Messages in this thread | | | From | Dmitry Vyukov <> | Date | Wed, 11 Mar 2020 18:34:53 +0100 | Subject | Re: [PATCH] UML: add support for KASAN under x86_64 |
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On Wed, Mar 11, 2020 at 12:19 PM Johannes Berg <johannes@sipsolutions.net> wrote: > > On Wed, 2020-03-11 at 11:32 +0100, Johannes Berg wrote: > > > > I do see issues with modules though, e.g. > > https://p.sipsolutions.net/1a2df5f65d885937.txt > > > > where we seem to get some real confusion when lockdep is storing the > > stack trace?? > > > > And https://p.sipsolutions.net/9a97e8f68d8d24b7.txt, where something > > convinces ASAN that an address is a user address (it might even be > > right?) and it disallows kernel access to it? > > I can work around both of these by not freeing the original module copy > in kernel/module.c: > > /* Get rid of temporary copy. */ > // free_copy(info); > > but I really have no idea why we get this in the first place? > > Another interesting data point is that it never happens on the first > module. > > Also, I've managed to get a report like this: > > Memory state around the buggy address: > 000000007106cf00: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b > 000000007106cf80: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b > >000000007106d000: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b > ^ > 000000007106d080: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b > 000000007106d100: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b > > > which indicates that something's _really_ off with the KASAN shadow? > > > Ohhh ... > > $ gdb -p ... > (gdb) p/x task_size > $1 = 0x7fc0000000 > (gdb) p/x __end_of_fixed_addresses > $2 = 0x0 > (gdb) p/x end_iomem > $3 = 0x70000000 > (gdb) p/x __va_space > > #define TASK_SIZE (task_size) > #define FIXADDR_TOP (TASK_SIZE - 2 * PAGE_SIZE) > > #define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE) > #define FIXADDR_SIZE (__end_of_fixed_addresses << PAGE_SHIFT) > > #define VMALLOC_END (FIXADDR_START-2*PAGE_SIZE) > > #define MODULES_VADDR VMALLOC_START > #define MODULES_END VMALLOC_END > #define VMALLOC_START ((end_iomem + VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1)) > #define VMALLOC_OFFSET (__va_space) > #define __va_space (8*1024*1024) > > > So from that, it would look like the UML vmalloc area is from > 0x 70800000 all the way to > 0x7fbfffc000, which obviously clashes with the KASAN_SHADOW_OFFSET being > just 0x7fff8000. > > > I'm guessing that basically the module loading overwrote the kasan > shadow then?
Well, ok, this is definitely not going to fly :)
I don't know if it's easy to move modules to a different location. It would be nice because 0x7fbfffc000 is the shadow start that's used in userspace asan and it allows to faster instrumentation (if offset is within first 2 gigs, the instruction encoding is much more compact, for >2gigs it will require several instructions). But if it's not really easy, I guess we go with a large shadow start (at least initially). A slower but working KASAN is better than fast non-working KASAN :)
> I tried changing it > > config KASAN_SHADOW_OFFSET > hex > depends on KASAN > - default 0x7fff8000 > + default 0x8000000000 > > > and also put a check in like this: > > +++ b/arch/um/kernel/um_arch.c > @@ -13,6 +13,7 @@ > #include <linux/sched.h> > #include <linux/sched/task.h> > #include <linux/kmsg_dump.h> > +#include <linux/kasan.h> > > #include <asm/pgtable.h> > #include <asm/processor.h> > @@ -267,9 +268,11 @@ int __init linux_main(int argc, char **argv) > /* > * TASK_SIZE needs to be PGDIR_SIZE aligned or else exit_mmap craps > * out > */ > task_size = host_task_size & PGDIR_MASK; > > + if (task_size > KASAN_SHADOW_OFFSET) > + panic("KASAN shadow offset must be bigger than task size"); > > > but now I just crash accessing the shadow even though it was mapped fine?
Yes, this is puzzling. I noticed that RIP is the same in both cases and it relates to vmap code. A support for shadow for vmalloced-memory was added to KASAN recently and I suspect it may conflict with UML. See: https://elixir.bootlin.com/linux/v5.6-rc5/ident/kasan_populate_vmalloc
I think we simply don't need any of that because we already mapped shadow for all potentially used memory. A simple thing to try is to disable CONFIG_KASAN_VMALLOC. If it fixes the problem, we need to either force-disable CONFIG_KASAN_VMALLOC under UML or return early from these functions.
