On 08/09/26 12:19 pm, Mukesh Kumar Chaurasiya (IBM) wrote:
powerpc unconditionally selects GENERIC_ENTRY. The GENERIC_ENTRY
infrastructure relies on the compiler emitting __asan_mem*() calls at
instrumented mem*() sites rather than plain memset/memcpy/memmove, so
that entry/exit paths calling those functions are not instrumented.
This assumption is encoded in two places:
mm/kasan/shadow.c:
#if !defined(CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX) &&
!defined(CONFIG_GENERIC_ENTRY)
include/linux/fortify-string.h:
#if !defined(CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX) &&
!defined(CONFIG_GENERIC_ENTRY)
When GENERIC_ENTRY is set, both guards suppress the C wrappers for
memset/memcpy/memmove and the __underlying_mem*() redirections. This
is only safe when the compiler supports the prefixed __asan_mem*()
intrinsics. On older toolchains (e.g. GCC 9) that lack this support,
plain mem*() calls from instrumented code fall through to the raw
assembly implementations in mem_64.S / copy_32.S, completely bypassing
the KASAN shadow check.
Other arches with GENERIC_ENTRY (x86, s390, loongarch, riscv) do not
hit this because their CI toolchains are always new enough to support
the prefix flag.
Background: the !GENERIC_ENTRY guard was introduced by commit 69d4c0d32186
("entry, kasan, x86: Disallow overriding mem*() functions", Peter Zijlstra,
Jan 2023). The root problem is that the KASAN C wrappers override the
linker symbol memset/memcpy/memmove globally, so any call from noinstr or
__no_sanitize_address code (e.g. irqentry_enter/irqentry_exit) would still
reach the KASAN shadow-check wrapper -- at a point where KASAN invariants
may not hold. The compiler prefix approach (Marco Elver, Feb 2023,
commit 51287dcb00cc) solves this by having the compiler emit __asan_memset
at instrumented call sites and bare memset inside __no_sanitize_address
functions, splitting the decision at code-generation time rather than at
link time.
A manual C-level override cannot replicate this split: a single linker
symbol cannot be made to resolve differently depending on the caller.
x86 also placed its raw memset/memcpy/memmove implementations in
.noinstr.text (same commit, 69d4c0d32186), which is the other half of
the fix: noinstr callers hit the raw assembly directly, safely bypassing
KASAN. PowerPC has not done this. Placing mem_64.S / memcpy_64.S /
copy_32.S implementations in .noinstr.text would be the complementary
long-term fix that could re-enable KASAN on older toolchains, but it
requires care around linker stub overflow on large PPC64 kernels (the
same reason powerpc uses NOKPROBE_SYMBOL rather than noinstr for its
interrupt handlers -- see the comment in asm/interrupt.h). That work
is left as a follow-up.
For now, introduce PPC_CC_HAS_KASAN_MEMINTRINSIC_PREFIX, an arch-local
compiler probe that mirrors the same check as CC_HAS_KASAN_MEMINTRINSIC_PREFIX
in lib/Kconfig.kasan but lives outside the 'if KASAN' block to avoid a
recursive dependency (CC_HAS_KASAN_MEMINTRINSIC_PREFIX depends on KASAN
which depends on HAVE_ARCH_KASAN). Gate the three HAVE_ARCH_KASAN selects
on this new symbol so that KASAN is not offered as a config option on
toolchains that cannot support it correctly with GENERIC_ENTRY.
Since KASAN on powerpc now unconditionally implies
CC_HAS_KASAN_MEMINTRINSIC_PREFIX, the old !CC_HAS_KASAN_MEMINTRINSIC_PREFIX
code paths in asm/kasan.h and asm/string.h are dead. Clean them up:
- asm/kasan.h: remove the dual-entry-point variant of _GLOBAL_KASAN /
_GLOBAL_TOC_KASAN / EXPORT_SYMBOL_KASAN that emitted both memset and
__memset as entry points to the same assembly. These aliases were only
needed so the C KASAN wrappers in shadow.c could call __memset() to
reach raw memory ops; with the compiler prefix approach the wrappers are
not used at all for mem* on powerpc.
- asm/string.h: remove the separate __memset/__memcpy/__memmove symbol
declarations and the memset/memcpy/memmove macro redirections for
uninstrumented files that were needed on old toolchains. Simplify the
CONFIG_KASAN block to just the three #define aliases (which are still
used by shadow.c as raw backends).
