Re: [PATCH] kunit/stackinit: Use fill byte different from Clang i386 pattern

From: Kees Cook
Date: Tue Mar 04 2025 - 18:56:20 EST

On Tue, Mar 04, 2025 at 03:13:20PM -0800, Justin Stitt wrote:
> Hi,
>
> 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@xxxxxxxxxx>
> > ---
> > Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> > Cc: Bill Wendling <morbo@xxxxxxxxxx>
> > Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
> > Cc: llvm@xxxxxxxxxxxxxxx
> > ---
> > lib/tests/stackinit_kunit.c | 30 ++++++++++++++++++++----------
>
> Ah, couldn't find this file at first. Depends on [1].
>
> > 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); \
>
> KUNIT_ASSERT_EQ_MSG(test, sum, 0, \
> - "leaf fill was not 0xFF!?\n"); \
> + "leaf fill was not 0x%02X!??!?!?!?!?!?!?!\n", \
> + FILL_BYTE); \

OMGWTFBBQ!!

>
> > \
> > /* 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
> >
> >
>
> I don't understand the stack init pattern tendencies enough to give a RB
> but I looked at your patch and it seems to fully replace the fill test
> values from 0xff to 0x99, so if 0x99 is OK then OK.

Yeah, I had to go do some godbolting to remind myself what value were
being used. :)

Thanks for the double-check!

-Kees

>
> [1]: https://lore.kernel.org/all/20250211003136.2860503-3-kees@xxxxxxxxxx/
>
> Justin

--
Kees Cook