What does pte-manipulation code even do under UML?
Looking at the code around, kasan_mem_notifier may be a problem too, or at least excessive and confusing. We already have shadow for everything, we don't need _any_ of dynamic/lazy shadow mapping.
> Pid: 504, comm: modprobe Tainted: G O 5.5.0-rc6-00009-g09462ab4014b-dirty > RIP: > RSP: 000000006d68fa90 EFLAGS: 00010202 > RAX: 000000800e0210cd RBX: 000000007010866f RCX: 00000000601a9777 > RDX: 000000800e0210ce RSI: 0000000000000004 RDI: 000000007010866c > RBP: 000000006d68faa0 R08: 000000800e0210cd R09: 0000000060041432 > R10: 000000800e0210ce R11: 0000000000000001 R12: 000000800e0210cd > R13: 0000000000000000 R14: 0000000000000001 R15: 00000000601c2e82 > Kernel panic - not syncing: Kernel mode fault at addr 0x800e0210cd, ip 0x601c332b > CPU: 0 PID: 504 Comm: modprobe Tainted: G O 5.5.0-rc6-00009-g09462ab4014b-dirty #24 > Stack: > 601c2f89 70108638 6d68fab0 601c1209 > 6d68fad0 601a9777 6cf2b240 7317f000 > 6d68fb40 601a2ae9 6f15b118 00000001 > Call Trace: > ? __asan_load8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > __kasan_check_write (/home/tester/vlab/linux/mm/kasan/common.c:102) > __free_pages (/home/tester/vlab/linux/./arch/x86/include/asm/atomic.h:125 /home/tester/vlab/linux/./include/asm-generic/atomic-instrumented.h:748 /home/tester/vlab/linux/./include/linux/page_ref.h:139 /home/tester/vlab/linux/./include/linux/mm.h:593 /home/tester/vlab/linux/mm/page_alloc.c:4823) > __vunmap (/home/tester/vlab/linux/mm/vmalloc.c:2303 (discriminator 2)) > ? __asan_load4 (/home/tester/vlab/linux/mm/kasan/generic.c:251) > ? sysfs_create_bin_file (/home/tester/vlab/linux/fs/sysfs/file.c:537) > __vfree (/home/tester/vlab/linux/mm/vmalloc.c:2356) > ? delete_object_full (/home/tester/vlab/linux/mm/kmemleak.c:693) > vfree (/home/tester/vlab/linux/mm/vmalloc.c:2386) > ? sysfs_create_bin_file (/home/tester/vlab/linux/fs/sysfs/file.c:537) > ? __asan_load8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > load_module (/home/tester/vlab/linux/./include/linux/jump_label.h:254 /home/tester/vlab/linux/./include/linux/jump_label.h:264 /home/tester/vlab/linux/./include/trace/events/module.h:31 /home/tester/vlab/linux/kernel/module.c:3927) > ? kernel_read_file_from_fd (/home/tester/vlab/linux/fs/exec.c:993) > ? __asan_load8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > __do_sys_finit_module (/home/tester/vlab/linux/kernel/module.c:4019) > ? sys_finit_module (/home/tester/vlab/linux/kernel/module.c:3995) > ? __asan_store8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > sys_finit_module (/home/tester/vlab/linux/kernel/module.c:3995) > handle_syscall (/home/tester/vlab/linux/arch/um/kernel/skas/syscall.c:44) > userspace (/home/tester/vlab/linux/arch/um/os-Linux/skas/process.c:173 /home/tester/vlab/linux/arch/um/os-Linux/skas/process.c:416) > ? save_registers (/home/tester/vlab/linux/arch/um/os-Linux/registers.c:18) > ? arch_prctl (/home/tester/vlab/linux/arch/x86/um/syscalls_64.c:65) > ? calculate_sigpending (/home/tester/vlab/linux/kernel/signal.c:200) > ? __asan_load8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > ? __asan_load8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > ? __asan_load8 (/home/tester/vlab/linux/mm/kasan/generic.c:252) > fork_handler (/home/tester/vlab/linux/arch/um/kernel/process.c:154) > > johannes >
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