On Wed, May 27, 2020 at 8:52 AM Alexander Potapenko <gli...@google.com> wrote: > > This basically means that BPF's output register was uninitialized when > ___bpf_prog_run() returned. > > When I replace the lines initializing registers A and X in net/core/filter.c: > > - *new_insn++ = BPF_ALU32_REG(BPF_XOR, BPF_REG_A, BPF_REG_A); > - *new_insn++ = BPF_ALU32_REG(BPF_XOR, BPF_REG_X, BPF_REG_X); > > with > > + *new_insn++ = BPF_MOV32_IMM(BPF_REG_A, 0); > + *new_insn++ = BPF_MOV32_IMM(BPF_REG_X, 0); > > , the bug goes away, therefore I think it's being caused by XORing the > initially uninitialized registers with themselves. > > kernel/bpf/core.c:1408, where the uninitialized value was stored to > memory, points to the "ALU(ADD, +)" macro in ___bpf_prog_run(). > But the debug info seems to be incorrect here: if I comment this line > out and unroll the macro manually, KMSAN points to "ALU(SUB, -)". > Most certainly it's actually one of the XOR instruction declarations. > > Do you think it makes sense to use the UB-proof BPF_MOV32_IMM > instructions to initialize the registers?
I think it's better for UBsan to get smarter about xor-ing.
