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SubjectRe: kasan: false use-after-scope warnings with KCOV
On Tue, Nov 28, 2017 at 02:13:55PM +0000, Mark Rutland wrote:
> On Tue, Nov 28, 2017 at 01:57:49PM +0100, Dmitry Vyukov wrote:
> > On Tue, Nov 28, 2017 at 1:35 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> > > As a heads-up, I'm seeing a number of what appear to be false-positive
> > > use-after-scope warnings when I enable both KCOV and KASAN (inline or outline),
> > > when using the Linaro 17.08 GCC7.1.1 for arm64. So far I haven't spotted these
> > > without KCOV selected, and I'm only seeing these for sanitize-use-after-scope.
> > >
> > > The reports vary depending on configuration even with the same trigger. I'm not
> > > sure if it's the reporting that's misleading, or whether the detection is going
> > > wrong.

> ... it looks suspiciously like something is setting up non-zero shadow
> bytes, but not zeroing them upon return.

It looks like this is the case.

The hack below detects leftover poison on an exception return *before*
the false-positive warning (example splat at the end of the email). With
scripts/Makefile.kasan hacked to not pass
-fsanitize-address-use-after-scope, I see no leftover poison.

Unfortunately, there's not enough information left to say where exactly
that happened.

Given the report that Andrey linked to [1], it looks like the compiler
is doing something wrong, and failing to clear some poison in some
cases. Dennis noted [2] that this appears to be the case where inline
functions are called in a loop.

It sounds like this is a general GCC 7.x problem, on both x86_64 and
arm64. As we don't have a smoking gun, it's still possible that
something else is corrupting the shadow, but it seems unlikely.

[1] https://lkml.kernel.org/r/20171128124534.3jvuala525wvn64r@wfg-t540p.sh.intel.com
[2] https://lkml.kernel.org/r/20171127210301.GA55812@localhost.corp.microsoft.com

Thanks,
Mark.

Hack
--------
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 6d14b8f29b5f..8191e122d6f4 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -220,6 +220,8 @@ alternative_else_nop_endif
.endm

.macro kernel_exit, el
+ mov x0, sp
+ bl kasan_assert_task_stack_is_clean_below
.if \el != 0
disable_daif

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 405bba487df5..dab8a51ee52f 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -37,6 +37,8 @@
#include <linux/vmalloc.h>
#include <linux/bug.h>

+#include <asm/stacktrace.h>
+
#include "kasan.h"
#include "../slab.h"

@@ -241,6 +243,33 @@ static __always_inline bool memory_is_poisoned(unsigned long addr, size_t size)
return memory_is_poisoned_n(addr, size);
}

