Re: [PATCH] powerpc/bpf: fix write protecting JIT code

[Michael Ellerman <mpe@ellerman.id.au>]

"Naveen N. Rao" <naveen.n.rao@linux.ibm.com> writes:
> Michael Ellerman wrote:
>> Daniel Borkmann <daniel@iogearbox.net> writes:
>>> On 10/25/21 8:15 AM, Naveen N. Rao wrote:
>>>> Hari Bathini wrote:
>>>>> Running program with bpf-to-bpf function calls results in data access
>>>>> exception (0x300) with the below call trace:
>>>>>
>>>>>     [c000000000113f28] bpf_int_jit_compile+0x238/0x750 (unreliable)
>>>>>     [c00000000037d2f8] bpf_check+0x2008/0x2710
>>>>>     [c000000000360050] bpf_prog_load+0xb00/0x13a0
>>>>>     [c000000000361d94] __sys_bpf+0x6f4/0x27c0
>>>>>     [c000000000363f0c] sys_bpf+0x2c/0x40
>>>>>     [c000000000032434] system_call_exception+0x164/0x330
>>>>>     [c00000000000c1e8] system_call_vectored_common+0xe8/0x278
>>>>>
>>>>> as bpf_int_jit_compile() tries writing to write protected JIT code
>>>>> location during the extra pass.
>>>>>
>>>>> Fix it by holding off write protection of JIT code until the extra
>>>>> pass, where branch target addresses fixup happens.
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>> Fixes: 62e3d4210ac9 ("powerpc/bpf: Write protect JIT code")
>>>>> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
>>>>> ---
>>>>>  arch/powerpc/net/bpf_jit_comp.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>> 
>>>> Thanks for the fix!
>>>> 
>>>> Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>>>
>>> LGTM, I presume this fix will be routed via Michael.
>> 
>> Thanks for reviewing, I've picked it up.
>> 
>>> BPF selftests have plenty of BPF-to-BPF calls in there, too bad this was
>>> caught so late. :/
>> 
>> Yeah :/
>> 
>> STRICT_KERNEL_RWX is not on by default in all our defconfigs, so that's
>> probably why no one caught it.
>
> Yeah, sorry - we should have caught this sooner.
>
>> 
>> I used to run the BPF selftests but they stopped building for me a while
>> back, I'll see if I can get them going again.
>
> Ravi had started looking into getting the selftests working well before 
> he left. I will take a look at this.

Thanks.

I got them building with something like:

 - turning on DEBUG_INFO and DEBUG_INFO_BTF and rebuilding vmlinux
 - grabbing clang 13 from: 
   https://github.com/llvm/llvm-project/releases/download/llvmorg-13.0.0/clang+llvm-13.0.0-powerpc64le-linux-ubuntu-18.04.tar.xz
 - PATH=$HOME/clang+llvm-13.0.0-powerpc64le-linux-ubuntu-18.04/bin/:$PATH
 - apt install:
   - libelf-dev
   - dwarves
   - python-docutils
   - libcap-dev


The DEBUG_INFO requirement is a bit of a pain for me. I generally don't
build with that enabled, because the resulting kernels are stupidly
large. I'm not sure if that's a hard requirement, or if the vmlinux has
to match the running kernel exactly?

There is logic in tools/testing/bpf/Makefile to use VMLINUX_H instead of
extracting the BTF from the vmlinux (line 247), but AFAICS that's
unreachable since 1a3449c19407 ("selftests/bpf: Clarify build error if
no vmlinux"), which makes it a hard error to not have a VMLINUX_BTF.

cheers