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

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Message-ID: <30affad5-4f26-4e22-9d64-b8ece1199773@zytor.com>
Date: Thu, 15 May 2025 11:11:09 -0700
From: Xin Li <xin@...or.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        linux-acpi@...r.kernel.org, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        peterz@...radead.org, jgross@...e.com, boris.ostrovsky@...cle.com,
        rafael@...nel.org, lenb@...nel.org
Subject: Re: [PATCH v1 2/3] x86/xen/msr: Fix uninitialized symbol 'err'

On 5/15/2025 8:29 AM, Ingo Molnar wrote:
> 
> * Xin Li (Intel) <xin@...or.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@...aro.org>
>> Closes: https://lore.kernel.org/xen-devel/aBxNI_Q0-MhtBSZG@stanley.mountain/
>> Signed-off-by: Xin Li (Intel) <xin@...or.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().

Thanks!
     Xin