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.
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.
>
> 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) \
> >
