Martin KaFai Lau wrote:
> On Tue, Jul 28, 2020 at 01:55:22PM -0700, 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@xxxxxxxxx>
> > > > ---
> > > > 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.
OTOH it looks like we will cause the bpf_jit_blind_insn() to abort on those
instructions.
I'm not sure it matters for performance see'ing we are in a bit of an
edge case. iirc Daniel wrote that code so maybe its best to see if he has
any opinions.
@Daniel, Do you have a preference? If we use REG_RAX it seems the insns
will be skipped over by bpf_jit_blind_insn otoh its slightly faster I guess
to skip the load/store.
> >
> > 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.
> Agree that it would be nice to do the same in SOCK_OPS_SET_FIELD() also
> and this improvement could be done in bpf-next.
>
> >
> > >
> > > 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.
> Thanks for taking care of it!
