> { >> - switch (env->prog->type) { >> + struct bpf_prog *prog = env->prog; >> + enum bpf_prog_type prog_type = prog->aux->linked_prog ? >> + prog->aux->linked_prog->type : prog->type; > > I checked the verifier code. There are several places where > prog->type is checked and EXT program type will behave differently > from the linked program. > > Maybe abstract the the above logic to one static function like > > static enum bpf_prog_type resolved_prog_type(struct bpf_prog *prog) > { > return prog->aux->linked_prog ? prog->aux->linked_prog->type > : prog->type; > } > > This function can then be used in different places to give the resolved > prog type. > > Besides here checking pkt access permission, > another possible places to consider is return value > in function check_return_code(). Currently, > for EXT program, the result value can be anything. This may need to > be enforced. Could you take a look? It could be others as well. > You can take a look at verifier.c by searching "prog->type". Note that if the EXT program tries to replace a global subprogram, then return value cannot be enforced, just as what Patch #2 example shows. > >> + >> + switch (prog_type) { >> /* Program types only with direct read access go here! */ >> case BPF_PROG_TYPE_LWT_IN: >> case BPF_PROG_TYPE_LWT_OUT: >>
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