The signed-load mnemonic table has entries for byte, half-word, and word loads because BPF_MEMSX does not support double-word loads. A BPF_MEMSX | BPF_DW instruction nevertheless selects index 3, past the end of this table. Program Structure diagnostics can disassemble a malformed instruction before check_and_resolve_insns() rejects its opcode. Placing the invalid signed double-word load at the end of a program therefore triggers an out-of-bounds access while reporting subprogram fallthrough. Treat signed double-word loads as invalid in the disassembler and use the existing BUG_ldx fallback instead. Fixes: a8f427835394 ("bpf: Report Program Structure CFG errors") Reported-by: syzbot+3544d9b2a9206be8ba37@syzkaller.appspotmail.com Signed-off-by: Kumar Kartikeya Dwivedi --- kernel/bpf/disasm.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c index 50b3ca5149a0..b1a3fbe3fda5 100644 --- a/kernel/bpf/disasm.c +++ b/kernel/bpf/disasm.c @@ -295,7 +295,8 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs, verbose(cbs->private_data, "BUG_st_%02x", insn->code); } } else if (class == BPF_LDX) { - if (BPF_MODE(insn->code) != BPF_MEM && BPF_MODE(insn->code) != BPF_MEMSX) { + if ((BPF_MODE(insn->code) != BPF_MEM && BPF_MODE(insn->code) != BPF_MEMSX) || + (BPF_MODE(insn->code) == BPF_MEMSX && BPF_SIZE(insn->code) == BPF_DW)) { verbose(cbs->private_data, "BUG_ldx_%02x", insn->code); return; } -- 2.53.0 An invalid BPF_MEMSX | BPF_DW instruction can reach Program Structure diagnostics before opcode validation when placed at the end of a subprogram. Exercise this path and require the disassembler fallback so table bounds regressions are caught. Signed-off-by: Kumar Kartikeya Dwivedi --- tools/testing/selftests/bpf/progs/verifier_cfg.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/verifier_cfg.c b/tools/testing/selftests/bpf/progs/verifier_cfg.c index c1f55e1d80a4..3c3bb03e8217 100644 --- a/tools/testing/selftests/bpf/progs/verifier_cfg.c +++ b/tools/testing/selftests/bpf/progs/verifier_cfg.c @@ -3,6 +3,7 @@ #include #include +#include "../../../include/linux/filter.h" #include "bpf_misc.h" SEC("socket") @@ -55,6 +56,19 @@ __naked void out_of_range_jump2(void) " ::: __clobber_all); } +SEC("socket") +__description("invalid DW LDSX instruction in diagnostics") +__failure __msg("BUG_ldx_99") +__log_level(2) +__naked void invalid_dw_ldsx(void) +{ + asm volatile (" \ + .8byte %[ldsx_dw]; \ +" : + : __imm_insn(ldsx_dw, BPF_RAW_INSN(BPF_LDX | BPF_MEMSX | BPF_DW, BPF_REG_0, BPF_REG_0, 0, 0)) + : __clobber_all); +} + SEC("socket") __description("loop (back-edge)") __failure __msg("unreachable insn 1") -- 2.53.0