Date: Fri, 5 May 2017 11:38:39 +0100 From: Mark Rutland <mark.rutland@....com> To: Daniel Micay <danielmicay@...il.com> Cc: Kees Cook <keescook@...omium.org>, kernel-hardening@...ts.openwall.com, ard.biesheuvel@...aro.org, matt@...eblueprint.co.uk Subject: Re: [PATCH] add the option of fortified string.h functions On Thu, May 04, 2017 at 07:09:17PM +0100, Mark Rutland wrote: > On Thu, May 04, 2017 at 01:49:44PM -0400, Daniel Micay wrote: > > On Thu, 2017-05-04 at 16:48 +0100, Mark Rutland wrote: > > > ... with an EFI stub fortify_panic() hacked in, I can build an arm64 > > > kernel with this applied. It dies at some point after exiting EFI > > > boot services; i don't know whether it made it out of the stub and > > > into the kernel proper. > > > > Could start with #define __NO_FORTIFY above the #include sections there > > instead (or -D__NO_FORTIFY as a compiler flag), which will skip > > fortifying those for now. > > Neat. Given there are a few files, doing the latter for the stub is the > simplest option. > > > I'm successfully using this on a non-EFI ARM64 3.18 LTS kernel, so it > > should be close to working on other systems (but not necessarily with > > messy drivers). The x86 EFI workaround works. > > FWIW, I've been playing atop of next-20170504, with a tonne of other > debug options enabled (including KASAN_INLINE). > > From a quick look with a JTAG debugger, the CPU got out of the stub and > into the kernel. It looks like it's dying initialising KASAN, where the > vectors appear to have been corrupted. >From a walk up the call chain, I saw mm/kasan/kasan.c's memcpy was being called recursively. Somehow the fortified memcpy() instrumentation results in kasan's memcpy() calling itself rather than __memcpy(). The resulting stack overflow ends up clobbering the vectors (adn everythigg else) as this is happening early at boot when everything is mapepd RW. That can be avoided with: ---->8---- diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile index f742596..b5327f5 100644 --- a/drivers/firmware/efi/libstub/Makefile +++ b/drivers/firmware/efi/libstub/Makefile @@ -18,7 +18,8 @@ cflags-$(CONFIG_EFI_ARMSTUB) += -I$(srctree)/scripts/dtc/libfdt KBUILD_CFLAGS := $(cflags-y) -DDISABLE_BRANCH_PROFILING \ $(call cc-option,-ffreestanding) \ - $(call cc-option,-fno-stack-protector) + $(call cc-option,-fno-stack-protector) \ + -D__NO_FORTIFY GCOV_PROFILE := n KASAN_SANITIZE := n ---->8---- ... though it's a worring that __memcpy() is overridden. I think we need to be more careful with the way we instrument the string functions. FWIW, with that, and the previous bits, I can boot a next-20170504 kernel with this applied. However, I get a KASAN splat from the SLUB init code, even though that's deliberately not instrumented by KASAN: [ 0.000000] ================================================================== [ 0.000000] BUG: KASAN: slab-out-of-bounds in kmem_cache_alloc+0x2ec/0x438 [ 0.000000] Write of size 352 at addr ffff800936802000 by task swapper/0 [ 0.000000] [ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.11.0-next-20170504-00002-g760cfdb-dirty #15 [ 0.000000] Hardware name: ARM Juno development board (r1) (DT) [ 0.000000] Call trace: [ 0.000000] [<ffff2000080949c8>] dump_backtrace+0x0/0x538 [ 0.000000] [<ffff2000080951a0>] show_stack+0x20/0x30 [ 0.000000] [<ffff200008c125a0>] dump_stack+0x120/0x188 [ 0.000000] [<ffff20000857ac04>] print_address_description+0x10c/0x380 [ 0.000000] [<ffff20000857b354>] kasan_report+0x12c/0x3b8 [ 0.000000] [<ffff200008579d54>] check_memory_region+0x144/0x1a0 [ 0.000000] [<ffff20000857a1f4>] memset+0x2c/0x50 [ 0.000000] [<ffff2000085730bc>] kmem_cache_alloc+0x2ec/0x438 [ 0.000000] [<ffff20000a937528>] bootstrap+0x34/0x28c [ 0.000000] [<ffff20000a937804>] kmem_cache_init+0x84/0x118 [ 0.000000] [<ffff20000a9014bc>] start_kernel+0x2f8/0x644 [ 0.000000] [<ffff20000a9001e8>] __primary_switched+0x6c/0x74 [ 0.000000] [ 0.000000] Allocated by task 0: [ 0.000000] (stack is not available) [ 0.000000] [ 0.000000] Freed by task 0: [ 0.000000] (stack is not available) [ 0.000000] [ 0.000000] The buggy address belongs to the object at ffff800936802000 [ 0.000000] which belongs to the cache kmem_cache of size 352 [ 0.000000] The buggy address is located 0 bytes inside of [ 0.000000] 352-byte region [ffff800936802000, ffff800936802160) [ 0.000000] The buggy address belongs to the page: [ 0.000000] page:ffff7e0024da0080 count:1 mapcount:0 mapping: (null) index:0x0 compound_mapcount: 0 [ 0.000000] flags: 0x1fffc00000008100(slab|head) [ 0.000000] raw: 1fffc00000008100 0000000000000000 0000000000000000 0000000180100010 [ 0.000000] raw: dead000000000100 dead000000000200 ffff20000aa2c068 0000000000000000 [ 0.000000] page dumped because: kasan: bad access detected [ 0.000000] [ 0.000000] Memory state around the buggy address: [ 0.000000] ffff800936801f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [ 0.000000] ffff800936801f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [ 0.000000] >ffff800936802000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc [ 0.000000] ^ [ 0.000000] ffff800936802080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc [ 0.000000] ffff800936802100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc [ 0.000000] ================================================================== Thanks, Mark.
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