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

[Steven Price]

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