Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()

On Fri, Jul 9, 2021 at 4:11 AM He Fengqing [off-list ref] wrote:

在 2021/7/8 11:09, Alexei Starovoitov 写道:

quoted

On Wed, Jul 7, 2021 at 8:00 PM He Fengqing [off-list ref] wrote:

quoted

Ok, I will change this in next version.

before you spam the list with the next version
please explain why any of these changes are needed?
I don't see an explanation in the patches and I don't see a bug in the code.
Did you check what is the prog clone ?
When is it constructed? Why verifier has anything to do with it?
.

I'm sorry, I didn't describe these errors clearly.

bpf_check(bpf_verifier_env)
     |
     |->do_misc_fixups(env)
     |    |
     |    |->bpf_patch_insn_data(env)
     |    |    |
     |    |    |->bpf_patch_insn_single(env->prog)
     |    |    |    |
     |    |    |    |->bpf_prog_realloc(env->prog)
     |    |    |    |    |
     |    |    |    |    |->construct new_prog
     |    |    |    |    |    free old_prog(env->prog)
     |    |    |    |    |
     |    |    |    |    |->return new_prog;
     |    |    |    |
     |    |    |    |->return new_prog;
     |    |    |
     |    |    |->adjust_insn_aux_data
     |    |    |    |
     |    |    |    |->return ENOMEM;
     |    |    |
     |    |    |->return NULL;
     |    |
     |    |->return ENOMEM;

bpf_verifier_env->prog had been freed in bpf_prog_realloc function.


There are two errors here, the first is memleak in the
bpf_patch_insn_data function, and the second is use after free in the
bpf_check function.

memleak in bpf_patch_insn_data:

Look at the call chain above, if adjust_insn_aux_data function return
ENOMEM, bpf_patch_insn_data will return NULL, but we do not free the
new_prog.

So in the patch 2, before bpf_patch_insn_data return NULL, we free the
new_prog.

use after free in bpf_check:

If bpf_patch_insn_data function return NULL, we will not assign new_prog
to the bpf_verifier_env->prog, but bpf_verifier_env->prog has been freed
in the bpf_prog_realloc function. Then in bpf_check function, we will
use bpf_verifier_env->prog after do_misc_fixups function.

In the patch 3, I added a free_old parameter to bpf_prog_realloc, in
this scenario we don't free old_prog. Instead, we free it in the
do_misc_fixups function when bpf_patch_insn_data return a valid new_prog.

Thanks for explaining.
Why not to make adjust_insn_aux_data() in bpf_patch_insn_data() first then?
Just changing the order will resolve both issues, no?