linux-kernel - Re: [PATCH] KVM: arm64: vgic-its: Remove VLA usage

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Date:   Mon, 2 Jul 2018 10:15:27 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     Christoffer Dall <christoffer.dall@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Andre Przywara <andre.przywara@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] KVM: arm64: vgic-its: Remove VLA usage

On Mon, Jul 2, 2018 at 12:36 AM, Auger Eric <eric.auger@...hat.com> wrote:
> Hi Kees,
>
> On 06/29/2018 08:46 PM, Kees Cook wrote:
>> In the quest to remove all stack VLA usage from the kernel[1], this
>> switches to using a maximum size and adds sanity checks. Additionally
>> cleans up some of the int-vs-u32 usage and adds additional bounds checking.
>> As it currently stands, this will always be 8 bytes until the ABI changes.
>>
>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>
>> Cc: Christoffer Dall <christoffer.dall@....com>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Eric Auger <eric.auger@...hat.com>
>> Cc: Andre Przywara <andre.przywara@....com>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: kvmarm@...ts.cs.columbia.edu
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  virt/kvm/arm/vgic/vgic-its.c | 19 +++++++++++++++----
>>  1 file changed, 15 insertions(+), 4 deletions(-)
>>
>> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
>> index 4ed79c939fb4..3143fc047fcf 100644
>> --- a/virt/kvm/arm/vgic/vgic-its.c
>> +++ b/virt/kvm/arm/vgic/vgic-its.c
>> @@ -168,8 +168,14 @@ struct vgic_its_abi {
>>       int (*commit)(struct vgic_its *its);
>>  };
>>
>> +#define ABI_0_ESZ    8
>> +#define ESZ_MAX              ABI_0_ESZ
>> +
>>  static const struct vgic_its_abi its_table_abi_versions[] = {
>> -     [0] = {.cte_esz = 8, .dte_esz = 8, .ite_esz = 8,
>> +     [0] = {
>> +      .cte_esz = ABI_0_ESZ,
>> +      .dte_esz = ABI_0_ESZ,
>> +      .ite_esz = ABI_0_ESZ,
>>        .save_tables = vgic_its_save_tables_v0,
>>        .restore_tables = vgic_its_restore_tables_v0,
>>        .commit = vgic_its_commit_v0,
>> @@ -180,10 +186,12 @@ static const struct vgic_its_abi its_table_abi_versions[] = {
>>
>>  inline const struct vgic_its_abi *vgic_its_get_abi(struct vgic_its *its)
>>  {
>> +     if (WARN_ON(its->abi_rev >= NR_ITS_ABIS))
>> +             return NULL;
>>       return &its_table_abi_versions[its->abi_rev];
>>  }
>>
>> -int vgic_its_set_abi(struct vgic_its *its, int rev)
>> +static int vgic_its_set_abi(struct vgic_its *its, u32 rev)
>>  {
> if vgic_its_get_abi is likely to return NULL, don't we need to check abi
> != NULL in all call sites.

My thinking was that since it should never happen, a WARN_ON would be
sufficient. But I can drop all these changes if you want. I just
wanted to see the VLA removed. :)

-Kees

>
> abi_rev is actually set by vgic_its_set_abi() which is actually called
> by vgic_mmio_uaccess_write_its_iidr() and vgic_its_create().
>
> Only vgic_mmio_uaccess_write_its_iidr allows the userspace to overwrite
> the default abi_rev. At this point a check against NR_ITS_ABIS is
> already done. So to me the check is done at the source?
>
> Thanks
>
> Eric
>>       const struct vgic_its_abi *abi;
>>
>> @@ -1881,16 +1889,19 @@ typedef int (*entry_fn_t)(struct vgic_its *its, u32 id, void *entry,
>>   * Return: < 0 on error, 0 if last element was identified, 1 otherwise
>>   * (the last element may not be found on second level tables)
>>   */
>> -static int scan_its_table(struct vgic_its *its, gpa_t base, int size, int esz,
>> +static int scan_its_table(struct vgic_its *its, gpa_t base, int size, u32 esz,
>>                         int start_id, entry_fn_t fn, void *opaque)
>>  {
>>       struct kvm *kvm = its->dev->kvm;
>>       unsigned long len = size;
>>       int id = start_id;
>>       gpa_t gpa = base;
>> -     char entry[esz];
>> +     char entry[ESZ_MAX];
>>       int ret;
>>
>> +     if (WARN_ON(esz > ESZ_MAX))
>> +             return -EINVAL;
>> +
>>       memset(entry, 0, esz);
>>
>>       while (len > 0) {
>>



-- 
Kees Cook
Pixel Security