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SubjectRe: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()
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在 2021/7/8 11:09, Alexei Starovoitov 写道:
> On Wed, Jul 7, 2021 at 8:00 PM He Fengqing <hefengqing@huawei.com> wrote:
>>
>> 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 your reviews.

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