linux-hardening - Re: [PATCH] kasan: test: fix compatibility with FORTIFY

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Date:   Mon, 24 Jan 2022 13:56:30 -0500
From:   Nico Pache <npache@...hat.com>
To:     Marco Elver <elver@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Kees Cook <keescook@...omium.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] kasan: test: fix compatibility with FORTIFY_SOURCE



On 1/24/22 11:07, Marco Elver wrote:
> With CONFIG_FORTIFY_SOURCE enabled, string functions will also perform
> dynamic checks using __builtin_object_size(ptr), which when failed will
> panic the kernel.
> 
> Because the KASAN test deliberately performs out-of-bounds operations,
> the kernel panics with FORITY_SOURCE, for example:
> 
>  | kernel BUG at lib/string_helpers.c:910!
>  | invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
>  | CPU: 1 PID: 137 Comm: kunit_try_catch Tainted: G    B             5.16.0-rc3+ #3
>  | Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
>  | RIP: 0010:fortify_panic+0x19/0x1b
>  | ...
>  | Call Trace:
>  |  <TASK>
>  |  kmalloc_oob_in_memset.cold+0x16/0x16
>  |  ...
> 
> Fix it by also hiding `ptr` from the optimizer, which will ensure that
> __builtin_object_size() does not return a valid size, preventing
> fortified string functions from panicking.
> 
> Reported-by: Nico Pache <npache@...hat.com>
> Signed-off-by: Marco Elver <elver@...gle.com>

Looks good! Thanks for posting this Marco :)

Reviewed-by: Nico Pache <npache@...hat.com>

> ---
>  lib/test_kasan.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> index 847cdbefab46..26a5c9007653 100644
> --- a/lib/test_kasan.c
> +++ b/lib/test_kasan.c
> @@ -492,6 +492,7 @@ static void kmalloc_oob_in_memset(struct kunit *test)
>  	ptr = kmalloc(size, GFP_KERNEL);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>  
> +	OPTIMIZER_HIDE_VAR(ptr);
>  	OPTIMIZER_HIDE_VAR(size);
>  	KUNIT_EXPECT_KASAN_FAIL(test,
>  				memset(ptr, 0, size + KASAN_GRANULE_SIZE));
> @@ -515,6 +516,7 @@ static void kmalloc_memmove_negative_size(struct kunit *test)
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>  
>  	memset((char *)ptr, 0, 64);
> +	OPTIMIZER_HIDE_VAR(ptr);
>  	OPTIMIZER_HIDE_VAR(invalid_size);
>  	KUNIT_EXPECT_KASAN_FAIL(test,
>  		memmove((char *)ptr, (char *)ptr + 4, invalid_size));
> @@ -531,6 +533,7 @@ static void kmalloc_memmove_invalid_size(struct kunit *test)
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>  
>  	memset((char *)ptr, 0, 64);
> +	OPTIMIZER_HIDE_VAR(ptr);
>  	KUNIT_EXPECT_KASAN_FAIL(test,
>  		memmove((char *)ptr, (char *)ptr + 4, invalid_size));
>  	kfree(ptr);
> @@ -893,6 +896,7 @@ static void kasan_memchr(struct kunit *test)
>  	ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>  
> +	OPTIMIZER_HIDE_VAR(ptr);
>  	OPTIMIZER_HIDE_VAR(size);
>  	KUNIT_EXPECT_KASAN_FAIL(test,
>  		kasan_ptr_result = memchr(ptr, '1', size + 1));
> @@ -919,6 +923,7 @@ static void kasan_memcmp(struct kunit *test)
>  	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
>  	memset(arr, 0, sizeof(arr));
>  
> +	OPTIMIZER_HIDE_VAR(ptr);
>  	OPTIMIZER_HIDE_VAR(size);
>  	KUNIT_EXPECT_KASAN_FAIL(test,
>  		kasan_int_result = memcmp(ptr, arr, size+1));
>