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Message-ID: <CAPnx3XPRnpPQyW7UO_TTmQrHwitDw+_i3ESVkaGq+JyiY9Pu0w@mail.gmail.com>
Date: Tue, 26 Jan 2021 22:13:25 +0800
From: Candle Sun <candlesea@...il.com>
To: David Laight <David.Laight@...lab.com>
Cc: "keescook@...omium.org" <keescook@...omium.org>,
"arnd@...db.de" <arnd@...db.de>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
"natechancellor@...il.com" <natechancellor@...il.com>,
"ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
"candle.sun@...soc.com" <candle.sun@...soc.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"clang-built-linux@...glegroups.com"
<clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] lkdtm: fix memory copy size for WRITE_KERN
On Mon, Jan 25, 2021 at 6:37 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Candle Sun
> > Sent: 25 January 2021 08:56
> >
> > From: Candle Sun <candle.sun@...soc.com>
> >
> > Though do_overwritten() follows do_nothing() in source code, the final
> > memory address order is determined by compiler. We can't always assume
> > address of do_overwritten() is bigger than do_nothing(). At least the
> > Clang we are using places do_overwritten() before do_nothing() in the
> > object. This causes the copy size in lkdtm_WRITE_KERN() is *really*
> > big and WRITE_KERN test on ARM32 arch will fail.
> >
> > Compare the address order before doing the subtraction.
>
> It isn't clear that helps.
> Compile with -ffunction-sections and/or do LTO an there
> is no reason at all why the functions should be together.
>
> Even without that lkdtm_WRITE_KERN() could easily be between them.
>
> You need to get the size of the 'empty function' from the
> symbol table.
>
> David
Thanks David.
I think using abs() by Nick's advice would be better. But could you
point out which kernel function can get function size?
Regards,
Candle
>
> >
> > Signed-off-by: Candle Sun <candle.sun@...soc.com>
> > ---
> > drivers/misc/lkdtm/perms.c | 19 +++++++++----------
> > 1 file changed, 9 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> > index 2dede2ef658f..fbfbdf89d668 100644
> > --- a/drivers/misc/lkdtm/perms.c
> > +++ b/drivers/misc/lkdtm/perms.c
> > @@ -31,13 +31,13 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
> > * This just returns to the caller. It is designed to be copied into
> > * non-executable memory regions.
> > */
> > -static void do_nothing(void)
> > +static noinline void do_nothing(void)
> > {
> > return;
> > }
> >
> > /* Must immediately follow do_nothing for size calculuations to work out. */
> > -static void do_overwritten(void)
> > +static noinline void do_overwritten(void)
> > {
> > pr_info("do_overwritten wasn't overwritten!\n");
> > return;
> > @@ -110,15 +110,14 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)
> >
> > void lkdtm_WRITE_KERN(void)
> > {
> > - size_t size;
> > - volatile unsigned char *ptr;
> > + unsigned long value_dow = (unsigned long)do_overwritten;
> > + unsigned long value_do = (unsigned long)do_nothing;
> > + size_t size = (size_t)(value_dow > value_do ?
> > + value_dow - value_do : value_do - value_dow);
> >
> > - size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
> > - ptr = (unsigned char *)do_overwritten;
> > -
> > - pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> > - memcpy((void *)ptr, (unsigned char *)do_nothing, size);
> > - flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
> > + pr_info("attempting bad %zu byte write at %px\n", size, do_overwritten);
> > + memcpy((void *)value_dow, (void *)value_do, size);
> > + flush_icache_range(value_dow, value_dow + (unsigned long)size);
> > pr_err("FAIL: survived bad write\n");
> >
> > do_overwritten();
> > --
> > 2.17.0
>
> -
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