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Message-ID: <20250305151251.GA1538566@ax162>
Date: Wed, 5 Mar 2025 16:12:51 +0100
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
llvm@...ts.linux.dev, Geert Uytterhoeven <geert+renesas@...der.be>,
David Gow <davidgow@...gle.com>,
"Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
Jeff Johnson <quic_jjohnson@...cinc.com>,
Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
linux-hardening@...r.kernel.org
Subject: Re: [PATCH] kunit/stackinit: Use fill byte different from Clang i386
pattern
On Tue, Mar 04, 2025 at 02:56:11PM -0800, Kees Cook wrote:
> The byte initialization values used with -ftrivial-auto-var-init=pattern
> (CONFIG_INIT_STACK_ALL_PATTERN=y) depends on the compiler, architecture,
> and byte position relative to struct member types. On i386 with Clang,
> this includes the 0xFF value, which means it looks like nothing changes
> between the leaf byte filling pass and the expected "stack wiping"
> pass of the stackinit test.
>
> Use the byte fill value of 0x99 instead, fixing the test for i386 Clang
> builds.
>
> Reported-by: ernsteiswuerfel
> Closes: https://github.com/ClangBuiltLinux/linux/issues/2071
> Fixes: 8c30d32b1a32 ("lib/test_stackinit: Handle Clang auto-initialization pattern")
> Signed-off-by: Kees Cook <kees@...nel.org>
stackinit passes with arm, arm64, i386, and x86_64 when using
LLVM 20.1.0-rc3. Hopefully they do not change the init pattern :)
Tested-by: Nathan Chancellor <nathan@...nel.org>
> ---
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Bill Wendling <morbo@...gle.com>
> Cc: Justin Stitt <justinstitt@...gle.com>
> Cc: llvm@...ts.linux.dev
> ---
> lib/tests/stackinit_kunit.c | 30 ++++++++++++++++++++----------
> 1 file changed, 20 insertions(+), 10 deletions(-)
>
> diff --git a/lib/tests/stackinit_kunit.c b/lib/tests/stackinit_kunit.c
> index 135322592faf..63aa78e6f5c1 100644
> --- a/lib/tests/stackinit_kunit.c
> +++ b/lib/tests/stackinit_kunit.c
> @@ -184,6 +184,15 @@ static bool stackinit_range_contains(char *haystack_start, size_t haystack_size,
> #define INIT_UNION_assigned_copy(var_type) \
> INIT_STRUCT_assigned_copy(var_type)
>
> +/*
> + * The "did we actually fill the stack?" check value needs
> + * to be neither 0 nor any of the "pattern" bytes. The
> + * pattern bytes are compiler, architecture, and type based,
> + * so we have to pick a value that never appears for those
> + * combinations. Use 0x99 which is not 0xFF, 0xFE, nor 0xAA.
> + */
> +#define FILL_BYTE 0x99
> +
> /*
> * @name: unique string name for the test
> * @var_type: type to be tested for zeroing initialization
> @@ -206,12 +215,12 @@ static noinline void test_ ## name (struct kunit *test) \
> ZERO_CLONE_ ## which(zero); \
> /* Clear entire check buffer for 0xFF overlap test. */ \
> memset(check_buf, 0x00, sizeof(check_buf)); \
> - /* Fill stack with 0xFF. */ \
> + /* Fill stack with FILL_BYTE. */ \
> ignored = leaf_ ##name((unsigned long)&ignored, 1, \
> FETCH_ARG_ ## which(zero)); \
> - /* Verify all bytes overwritten with 0xFF. */ \
> + /* Verify all bytes overwritten with FILL_BYTE. */ \
> for (sum = 0, i = 0; i < target_size; i++) \
> - sum += (check_buf[i] != 0xFF); \
> + sum += (check_buf[i] != FILL_BYTE); \
> /* Clear entire check buffer for later bit tests. */ \
> memset(check_buf, 0x00, sizeof(check_buf)); \
> /* Extract stack-defined variable contents. */ \
> @@ -222,7 +231,8 @@ static noinline void test_ ## name (struct kunit *test) \
> * possible between the two leaf function calls. \
> */ \
> KUNIT_ASSERT_EQ_MSG(test, sum, 0, \
> - "leaf fill was not 0xFF!?\n"); \
> + "leaf fill was not 0x%02X!?\n", \
> + FILL_BYTE); \
> \
> /* Validate that compiler lined up fill and target. */ \
> KUNIT_ASSERT_TRUE_MSG(test, \
> @@ -234,9 +244,9 @@ static noinline void test_ ## name (struct kunit *test) \
> (int)((ssize_t)(uintptr_t)fill_start - \
> (ssize_t)(uintptr_t)target_start)); \
> \
> - /* Look for any bytes still 0xFF in check region. */ \
> + /* Validate check region has no FILL_BYTE bytes. */ \
> for (sum = 0, i = 0; i < target_size; i++) \
> - sum += (check_buf[i] == 0xFF); \
> + sum += (check_buf[i] == FILL_BYTE); \
> \
> if (sum != 0 && xfail) \
> kunit_skip(test, \
> @@ -271,12 +281,12 @@ static noinline int leaf_ ## name(unsigned long sp, bool fill, \
> * stack frame of SOME kind... \
> */ \
> memset(buf, (char)(sp & 0xff), sizeof(buf)); \
> - /* Fill variable with 0xFF. */ \
> + /* Fill variable with FILL_BYTE. */ \
> if (fill) { \
> fill_start = &var; \
> fill_size = sizeof(var); \
> memset(fill_start, \
> - (char)((sp & 0xff) | forced_mask), \
> + FILL_BYTE & forced_mask, \
> fill_size); \
> } \
> \
> @@ -469,7 +479,7 @@ static int noinline __leaf_switch_none(int path, bool fill)
> fill_start = &var;
> fill_size = sizeof(var);
>
> - memset(fill_start, forced_mask | 0x55, fill_size);
> + memset(fill_start, (forced_mask | 0x55) & FILL_BYTE, fill_size);
> }
> memcpy(check_buf, target_start, target_size);
> break;
> @@ -480,7 +490,7 @@ static int noinline __leaf_switch_none(int path, bool fill)
> fill_start = &var;
> fill_size = sizeof(var);
>
> - memset(fill_start, forced_mask | 0xaa, fill_size);
> + memset(fill_start, (forced_mask | 0xaa) & FILL_BYTE, fill_size);
> }
> memcpy(check_buf, target_start, target_size);
> break;
> --
> 2.34.1
>
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