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bpf: fix incorrect sign extension in check_alu_op()

Distinguish between
BPF_ALU64|BPF_MOV|BPF_K (load 32-bit immediate, sign-extended to 64-bit)
and BPF_ALU|BPF_MOV|BPF_K (load 32-bit immediate, zero-padded to 64-bit);
only perform sign extension in the first case.

Starting with v4.14, this is exploitable by unprivileged users as long as
the unprivileged_bpf_disabled sysctl isn't set.

Debian assigned CVE-2017-16995 for this issue.

v3:
 - add CVE number (Ben Hutchings)

Fixes: 4846113 ("bpf: allow access into map value arrays")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
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thejh authored and borkmann committed Dec 19, 2017
1 parent 4374f25 commit 95a762e2c8c942780948091f8f2a4f32fce1ac6f
Showing with 7 additions and 1 deletion.
  1. +7 −1 kernel/bpf/verifier.c
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@@ -2408,7 +2408,13 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
* remember the value we stored into this reg
*/
regs[insn->dst_reg].type = SCALAR_VALUE;
__mark_reg_known(regs + insn->dst_reg, insn->imm);
if (BPF_CLASS(insn->code) == BPF_ALU64) {
__mark_reg_known(regs + insn->dst_reg,
insn->imm);
} else {
__mark_reg_known(regs + insn->dst_reg,
(u32)insn->imm);
}
}
} else if (opcode > BPF_END) {

1 comment on commit 95a762e

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charliezzo Apr 1, 2018

  •   	__mark_reg_known(regs + insn->dst_reg, insn->imm);
    
  •   	if (BPF_CLASS(insn->code) == BPF_ALU64) {
    
  •   		__mark_reg_known(regs + insn->dst_reg,insn->imm);
    
  •   	} else {
    
  •   		__mark_reg_known(regs + insn->dst_reg, (u32)insn->imm);
    
  •   	}
    

yes,its not good for Ubuntu16.04.1-16.04.4

charliezzo commented on 95a762e Apr 1, 2018

  •   	__mark_reg_known(regs + insn->dst_reg, insn->imm);
    
  •   	if (BPF_CLASS(insn->code) == BPF_ALU64) {
    
  •   		__mark_reg_known(regs + insn->dst_reg,insn->imm);
    
  •   	} else {
    
  •   		__mark_reg_known(regs + insn->dst_reg, (u32)insn->imm);
    
  •   	}
    

yes,its not good for Ubuntu16.04.1-16.04.4

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