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| author | Yonghong Song <yhs@fb.com> | 2020-06-18 16:46:31 -0700 |
|---|---|---|
| committer | Daniel Borkmann <daniel@iogearbox.net> | 2020-06-19 23:34:42 +0200 |
| commit | 6c6935419e2fd8ef1fcde71c4e50b9095e520900 (patch) | |
| tree | 0e827993fe8351e8492d71f2a4d13fa680cd35f9 /kernel/bpf/verifier.c | |
| parent | 7c7982cbadbb63eb76401ddc4ef090cf7ae274b4 (diff) | |
| download | lwn-6c6935419e2fd8ef1fcde71c4e50b9095e520900.tar.gz lwn-6c6935419e2fd8ef1fcde71c4e50b9095e520900.zip | |
bpf: Avoid verifier failure for 32bit pointer arithmetic
When do experiments with llvm (disabling instcombine and
simplifyCFG), I hit the following error with test_seg6_loop.o.
; R1=pkt(id=0,off=0,r=48,imm=0), R7=pkt(id=0,off=40,r=48,imm=0)
w2 = w7
; R2_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff))
w2 -= w1
R2 32-bit pointer arithmetic prohibited
The corresponding source code is:
uint32_t srh_off
// srh and skb->data are all packet pointers
srh_off = (char *)srh - (char *)(long)skb->data;
The verifier does not support 32-bit pointer/scalar arithmetic.
Without my llvm change, the code looks like
; R3=pkt(id=0,off=40,r=48,imm=0), R8=pkt(id=0,off=0,r=48,imm=0)
w3 -= w8
; R3_w=inv(id=0)
This is explicitly allowed in verifier if both registers are
pointers and the opcode is BPF_SUB.
To fix this problem, I changed the verifier to allow
32-bit pointer/scaler BPF_SUB operations.
At the source level, the issue could be workarounded with
inline asm or changing "uint32_t srh_off" to "uint64_t srh_off".
But I feel that verifier change might be the right thing to do.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200618234631.3321118-1-yhs@fb.com
Diffstat (limited to 'kernel/bpf/verifier.c')
| -rw-r--r-- | kernel/bpf/verifier.c | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 34cde841ab68..a1857c4ffaaf 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5031,6 +5031,11 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env, if (BPF_CLASS(insn->code) != BPF_ALU64) { /* 32-bit ALU ops on pointers produce (meaningless) scalars */ + if (opcode == BPF_SUB && env->allow_ptr_leaks) { + __mark_reg_unknown(env, dst_reg); + return 0; + } + verbose(env, "R%d 32-bit pointer arithmetic prohibited\n", dst); |