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

[Naveen N. Rao]

Michael Ellerman wrote:
> Daniel Borkmann <daniel at 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 at vger.kernel.org
>>>> Fixes: 62e3d4210ac9 ("powerpc/bpf: Write protect JIT code")
>>>> Signed-off-by: Hari Bathini <hbathini at 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 at 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,
Naveen