A load-acquire from an arena pointer is converted to BPF_PROBE_ATOMIC and gets an exception table entry, but the entry is filled in as if it were a store, since populate_extable() decides based on instruction class alone and a load-acquire is of BPF_STX class: if (BPF_CLASS(insn->code) == BPF_LDX) { arena_reg = reg2pt_regs[src_reg]; fixup_reg = reg2pt_regs[dst_reg]; } else { arena_reg = reg2pt_regs[dst_reg]; fixup_reg = DONT_CLEAR; } For a load-acquire dst_reg holds the loaded value and src_reg holds the address, so both assignments in the else branch are wrong. On a fault over an unmapped arena page ex_handler_bpf() then: - computes the reported address from the value register instead of the address register - reports the access as a WRITE, since it derives the direction from fixup_reg == DONT_CLEAR - leaves dst_reg untouched, so the program continues with a stale value instead of the 0 that BPF_PROBE_* loads deliver The access itself is emitted correctly, emit_atomic_ld_st_index() uses src_reg as the address, so this is a broken probe contract and a wrong diagnostic rather than a memory safety issue. Use bpf_atomic_is_load_acq() helper so a load-acquire takes the load path. Fixes: 5341c9a4d833 ("bpf, x86: Support load-acquire and store-release instructions") Signed-off-by: Daniel Borkmann Cc: Peilin Ye --- arch/x86/net/bpf_jit_comp.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 01e7ce569c1e..88ed95b2eaa7 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -2331,8 +2331,13 @@ st: insn_off = insn->off; * BPF_PROBE_ATOMIC) before being used for the memory access. Pass * the reg holding the unmodified 32-bit address to * ex_handler_bpf(). + * + * A load-acquire is of BPF_STX class, but reads from src_reg + * into dst_reg like a BPF_LDX does, hence it must not be + * treated as a store here. */ - if (BPF_CLASS(insn->code) == BPF_LDX) { + if (BPF_CLASS(insn->code) == BPF_LDX || + bpf_atomic_is_load_acq(insn)) { arena_reg = reg2pt_regs[src_reg]; fixup_reg = reg2pt_regs[dst_reg]; } else { -- 2.43.0