Re: [PATCH v1 2/3] x86/xen/msr: Fix uninitialized symbol 'err'

[Xin Li <xin@zytor.com>]

On 5/16/2025 6:42 AM, Jürgen Groß wrote:
> On 15.05.25 20:11, Xin Li wrote:
>> On 5/15/2025 8:29 AM, Ingo Molnar wrote:
>>>
>>> * Xin Li (Intel) <xin@zytor.com> wrote:
>>>
>>>> xen_read_msr_safe() currently passes an uninitialized argument err to
>>>> xen_do_read_msr().  But as xen_do_read_msr() may not set the argument,
>>>> xen_read_msr_safe() could return err with an unpredictable value.
>>>>
>>>> To ensure correctness, initialize err to 0 (representing success)
>>>> in xen_read_msr_safe().
>>>>
>>>> Because xen_read_msr_safe() is essentially a wrapper of 
>>>> xen_do_read_msr(),
>>>> the latter should be responsible for initializing the value of *err 
>>>> to 0.
>>>> Thus initialize *err to 0 in xen_do_read_msr().
>>>>
>>>> Fixes: 502ad6e5a619 ("x86/msr: Change the function type of 
>>>> native_read_msr_safe()")
>>>> Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
>>>> Closes: https://lore.kernel.org/xen-devel/aBxNI_Q0- 
>>>> MhtBSZG@stanley.mountain/
>>>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>>>> ---
>>>>   arch/x86/xen/enlighten_pv.c | 5 ++++-
>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>>>> index 3be38350f044..01f1d441347e 100644
>>>> --- a/arch/x86/xen/enlighten_pv.c
>>>> +++ b/arch/x86/xen/enlighten_pv.c
>>>> @@ -1091,6 +1091,9 @@ static u64 xen_do_read_msr(u32 msr, int *err)
>>>>   {
>>>>       u64 val = 0;    /* Avoid uninitialized value for safe variant. */
>>>> +    if (err)
>>>> +        *err = 0;
>>>> +
>>>>       if (pmu_msr_chk_emulated(msr, &val, true))
>>>>           return val;
>>>> @@ -1162,7 +1165,7 @@ static void xen_do_write_msr(u32 msr, u64 val, 
>>>> int *err)
>>>>   static int xen_read_msr_safe(u32 msr, u64 *val)
>>>>   {
>>>> -    int err;
>>>> +    int err = 0;
>>>>       *val = xen_do_read_msr(msr, &err);
>>>>       return err;
>>>
>>> So why not initialize 'err' with 0 in both callers, xen_read_msr_safe()
>>> and xen_read_msr(), and avoid all the initialization trouble in
>>> xen_do_read_msr()?
>>
>> Yeah, I should make the change in xen_read_msr() too.
>>
>> However xen_do_read_msr() should be implemented in a defensive way to
>> set *err properly as it's part of its return value.  Actually it was so,
>> but one of my previous cleanup patch removed it because err is no longer
>> passed to pmu_msr_chk_emulated().
> 
> xen_do_read_msr() is usable only in enlighten_pv.c as it is static.
> 
> So I'd prefer to drop setting err to 0 in xen_do_read_msr() initially
> and to set err to 0 in all callers.

Okay, I will send v1A to address this comment then.

Thanks!
     Xin

> Juergen