linux-kernel - Re: [PATCH 1/2] x86: mm: ptdump: Calculate effective permissions correctly

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Date:   Tue, 26 May 2020 11:41:41 +0100
From:   Steven Price <steven.price@....com>
To:     Qian Cai <cai@....pw>, Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        Jan Beulich <jbeulich@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/2] x86: mm: ptdump: Calculate effective permissions
 correctly

On 22/05/2020 19:07, Qian Cai wrote:
> On Thu, May 21, 2020 at 04:23:07PM +0100, Steven Price wrote:
>> --- a/arch/x86/mm/dump_pagetables.c
>> +++ b/arch/x86/mm/dump_pagetables.c
>> @@ -249,10 +249,22 @@ static void note_wx(struct pg_state *st, unsigned long addr)
>> @@ -270,16 +282,10 @@ static void note_page(struct ptdump_state *pt_st, unsigned long addr, int level,
>>   	struct seq_file *m = st->seq;
>>   
>>   	new_prot = val & PTE_FLAGS_MASK;
>> +	new_eff = st->prot_levels[level];
> 
> This will trigger,
> 
> .config (if ever matters):
> https://raw.githubusercontent.com/cailca/linux-mm/master/x86.config
> 
> [  104.532621] UBSAN: array-index-out-of-bounds in arch/x86/mm/dump_pagetables.c:284:27
> [  104.542620] index -1 is out of range for type 'pgprotval_t [5]'

Doh! In this case the result (new_eff) is never actually used: the -1 is 
used just to flush the last proper entry out, so in this case val == 0 
which means the following statement sets new_eff = 0. But obviously an 
out-of-bounds access isn't great. Thanks for testing!

The following patch should fix it up by making the assignment 
conditional on val != 0.

Andrew: Would you like me to resend, or can you squash in the below?

Steve

-----8<----
diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index 17aa7ac94a34..ea9010113f69 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -282,10 +282,10 @@ static void note_page(struct ptdump_state *pt_st, 
unsigned long addr, int level,
  	struct seq_file *m = st->seq;

  	new_prot = val & PTE_FLAGS_MASK;
-	new_eff = st->prot_levels[level];
-
  	if (!val)
  		new_eff = 0;
+	else
+		new_eff = st->prot_levels[level];

  	/*
  	 * If we have a "break" in the series, we need to flush the state that