Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] lkdtm: fix memory copy size for WRITE_KERN | Date | Mon, 25 Jan 2021 10:37:36 +0000 |
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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
> > 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
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