Re: [PATCH v2 1/1] hw/arm: do not free machine->fdt in arm_load_dtb()

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 3/28/23 04:01, Markus Armbruster wrote:
> Daniel Henrique Barboza <danielhb413@gmail.com> writes:
> 
>> At this moment, arm_load_dtb() can free machine->fdt when
>> binfo->dtb_filename is NULL. If there's no 'dtb_filename', 'fdt' will be
>> retrieved by binfo->get_dtb(). If get_dtb() returns machine->fdt, as is
>> the case of machvirt_dtb() from hw/arm/virt.c, fdt now has a pointer to
>> machine->fdt. And, in that case, the existing g_free(fdt) at the end of
>> arm_load_dtb() will make machine->fdt point to an invalid memory region.
>>
>> After the command 'dumpdtb' were introduced a couple of releases ago,
>> running it with any ARM machine that uses arm_load_dtb() will crash
>> QEMU.
>>
>> Let's enable all arm_load_dtb() callers to use dumpdtb properly. Instead
>> of freeing 'fdt', assign it back to ms->fdt.
>>
>> Note that all current callers (sbsa-ref.c, virt.c, xlnx-versal-virt.c)
>> are assigning ms->fdt before arm_load_dtb() is called, regardless of
>> whether the user is inputting an external FDT via '-dtb'. To avoid
>> leaking the board FDT if '-dtb' is used (since we're assigning ms->fdt
>> in the end), free ms->fdt before load_device_tree().
>>
>> Cc: Peter Maydell <peter.maydell@linaro.org>
>> Cc: qemu-arm@nongnu.org
>> Fixes: bf353ad55590f ("qmp/hmp, device_tree.c: introduce dumpdtb")
>> Reported-by: Markus Armbruster <armbru@redhat.com>i
>> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
>> ---
>>   hw/arm/boot.c | 10 +++++++++-
>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/arm/boot.c b/hw/arm/boot.c
>> index 50e5141116..de18c0a969 100644
>> --- a/hw/arm/boot.c
>> +++ b/hw/arm/boot.c
>> @@ -549,6 +549,13 @@ int arm_load_dtb(hwaddr addr, const struct arm_boot_info *binfo,
>>               goto fail;
>>           }
>>   
>> +        /*
>> +         * If we're here we won't be using the ms->fdt from the board.
>> +         * We'll assign a new ms->fdt at the end, so free it now to
>> +         * avoid leaking the board FDT.
>> +         */
>> +        g_free(ms->fdt);
>> +
> 
> "We will" is not true: we will not if we goto fail.  Leaves ms->fdt
> dangling, doesn't it?

We can postpone this g_free() to execute after "if (!fdt) {}" to be sure that we're
not freeing ms->fdt right before 'goto fail'.

> 
>>           fdt = load_device_tree(filename, &size);
>>           if (!fdt) {
>>               fprintf(stderr, "Couldn't open dtb file %s\n", filename);
>                 g_free(filename);
>                 goto fail;
>             }
>             g_free(filename);
>         } else {
>             fdt = binfo->get_dtb(binfo, &size);
>             if (!fdt) {
>                 fprintf(stderr, "Board was unable to create a dtb blob\n");
>                 goto fail;
>             }
> 
> If we succeed, we'll assign @fdt to ms->fdt (next hunk).  Won't this
> leak old ms->fdt?


For all callers binfo->get_dtb() is returning ms->fdt, i.e. this line:

              fdt = binfo->get_dtb(binfo, &size);

Is equal to this:

              fdt = ms->fdt;

And this is why we can't unconditionally do a g_free(ms->fdt).


I believe we can improve the ARM boot code to not create ms->fdt at init(),
leaving it unassigned, and make get_dtb() return the machine FDT on a common
"void *" pointer. That would spare us from having go g_free(ms->fdt) to avoid
leaks and we would assign ms->fdt at the end of arm_load_dtb() normally. I made
a quick attempt at that but the ARM init() code is a little tricker than I've
anticipated. I might have a crack at it later.


Thanks,

Daniel


> 
>         }
> 
>> @@ -689,7 +696,8 @@ int arm_load_dtb(hwaddr addr, const struct arm_boot_info *binfo,
>>       qemu_register_reset_nosnapshotload(qemu_fdt_randomize_seeds,
>>                                          rom_ptr_for_as(as, addr, size));
>>   
>> -    g_free(fdt);
>> +    /* Set ms->fdt for 'dumpdtb' QMP/HMP command */
>> +    ms->fdt = fdt;
>>   
>>       return size;
>