linux-kernel - [PATCH] bpf: do not restore dst_reg when cur

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Message-Id: <eec7905a71d207f7612e9d86e1bf59c43efc65a4.1553156792.git.xuyu@linux.alibaba.com>
Date:   Thu, 21 Mar 2019 16:31:33 +0800
From:   Xu Yu <xuyu@...ux.alibaba.com>
To:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] bpf: do not restore dst_reg when cur_state is freed

Syzkaller hit 'KASAN: use-after-free Write in sanitize_ptr_alu' bug.
Call trace:
  dump_stack+0xbf/0x12e
  print_address_description+0x6a/0x280
  kasan_report+0x237/0x360
  sanitize_ptr_alu+0x85a/0x8d0
  adjust_ptr_min_max_vals+0x8f2/0x1ca0
  adjust_reg_min_max_vals+0x8ed/0x22e0
  do_check+0x1ca6/0x5d00
  bpf_check+0x9ca/0x2570
  bpf_prog_load+0xc91/0x1030
  __se_sys_bpf+0x61e/0x1f00
  do_syscall_64+0xc8/0x550
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
Fault injection trace:
  kfree+0xea/0x290
  free_func_state+0x4a/0x60
  free_verifier_state+0x61/0xe0
  push_stack+0x216/0x2f0	          <- inject failslab
  sanitize_ptr_alu+0x2b1/0x8d0
  adjust_ptr_min_max_vals+0x8f2/0x1ca0
  adjust_reg_min_max_vals+0x8ed/0x22e0
  do_check+0x1ca6/0x5d00
  bpf_check+0x9ca/0x2570
  bpf_prog_load+0xc91/0x1030
  __se_sys_bpf+0x61e/0x1f00
  do_syscall_64+0xc8/0x550
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

When kzalloc() fails in push_stack(), free_verifier_state() will free
current verifier state. As push_stack() returns, dst_reg was restored
if ptr_is_dst_reg is false. However, as member of the cur_state, dst_reg
is also freed, and error occurs when dereferencing dst_reg.

Simply fix it by checking whether cur_state is NULL before retoring
dst_reg.

Signed-off-by: Xu Yu <xuyu@...ux.alibaba.com>
---
 kernel/bpf/verifier.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ce166a0..018ce4f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3368,7 +3368,7 @@ static int sanitize_ptr_alu(struct bpf_verifier_env *env,
 		*dst_reg = *ptr_reg;
 	}
 	ret = push_stack(env, env->insn_idx + 1, env->insn_idx, true);
-	if (!ptr_is_dst_reg)
+	if (!ptr_is_dst_reg && env->cur_state)
 		*dst_reg = tmp;
 	return !ret ? -EFAULT : 0;
 }
-- 
1.8.3.1