Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 26 Jan 2021 10:39:47 -0800 | Subject | Re: [PATCH] lkdtm: fix memory copy size for WRITE_KERN |
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On Tue, Jan 26, 2021 at 6:13 AM Candle Sun <candlesea@gmail.com> wrote: > > 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?
The Elf symbol table should contain this info, IIUC.
Given a string literal of a symbol (such as a function identifier), kallsyms_lookup_name() can be used to return its address.
From there we'd want to fetch the Elf_Sym for the address which should contain a st_size field which I think corresponds to the size in bytes of the function. (At least, from playing with `llvm-readelf -s`) Probably would want to validate it's an STT_FUNC symbol type, too. We basically want something like kexec_purgatory_find_symbol(), but that knows about the entire kernel image, and not the purgatory image used during kexec. I don't see any such function currently in the kernel...but it's a large codebase to search through.
> > 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) > >
-- Thanks, ~Nick Desaulniers
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