[PATCH v2] bpf: do not restore dst_reg when cur_state is freed

[Xu Yu]

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:
 Âkfree+0xea/0x290
 Âfree_func_state+0x4a/0x60
 Âfree_verifier_state+0x61/0xe0
 Âpush_stack+0x216/0x2f0	          <- inject failslab
 Âsanitize_ptr_alu+0x2b1/0x8d0
 Âadjust_ptr_min_max_vals+0x8f2/0x1ca0
 Âadjust_reg_min_max_vals+0x8ed/0x22e0
 Âdo_check+0x1ca6/0x5d00
 Âbpf_check+0x9ca/0x2570
 Âbpf_prog_load+0xc91/0x1030
 Â__se_sys_bpf+0x61e/0x1f00
 Âdo_syscall_64+0xc8/0x550
 Âentry_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 testing ret of push_stack() before retoring dst_reg.

Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
---
 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..cb1b874 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 && ret)
 		*dst_reg = tmp;
 	return !ret ? -EFAULT : 0;
 }
-- 
1.8.3.1