From a82fe085f344ef20b452cd5f481010ff96b5c4cd Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Tue, 7 Dec 2021 11:02:02 +0000 Subject: bpf: Fix kernel address leakage in atomic cmpxchg's r0 aux reg The implementation of BPF_CMPXCHG on a high level has the following parameters: .-[old-val] .-[new-val] BPF_R0 = cmpxchg{32,64}(DST_REG + insn->off, BPF_R0, SRC_REG) `-[mem-loc] `-[old-val] Given a BPF insn can only have two registers (dst, src), the R0 is fixed and used as an auxilliary register for input (old value) as well as output (returning old value from memory location). While the verifier performs a number of safety checks, it misses to reject unprivileged programs where R0 contains a pointer as old value. Through brute-forcing it takes about ~16sec on my machine to leak a kernel pointer with BPF_CMPXCHG. The PoC is basically probing for kernel addresses by storing the guessed address into the map slot as a scalar, and using the map value pointer as R0 while SRC_REG has a canary value to detect a matching address. Fix it by checking R0 for pointers, and reject if that's the case for unprivileged programs. Fixes: 5ffa25502b5a ("bpf: Add instructions for atomic_[cmp]xchg") Reported-by: Ryota Shiga (Flatt Security) Acked-by: Brendan Jackman Signed-off-by: Daniel Borkmann Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 53d39db3b0fa..2d48159b58bd 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -4547,9 +4547,16 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i if (insn->imm == BPF_CMPXCHG) { /* Check comparison of R0 with memory location */ - err = check_reg_arg(env, BPF_REG_0, SRC_OP); + const u32 aux_reg = BPF_REG_0; + + err = check_reg_arg(env, aux_reg, SRC_OP); if (err) return err; + + if (is_pointer_value(env, aux_reg)) { + verbose(env, "R%d leaks addr into mem\n", aux_reg); + return -EACCES; + } } if (is_pointer_value(env, insn->src_reg)) { -- cgit