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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 4 Nov 2022 10:48:37 -0700
Message-ID: <CAKwvOdkdeLEvtOmX423oYaWCami0kAFatWe25DdJq7gbmGb+5g@mail.gmail.com>
Subject: Re: [PATCH] gcov: clang: fix the buffer overflow issue
To:     Mukesh Ojha <quic_mojha@quicinc.com>
Cc:     oberpar@linux.ibm.com, nathan@kernel.org, trix@redhat.com,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev
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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.

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?

>                 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
>


-- 
Thanks,
~Nick Desaulniers