From: Piotr Krysiuk piotras@gmail.com

commit e4d4d456436bfb2fe412ee2cd489f7658449b098 upstream.

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 piotras@gmail.com Reviewed-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/net/bpf_jit_comp.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 46ab928312517..924ca27a6139b 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -1019,7 +1019,16 @@ xadd: if (is_imm8(insn->off)) }

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; }