Re: [PATCH] gcov: clang: fix the buffer overflow issue

[Peter Oberparleiter <oberpar@linux.ibm.com>]

On 07.11.2022 20:38, Nick Desaulniers wrote:
> On Fri, Nov 4, 2022 at 12:58 PM Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>> On 11/4/2022 11:18 PM, Nick Desaulniers wrote:
>>> On Fri, Nov 4, 2022 at 6:23 AM Mukesh Ojha <quic_mojha@quicinc.com> wrote:
>>>>
>>>> Currently, in clang version of gcov code when module is getting removed
>>>> gcov_info_add() incorrectly adds the sfn_ptr->counter to all the
>>>> dst->functions and it result in the kernel panic in below crash report.
>>>> Fix this by properly handling it.
>>>>
>>>> [    8.899094][  T599] Unable to handle kernel write to read-only memory at virtual address ffffff80461cc000
>>>> [    8.899100][  T599] Mem abort info:
>>>> [    8.899102][  T599]   ESR = 0x9600004f
>>>> [    8.899103][  T599]   EC = 0x25: DABT (current EL), IL = 32 bits
>>>> [    8.899105][  T599]   SET = 0, FnV = 0
>>>> [    8.899107][  T599]   EA = 0, S1PTW = 0
>>>> [    8.899108][  T599]   FSC = 0x0f: level 3 permission fault
>>>> [    8.899110][  T599] Data abort info:
>>>> [    8.899111][  T599]   ISV = 0, ISS = 0x0000004f
>>>> [    8.899113][  T599]   CM = 0, WnR = 1
>>>> [    8.899114][  T599] swapper pgtable: 4k pages, 39-bit VAs, pgdp=00000000ab8de000
>>>> [    8.899116][  T599] [ffffff80461cc000] pgd=18000009ffcde003, p4d=18000009ffcde003, pud=18000009ffcde003, pmd=18000009ffcad003, pte=00600000c61cc787
>>>> [    8.899124][  T599] Internal error: Oops: 9600004f [#1] PREEMPT SMP
>>>> [    8.899265][  T599] Skip md ftrace buffer dump for: 0x1609e0
>>>> ....
>>>> ..,
>>>> [    8.899544][  T599] CPU: 7 PID: 599 Comm: modprobe Tainted: G S         OE     5.15.41-android13-8-g38e9b1af6bce #1
>>>> [    8.899547][  T599] Hardware name: XXX (DT)
>>>> [    8.899549][  T599] pstate: 82400005 (Nzcv daif +PAN -UAO +TCO -DIT -SSBS BTYPE=--)
>>>> [    8.899551][  T599] pc : gcov_info_add+0x9c/0xb8
>>>> [    8.899557][  T599] lr : gcov_event+0x28c/0x6b8
>>>> [    8.899559][  T599] sp : ffffffc00e733b00
>>>> [    8.899560][  T599] x29: ffffffc00e733b00 x28: ffffffc00e733d30 x27: ffffffe8dc297470
>>>> [    8.899563][  T599] x26: ffffffe8dc297000 x25: ffffffe8dc297000 x24: ffffffe8dc297000
>>>> [    8.899566][  T599] x23: ffffffe8dc0a6200 x22: ffffff880f68bf20 x21: 0000000000000000
>>>> [    8.899569][  T599] x20: ffffff880f68bf00 x19: ffffff8801babc00 x18: ffffffc00d7f9058
>>>> [    8.899572][  T599] x17: 0000000000088793 x16: ffffff80461cbe00 x15: 9100052952800785
>>>> [    8.899575][  T599] x14: 0000000000000200 x13: 0000000000000041 x12: 9100052952800785
>>>> [    8.899577][  T599] x11: ffffffe8dc297000 x10: ffffffe8dc297000 x9 : ffffff80461cbc80
>>>> [    8.899580][  T599] x8 : ffffff8801babe80 x7 : ffffffe8dc2ec000 x6 : ffffffe8dc2ed000
>>>> [    8.899583][  T599] x5 : 000000008020001f x4 : fffffffe2006eae0 x3 : 000000008020001f
>>>> [    8.899586][  T599] x2 : ffffff8027c49200 x1 : ffffff8801babc20 x0 : ffffff80461cb3a0
>>>> [    8.899589][  T599] Call trace:
>>>> [    8.899590][  T599]  gcov_info_add+0x9c/0xb8
>>>> [    8.899592][  T599]  gcov_module_notifier+0xbc/0x120
>>>> [    8.899595][  T599]  blocking_notifier_call_chain+0xa0/0x11c
>>>> [    8.899598][  T599]  do_init_module+0x2a8/0x33c
>>>> [    8.899600][  T599]  load_module+0x23cc/0x261c
>>>> [    8.899602][  T599]  __arm64_sys_finit_module+0x158/0x194
>>>> [    8.899604][  T599]  invoke_syscall+0x94/0x2bc
>>>> [    8.899607][  T599]  el0_svc_common+0x1d8/0x34c
>>>> [    8.899609][  T599]  do_el0_svc+0x40/0x54
>>>> [    8.899611][  T599]  el0_svc+0x94/0x2f0
>>>> [    8.899613][  T599]  el0t_64_sync_handler+0x88/0xec
>>>> [    8.899615][  T599]  el0t_64_sync+0x1b4/0x1b8
>>>> [    8.899618][  T599] Code: f905f56c f86e69ec f86e6a0f 8b0c01ec (f82e6a0c)
>>>> [    8.899620][  T599] ---[ end trace ed5218e9e5b6e2e6 ]---
>>>>
>>>> Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
>>>> ---
>>>>   kernel/gcov/clang.c | 13 +++++++++----
>>>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/kernel/gcov/clang.c b/kernel/gcov/clang.c
>>>> index cbb0bed..0aabb9a 100644
>>>> --- a/kernel/gcov/clang.c
>>>> +++ b/kernel/gcov/clang.c
>>>> @@ -271,15 +271,20 @@ int gcov_info_is_compatible(struct gcov_info *info1, struct gcov_info *info2)
>>>>    */
>>>>   void gcov_info_add(struct gcov_info *dst, struct gcov_info *src)
>>>>   {
>>>> -       struct gcov_fn_info *dfn_ptr;
>>>> -       struct gcov_fn_info *sfn_ptr = list_first_entry_or_null(&src->functions,
>>>> -                       struct gcov_fn_info, head);
>>>
>>> Hi Mukesh,
>>> Thanks for the report and patch!
>>>
>>> Looking closer at the existing implementation, it looks curious to me
>>> that we use list_first_entry_or_null() since that may return NULL,
>>> which we never check for.  I'm curious if that's safe to remove?
>>> Probably, since we haven't had any issues reported thus far.
>>>
>>>> +       struct gcov_fn_info *sfn_ptr;
>>>> +       struct gcov_fn_info *dfn_ptr = list_first_entry_or_null(
>>>> +                       &dst->functions, struct gcov_fn_info, head);
>>>>
>>>> -       list_for_each_entry(dfn_ptr, &dst->functions, head) {
>>>> +       list_for_each_entry(sfn_ptr, &src->functions, head) {
>>>
>>> This seems to be iterating BOTH src and dest, whereas previously we
>>> were only iterating dest AFAICT.  Is this correct?  Seems to be a
>>> change of behavior, at the least, which seems orthogonal to fixing the
>>> panic.
>>
>> Can you just check the implementation here once ?
>>
>> https://elixir.bootlin.com/linux/v6.1-rc3/source/kernel/gcov/gcc_4_7.c#L241
>>
>> By looking at the above link clang version does not seem to doing right ?
> 
> Oh, indeed, the GCC variant is looping over BOTH src+dest together,
> then the counters.
> 
> I expect this patch to change the counter values, but I suspect they
> haven't been correct previously and we've only noticed whether
> branches were taken vs not.

