From 26f55a59dc65ff77cd1c4b37991e26497fc68049 Mon Sep 17 00:00:00 2001 From: Piotr Krysiuk Date: Tue, 6 Apr 2021 21:59:39 +0100 Subject: [PATCH] bpf, x86: Validate computation of branch displacements for x86-32 The branch displacement logic in the BPF JIT compilers for x86 assumes that, for any generated branch instruction, the distance cannot increase between optimization passes. But this assumption can be violated due to how the distances are computed. Specifically, whenever a backward branch is processed in do_jit(), the distance is computed by subtracting the positions in the machine code from different optimization passes. This is because part of addrs[] is already updated for the current optimization pass, before the branch instruction is visited. And so the optimizer can expand blocks of machine code in some cases. This can confuse the optimizer logic, where it assumes that a fixed point has been reached for all machine code blocks once the total program size stops changing. And then the JIT compiler can output abnormal machine code containing incorrect branch displacements. To mitigate this issue, we assert that a fixed point is reached while populating the output image. This rejects any problematic programs. The issue affects both x86-32 and x86-64. We mitigate separately to ease backporting. Signed-off-by: Piotr Krysiuk Reviewed-by: Daniel Borkmann Signed-off-by: Daniel Borkmann --- arch/x86/net/bpf_jit_comp32.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/arch/x86/net/bpf_jit_comp32.c b/arch/x86/net/bpf_jit_comp32.c index d17b67c69f89..6a99def7d315 100644 --- a/arch/x86/net/bpf_jit_comp32.c +++ b/arch/x86/net/bpf_jit_comp32.c @@ -2276,7 +2276,16 @@ notyet: } if (image) { - if (unlikely(proglen + ilen > oldproglen)) { + /* + * When populating the image, assert that: + * + * i) We do not write beyond the allocated space, and + * ii) addrs[i] did not change from the prior run, in order + * to validate assumptions made for computing branch + * displacements. + */ + if (unlikely(proglen + ilen > oldproglen || + proglen + ilen != addrs[i])) { pr_err("bpf_jit: fatal error\n"); return -EFAULT; }