6.15-stable review patch. If anyone has any objections, please let me know.
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From: Paul Chaignon paul.chaignon@gmail.com
[ Upstream commit 6279846b9b2532e1b04559ef8bd0dec049f29383 ]
Syzbot reported a kernel warning due to a range invariant violation on the following BPF program.
0: call bpf_get_netns_cookie 1: if r0 == 0 goto <exit> 2: if r0 & Oxffffffff goto <exit>
The issue is on the path where we fall through both jumps.
That path is unreachable at runtime: after insn 1, we know r0 != 0, but with the sign extension on the jset, we would only fallthrough insn 2 if r0 == 0. Unfortunately, is_branch_taken() isn't currently able to figure this out, so the verifier walks all branches. The verifier then refines the register bounds using the second condition and we end up with inconsistent bounds on this unreachable path:
1: if r0 == 0 goto <exit> r0: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0xffffffffffffffff) 2: if r0 & 0xffffffff goto <exit> r0 before reg_bounds_sync: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0) r0 after reg_bounds_sync: u64=[0x1, 0] var_off=(0, 0)
Improving the range refinement for JSET to cover all cases is tricky. We also don't expect many users to rely on JSET given LLVM doesn't generate those instructions. So instead of improving the range refinement for JSETs, Eduard suggested we forget the ranges whenever we're narrowing tnums after a JSET. This patch implements that approach.
Reported-by: syzbot+c711ce17dd78e5d4fdcf@syzkaller.appspotmail.com Suggested-by: Eduard Zingerman eddyz87@gmail.com Acked-by: Yonghong Song yonghong.song@linux.dev Acked-by: Eduard Zingerman eddyz87@gmail.com Signed-off-by: Paul Chaignon paul.chaignon@gmail.com Link: https://lore.kernel.org/r/9d4fd6432a095d281f815770608fdcd16028ce0b.175217136... Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a1ecad2944a8..bdeed2a24910 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -16014,6 +16014,10 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state if (!is_reg_const(reg2, is_jmp32)) break; val = reg_const_value(reg2, is_jmp32); + /* Forget the ranges before narrowing tnums, to avoid invariant + * violations if we're on a dead branch. + */ + __mark_reg_unbounded(reg1); if (is_jmp32) { t = tnum_and(tnum_subreg(reg1->var_off), tnum_const(~val)); reg1->var_off = tnum_with_subreg(reg1->var_off, t);