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bpf: Fix kernel address leakage in atomic cmpxchg's r0 aux reg

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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: 5ffa25502b ("bpf: Add instructions for atomic_[cmp]xchg")
Reported-by: Ryota Shiga (Flatt Security)
Acked-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Daniel Borkmann 2021-12-07 11:02:02 +00:00 committed by Alexei Starovoitov
parent 180486b430
commit a82fe085f3
1 changed files with 8 additions and 1 deletions
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9
kernel/bpf/verifier.c
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@ -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)) {