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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <iwj@xenproject.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>,
	Julien Grall <julien@xen.org>,
	Dario Faggioli <dfaggioli@suse.com>,
	Xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH 1/6] xen/trace: Don't over-read trace objects
Date: Fri, 17 Sep 2021 14:26:35 +0100	[thread overview]
Message-ID: <0db4dc68-ccf0-0de6-61ce-7f155431a977@citrix.com> (raw)
In-Reply-To: <ed07d2de-9354-6c6d-b9f6-4b325ca328ce@suse.com>

On 17/09/2021 13:58, Jan Beulich wrote:
> On 17.09.2021 10:45, Andrew Cooper wrote:
>> --- a/xen/common/trace.c
>> +++ b/xen/common/trace.c
>> @@ -686,22 +686,21 @@ void __trace_var(u32 event, bool_t cycles, unsigned int extra,
>>      unsigned long flags;
>>      u32 bytes_to_tail, bytes_to_wrap;
>>      unsigned int rec_size, total_size;
>> -    unsigned int extra_word;
>>      bool_t started_below_highwater;
>>  
>>      if( !tb_init_done )
>>          return;
>>  
>> -    /* Convert byte count into word count, rounding up */
>> -    extra_word = (extra / sizeof(u32));
>> -    if ( (extra % sizeof(u32)) != 0 )
>> -        extra_word++;
>> -    
>> -    ASSERT(extra_word <= TRACE_EXTRA_MAX);
>> -    extra_word = min_t(int, extra_word, TRACE_EXTRA_MAX);
>> -
>> -    /* Round size up to nearest word */
>> -    extra = extra_word * sizeof(u32);
>> +    /*
>> +     * Trace records require extra data which is an exact multiple of
>> +     * uint32_t.  Reject out-of-spec records.  Any failure here is an error in
>> +     * the caller.
>> +     */
> Hmm, is "require" accurate?

In terms of "what will go wrong if this condition is violated", yes.

>  They may very well come without extra data
> afaics.

0 is fine, and used by plenty of records, and also permitted by the
filtering logic.

>
>> +    if ( extra % sizeof(uint32_t) ||
>> +         extra / sizeof(uint32_t) > TRACE_EXTRA_MAX )
>> +        return printk_once(XENLOG_WARNING
>> +                           "Trace event %#x bad size %u, discarding\n",
>> +                           event, extra);
> Any HVM guest looks to be able to trivially trigger this log message
> (via HVMOP_xentrace), thus pointing out an issue in a guest and hiding
> any other Xen related output. I'd like to suggest to adjust that call
> site in prereq patch (I'm not overly fussed which of the two relatively
> obvious ways).
>
> Further sched/rt.c:burn_budget() has a bool field last in a packed
> struct, yielding a sizeof() that's not a multiple of 4. All the uses of
> __packed there look at best suspicious anyway.

Ugh - I checked the __trace_var() users, but not trace_var().  Luckily,
there are far fewer of the latter.

HVMOP_xentrace has no business being a hypercall in the first place. 
That can be fixed by also enforcing the multiple-of-4 requirement.

But yes - burn_budget() needs fixing in this patch too, taking it from a
theoretical to real problem.

~Andrew



  reply	other threads:[~2021-09-17 13:27 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-17  8:45 [PATCH 0/6] xen/trace: Fix leakage of uninitialised stack into the tracebuffer Andrew Cooper
2021-09-17  8:45 ` [PATCH 1/6] xen/trace: Don't over-read trace objects Andrew Cooper
2021-09-17 12:58   ` Jan Beulich
2021-09-17 13:26     ` Andrew Cooper [this message]
2021-09-20  8:00       ` Jan Beulich
2021-09-20 10:24         ` Andrew Cooper
2021-09-17  8:45 ` [PATCH 2/6] xen/memory: Remove tail padding from TRC_MEM_* records Andrew Cooper
2021-09-17 13:04   ` Jan Beulich
2021-09-17  8:45 ` [PATCH 3/6] xen/credit2: Remove tail padding from TRC_CSCHED2_* records Andrew Cooper
2021-09-17 13:10   ` Jan Beulich
2021-09-17 13:28     ` Andrew Cooper
2021-09-17  8:45 ` [PATCH 4/6] x86/trace: Reduce stack usage from HVMTRACE_ND() Andrew Cooper
2021-09-20  9:05   ` Jan Beulich
2021-09-20 11:02     ` Andrew Cooper
2021-09-20 13:00       ` Jan Beulich
2021-09-17  8:45 ` [PATCH 5/6] xen/credit2: Clean up trace handling Andrew Cooper
2021-09-20  9:11   ` Jan Beulich
2021-09-17  8:45 ` [PATCH 6/6] xen/trace: Minor code cleanup Andrew Cooper
2021-09-20  9:15   ` Jan Beulich

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