Messages in this thread | | | From | Candle Sun <> | Date | Tue, 26 Jan 2021 22:13:25 +0800 | Subject | Re: [PATCH] lkdtm: fix memory copy size for WRITE_KERN |
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On Mon, Jan 25, 2021 at 6:37 PM David Laight <David.Laight@aculab.com> wrote: > > From: Candle Sun > > Sent: 25 January 2021 08:56 > > > > From: Candle Sun <candle.sun@unisoc.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@unisoc.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 > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
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