Messages in this thread Patch in this message | | | From | Dmitry Vyukov <> | Date | Wed, 29 Nov 2017 19:57:15 +0100 | Subject | Re: kasan: false use-after-scope warnings with KCOV |
| |
On Wed, Nov 29, 2017 at 5:54 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote: > On 11/28/2017 08:52 PM, Dmitry Vyukov wrote: >> On Tue, Nov 28, 2017 at 4:24 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. >> >> >> >> We use gcc 7.1 extensively on x86_64 and have not seen any problems. >> > > Yeah, it's probably two different problems. > > Today kbuild reported another use-after-scope - http://lkml.kernel.org/r/<20171129052106.rhgbjhhis53hkgfn@wfg-t540p.sh.intel.com> > No struct leak plugin and kcov instrumentation is also off. It's hard to tell whether it's false-positive or not, the code is a mess. > So until proven otherwise, I tend to think that this time it's a real bug. > .config attached, if someone want to look. It's easy reproducible, just boot qemu and wait.
I hacked a quick prototype for printing frame info:
--- a/mm/kasan/report.c +++ b/mm/kasan/report.c @@ -289,6 +289,7 @@ static void print_shadow_for_address(const void *addr) int i; const void *shadow = kasan_mem_to_shadow(addr); const void *shadow_row; + unsigned long *ptr;
shadow_row = (void *)round_down((unsigned long)shadow, SHADOW_BYTES_PER_ROW) @@ -320,6 +321,18 @@ static void print_shadow_for_address(const void *addr)
shadow_row += SHADOW_BYTES_PER_ROW; } + + + ptr = (unsigned long *)((unsigned long)addr & ~7); + for (i = 0; i < 1000; i++, ptr--) { + if (*ptr == 0x41b58ab3) { + pr_err("\n"); + pr_err("frame offset: %lu\n", (unsigned long)addr - (unsigned long)ptr); + pr_err("desc: '%s'\n", (const char*)*(ptr+1)); + pr_err("func: %pS\n", (void*)*(ptr+2)); + break; + } + } }
And this gave me:
[ 26.763495] ================================================================== [ 26.764454] BUG: KASAN: use-after-scope in __drm_mm_interval_first+0xc0/0x1e2 [ 26.765297] Read of size 8 at addr ffff88006cb3fbe0 by task swapper/0/1 [ 26.766081] [ 26.766278] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.14.0-04319-gd17a1d97dc20-dirty #12 [ 26.767760] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 [ 26.769419] Call Trace: [ 26.769895] dump_stack+0xdb/0x17a [ 26.770152] ? _atomic_dec_and_lock+0x12f/0x12f [ 26.770152] ? show_regs_print_info+0x5b/0x5b [ 26.770152] ? kasan_report+0x4d/0x247 [ 26.770152] ? __drm_mm_interval_first+0xc0/0x1e2 [ 26.770152] print_address_description+0x9a/0x232 [ 26.770152] ? __drm_mm_interval_first+0xc0/0x1e2 [ 26.770152] kasan_report+0x21e/0x247 [ 26.770152] __asan_report_load8_noabort+0x14/0x16 [ 26.770152] __drm_mm_interval_first+0xc0/0x1e2 [ 26.770152] assert_continuous+0x13e/0x22f [ 26.770152] __igt_insert+0x665/0xc87 [ 26.770152] ? igt_bottomup+0xaa0/0xaa0 [ 26.770152] ? sched_clock_local+0x3c/0xfb [ 26.770152] ? find_held_lock+0x33/0x103 [ 26.770152] ? next_prime_number+0x318/0x362 [ 26.770152] ? rcu_irq_enter_disabled+0xd/0xd [ 26.770152] ? next_prime_number+0x337/0x362 [ 26.770152] igt_replace+0x4b/0xb3 [ 26.770152] test_drm_mm_init+0x118/0x172 [ 26.770152] ? drm_kms_helper_init+0xb/0xb [ 26.770152] do_one_initcall+0x10f/0x21f [ 26.770152] ? initcall_blacklisted+0x185/0x185 [ 26.770152] ? down_write_nested+0xa1/0x164 [ 26.770152] ? kasan_poison_shadow+0x2f/0x31 [ 26.770152] ? kasan_unpoison_shadow+0x14/0x35 [ 26.770152] kernel_init_freeable+0x2ae/0x339 [ 26.770152] ? rest_init+0x250/0x250 [ 26.770152] kernel_init+0xc/0x105 [ 26.770152] ? rest_init+0x250/0x250 [ 26.770152] ret_from_fork+0x24/0x30 [ 26.770152] [ 26.770152] The buggy address belongs to the page: [ 26.770152] page:ffff88007f39c5c8 count:0 mapcount:0 mapping: (null) index:0x0 [ 26.770152] flags: 0x1a01fff800000() [ 26.770152] raw: 0001a01fff800000 0000000000000000 0000000000000000 00000000ffffffff [ 26.770152] raw: ffff88007f39c5e8 ffff88007f39c5e8 0000000000000000 [ 26.770152] page dumped because: kasan: bad access detected [ 26.790299] [ 26.790299] Memory state around the buggy address: [ 26.790299] ffff88006cb3fa80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 [ 26.790299] ffff88006cb3fb00: f1 f1 f1 00 f2 f2 f2 f2 f2 f2 f2 00 00 f2 f2 f2 [ 26.790299] >ffff88006cb3fb80: f2 f2 f2 f8 f8 f2 f2 f2 f2 f2 f2 f8 f8 f8 f8 f8 [ 26.790299] ^ [ 26.790299] ffff88006cb3fc00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f2 [ 26.790299] ffff88006cb3fc80: f2 f2 f2 00 00 00 00 00 00 00 00 00 00 00 00 00 [ 26.790299] [ 26.790299] frame offset: 232 [ 26.790299] desc: '5 32 8 3 __u 96 16 4 prng 160 16 7 state__ 224 160 3 tmp 416 224 2 mm ' [ 26.790299] func: __igt_insert+0x0/0xc87 [ 26.790299] ==================================================================
That desc string is: number of local objects, then for each object: offset, size, name length, name.
So that's variable tmp in __igt_insert. Not too surprising looking at the code:
for (mode = insert_modes; mode->name; mode++) { for (n = 0; n < count; n++) { struct drm_mm_node tmp; node = replace ? &tmp : &nodes[n]; memset(node, 0, sizeof(*node)); if (!expect_insert(&mm, node, size, 0, n, mode)) { pr_err("%s insert failed, size %llu step %d\n", mode->name, size, n); goto out; } if (replace) { drm_mm_replace_node(&tmp, &nodes[n]); if (drm_mm_node_allocated(&tmp)) { pr_err("replaced old-node still allocated! step %d\n", n); goto out; } if (!assert_node(&nodes[n], &mm, size, 0, n)) { pr_err("replaced node did not inherit parameters, size %llu step %d\n", size, n); goto out; } if (tmp.start != nodes[n].start) { pr_err("replaced node mismatch location expected [%llx + %llx], found [%llx + %llx]\n", tmp.start, size, nodes[n].start, nodes[n].size); goto out; } } }
I guess we need to finally do this for real. Also print global names.
| |