+/*
+ * In some contexts (e.g. when returning from an exception), all shadow beyond
+ * a certain point on the stack should be clear. This helper can be called by
+ * assembly code to verify this is the case.
+ */
+asmlinkage void kasan_assert_task_stack_is_clean_below(unsigned long watermark)
+{
+ unsigned long base;
+
+ /*
+ * This is an arm64-specific hack. This should be fixed properly to
+ * discover and check the bounds of the current stack in an
+ * arch-agnostic manner.
+ */
+ if (!on_task_stack(current, watermark))
+ return;
+
+ /*
+ * Calculate the task stack base address. Avoid using 'current'
+ * because this function is called by early resume code which hasn't
+ * yet set up the percpu register (%gs).
+ */
+ base = watermark & ~(THREAD_SIZE - 1);
+
+ WARN_ON_ONCE(memory_is_poisoned(base, watermark - base));
+}
+
static __always_inline void check_memory_region_inline(unsigned long addr,
size_t size, bool write,
unsigned long ret_ip)
--------
Splat
--------
[ 186.951300] WARNING: CPU: 1 PID: 2429 at mm/kasan/kasan.c:270 kasan_assert_task_stack_is_clean_below+0x144/0x150
[ 186.961418] Modules linked in:
[ 186.964468] CPU: 1 PID: 2429 Comm: perf Not tainted 4.15.0-rc1-00001-g7780802c256e #6
[ 186.972249] Hardware name: ARM Juno development board (r1) (DT)
[ 186.978133] task: ffff800933fe6900 task.stack: ffff80092c990000
[ 186.984019] pstate: 200003c5 (nzCv DAIF -PAN -UAO)
[ 186.988789] pc : kasan_assert_task_stack_is_clean_below+0x144/0x150
[ 186.995022] lr : ret_fast_syscall+0x34/0x98
[ 186.999177] sp : ffff80092c997ec0
[ 187.002472] x29: ffff80092c997ff0 x28: ffff800933fe6900
[ 187.007760] x27: ffff200009264000 x26: 00000000000000f1
[ 187.013047] x25: 0000000000000124 x24: 0000000000000015
[ 187.018334] x23: 0000000060000000 x22: 0000ffffae4b7554
[ 187.023621] x21: 00000000ffffffff x20: 000060092de30000
[ 187.028908] x19: 0000000000000000 x18: 0000ffffd2ec5330
[ 187.034195] x17: 0000ffffae4b7530 x16: ffff200008270508
[ 187.039482] x15: 0000ffffae538588 x14: 0000000000000000
[ 187.044769] x13: ffffffffffffffff x12: ffffffffffffffff
[ 187.050060] x11: 1ffff00125932f33 x10: ffff100125932f33
[ 187.055349] x9 : dfff200000000000 x8 : dfff200000000008
[ 187.060638] x7 : 1ffff00125932fd7 x6 : ffff100125932fd7
[ 187.065927] x5 : ffff80092c997ebf x4 : ffff100125932fd8
[ 187.071217] x3 : dfff200000000000 x2 : ffff100125932e30
[ 187.076506] x1 : ffff100125932e28 x0 : 00000000000000f8
[ 187.081793] Call trace:
[ 187.084238] kasan_assert_task_stack_is_clean_below+0x144/0x150
[ 187.090122] ---[ end trace 9c3a99d1de859687 ]---
[ 187.212571] ==================================================================
[ 187.219786] BUG: KASAN: use-after-scope in __save_stack_trace+0x1c8/0x2f0
[ 187.226537] Read of size 4 at addr ffff800930e4f048 by task true/2432
[ 187.232935]
[ 187.234430] CPU: 2 PID: 2432 Comm: true Tainted: G W 4.15.0-rc1-00001-g7780802c256e #6
[ 187.243507] Hardware name: ARM Juno development board (r1) (DT)
[ 187.249389] Call trace:
[ 187.251830] dump_backtrace+0x0/0x320
[ 187.255477] show_stack+0x20/0x30
[ 187.258782] dump_stack+0x108/0x174
[ 187.262256] print_address_description+0x60/0x270
[ 187.266936] kasan_report+0x210/0x2f0
[ 187.270584] __asan_load4+0x84/0xa8
[ 187.274059] __save_stack_trace+0x1c8/0x2f0
[ 187.278224] save_stack_trace+0x24/0x30
[ 187.282044] kasan_kmalloc+0xd0/0x180
[ 187.285688] kasan_slab_alloc+0x14/0x20
[ 187.289508] kmem_cache_alloc+0x128/0x1e8
[ 187.293499] perf_event_mmap+0x2dc/0x968
[ 187.297405] mmap_region+0x24c/0xa60
[ 187.300963] do_mmap+0x404/0x640
[ 187.304178] vm_mmap_pgoff+0x15c/0x190
[ 187.307909] vm_mmap+0x70/0xb0
[ 187.310951] elf_map+0x114/0x150
[ 187.314165] load_elf_binary+0x728/0x1b84
[ 187.318158] search_binary_handler+0xe4/0x3b8
[ 187.322495] do_execveat_common.isra.12+0xaa4/0xc60
[ 187.327349] SyS_execve+0x48/0x60
[ 187.330650] el0_svc_naked+0x20/0x24
[ 187.334202]
[ 187.335685] The buggy address belongs to the page:
[ 187.340453] page:ffff7e0024c393c0 count:0 mapcount:0 mapping: (null) index:0x0
[ 187.348414] flags: 0x1fffc00000000000()
[ 187.352240] raw: 1fffc00000000000 0000000000000000 0000000000000000 00000000ffffffff
[ 187.359947] raw: 0000000000000000 ffff7e0024c393e0 0000000000000000 0000000000000000
[ 187.367643] page dumped because: kasan: bad access detected
[ 187.373178]
[ 187.374661] Memory state around the buggy address:
[ 187.379428] ffff800930e4ef00: f1 f1 f8 f2 f2 f2 f2 f2 f2 f2 00 00 f2 f2 f2 f2
[ 187.386612] ffff800930e4ef80: f2 f2 00 00 f2 f2 f3 f3 f3 f3 f8 f8 f8 f8 f8 f8
[ 187.393795] >ffff800930e4f000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 00 00 00 00 00 00
[ 187.400973] ^
[ 187.406516] ffff800930e4f080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 187.413699] ffff800930e4f100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 187.420877] ==================================================================
--------
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