[PATCH 4.19 128/134] bpf: do not restore dst_reg when cur_state is freed

[Greg Kroah-Hartman]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>

commit 0803278b0b4d8eeb2b461fb698785df65a725d9e upstream.

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 restoring dst_reg.

Fixes: 979d63d50c0c ("bpf: prevent out of bounds speculation on pointer arithmetic")
Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/bpf/verifier.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2815,7 +2815,7 @@ do_sim:
 		*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;
 }