It seems that you need the following to force this issue:
- gcov_persists=1 on the kernel command line
- unload of a kernel module
- have an object file compiled into the kernel module where the first
  function has more basic blocks transitions than later functions

> Thanks for the patch.
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> 
> Peter, can you pick this up?

While this patch works correctly, it seems to contain changes that are
not necessary to fix the underlying problem.

The null-pointer check for !dfn_ptr in the loop body should not be
required because both lists are guaranteed to contain the same number of
elements (via a previous call to gcov_info_is_compatible()). As a
result, if dfn_ptr is null because dst->functions is empty, the loop
body will never be called because src->functions is also empty.

Also I don't understand why the patch changes the roles of dfn_ptr and
sfn_ptr - it should have no functional effect if you iterate over
src->functions or dst->functions.

The only required change seems to be the addition of the list_next_entry
statement for the non-loop-variable (sfn_ptr in the original code
version) to the end of the loop body. Without it, gcov_info_add() tries
to add counters of the very first function to the counter-arrays of all
other functions. This will cause the reported out-of-bounds write error
in case later functions in an object file provide fewer counts than the
first function.

@Mukesh: Please test if the minimal change outlined above also fixes the
problem you are seeing, and if it does, please send an updated patch.

Thanks!

> 
>>
>>>
>>> Otherwise it sounds like we could just add NULL ptr checks against
>>> sfn_ptr outside the loop, and against dfn_ptr inside the loop.
>>> Something like this?
>>> ```
>>> diff --git a/kernel/gcov/clang.c b/kernel/gcov/clang.c
>>> index cbb0bed958ab..5d4cb801aa9c 100644
>>> --- a/kernel/gcov/clang.c
>>> +++ b/kernel/gcov/clang.c
>>> @@ -275,10 +275,13 @@ void gcov_info_add(struct gcov_info *dst, struct
>>> gcov_info *src)
>>>          struct gcov_fn_info *sfn_ptr = list_first_entry_or_null(&src->functions,
>>>                          struct gcov_fn_info, head);
>>>
>>> -       list_for_each_entry(dfn_ptr, &dst->functions, head) {
>>> -               u32 i;
>>> +       if (!sfn_ptr)
>>> +               return;
>>>
>>> -               for (i = 0; i < sfn_ptr->num_counters; i++)
>>> +       list_for_each_entry(dfn_ptr, &dst->functions, head) {
>>> +               if (!dfn_ptr)
>>> +                       continue;
>>> +               for (u32 i = 0, e = sfn_ptr->num_counters; i != e; ++i)
>>>                          dfn_ptr->counters[i] += sfn_ptr->counters[i];
>>>          }
>>>   }
>>> ```
>>> Can you test the above hunk or comment on whether it addresses the issue?
>>
>>
>> BTW, it just handles NUL pointer issue and not the one which is
>> mentioned here.
>>
>> "Unable to handle kernel write to read-only memory at virtual address
>> ffffff80461cc000"
>>
>> -Mukesh
>>
>>>
>>>>                  u32 i;
>>>>
>>>> +               if (!dfn_ptr)
>>>> +                       return;
>>>> +
>>>>                  for (i = 0; i < sfn_ptr->num_counters; i++)
>>>>                          dfn_ptr->counters[i] += sfn_ptr->counters[i];
>>>> +
>>>> +               dfn_ptr = list_next_entry(dfn_ptr, head);
>>>>          }
>>>>   }
>>>>
>>>> --
>>>> 2.7.4
>>>>
>>>
>>>
> 
> 
> 

-- 
Peter Oberparleiter
Linux on IBM Z Development - IBM Germany R&D