- cputable.c, prom_init.c: update stale comments that said GCC replaces
memcpy() with __memcpy() under KASAN; with the prefix flag it emits
__asan_memcpy() instead.
Reported-by: Venkat Rao Bagalkote <[email protected]>
Closes:
https://lore.kernel.org/all/[email protected]
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
---
Tested-by: Venkat Rao Bagalkote <[email protected]>
Regards,
Venkat.
arch/powerpc/Kconfig | 10 +++++++---
arch/powerpc/include/asm/kasan.h | 19 ++++++++-----------
arch/powerpc/include/asm/string.h | 25 +++++--------------------
arch/powerpc/kernel/cputable.c | 6 +++---
arch/powerpc/kernel/prom_init.c | 4 ++--
5 files changed, 25 insertions(+), 39 deletions(-)
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 2580e27e4328..b27ed9739eea 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -7,6 +7,10 @@ config CC_HAS_ELFV2
config CC_HAS_PREFIXED
def_bool PPC64 && $(cc-option, -mcpu=power10 -mprefixed)
+config PPC_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
+ def_bool (CC_IS_CLANG && $(cc-option,-fsanitize=kernel-address -mllvm
-asan-kernel-mem-intrinsic-prefix=1)) || \
+ (CC_IS_GCC && $(cc-option,-fsanitize=kernel-address --param
asan-kernel-mem-intrinsic-prefix=1))
+
config CC_HAS_PCREL
# Clang has a bug (https://github.com/llvm/llvm-project/issues/62372)
# where pcrel code is not generated if -msoft-float, -mno-altivec, or
@@ -220,9 +224,9 @@ config PPC
select HAVE_ARCH_HUGE_VMAP if PPC_RADIX_MMU || PPC_8xx
select HAVE_ARCH_JUMP_LABEL
select HAVE_ARCH_JUMP_LABEL_RELATIVE
- select HAVE_ARCH_KASAN if PPC32 && PAGE_SHIFT <= 14
- select HAVE_ARCH_KASAN if PPC_RADIX_MMU
- select HAVE_ARCH_KASAN if PPC_BOOK3E_64
+ select HAVE_ARCH_KASAN if PPC32 && PAGE_SHIFT <= 14 &&
PPC_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
+ select HAVE_ARCH_KASAN if PPC_RADIX_MMU &&
PPC_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
+ select HAVE_ARCH_KASAN if PPC_BOOK3E_64 &&
PPC_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
select HAVE_ARCH_KCSAN
select HAVE_ARCH_KFENCE if ARCH_SUPPORTS_DEBUG_PAGEALLOC
diff --git a/arch/powerpc/include/asm/kasan.h b/arch/powerpc/include/asm/kasan.h
index a690e7da53c2..ffada5f51b2b 100644
--- a/arch/powerpc/include/asm/kasan.h
+++ b/arch/powerpc/include/asm/kasan.h
@@ -2,20 +2,17 @@
#ifndef __ASM_KASAN_H
#define __ASM_KASAN_H
-#if defined(CONFIG_KASAN) && !defined(CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX)
-#define _GLOBAL_KASAN(fn) \
- _GLOBAL(fn); \
- _GLOBAL(__##fn)
-#define _GLOBAL_TOC_KASAN(fn) \
- _GLOBAL_TOC(fn); \
- _GLOBAL_TOC(__##fn)
-#define EXPORT_SYMBOL_KASAN(fn) \
- EXPORT_SYMBOL(__##fn)
-#else /* CONFIG_KASAN && !CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX */
+/*
+ * powerpc requires CC_HAS_KASAN_MEMINTRINSIC_PREFIX whenever KASAN is
+ * enabled (see PPC_CC_HAS_KASAN_MEMINTRINSIC_PREFIX in arch/powerpc/Kconfig),
+ * so the compiler always emits __asan_mem*() at instrumented call sites and
+ * bare mem*() inside __no_sanitize_address / noinstr code. The old dual
+ * entry-point trick (_GLOBAL_KASAN emitting both memset and __memset) is
+ * therefore never needed.
+ */
#define _GLOBAL_KASAN(fn) _GLOBAL(fn)
#define _GLOBAL_TOC_KASAN(fn) _GLOBAL_TOC(fn)
#define EXPORT_SYMBOL_KASAN(fn)
-#endif /* CONFIG_KASAN && !CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX */
#ifndef __ASSEMBLER__
diff --git a/arch/powerpc/include/asm/string.h b/arch/powerpc/include/asm/string.h
index 1981bd4036b5..743126e3dcc1 100644
--- a/arch/powerpc/include/asm/string.h
+++ b/arch/powerpc/include/asm/string.h
@@ -29,29 +29,14 @@ extern void * memchr(const void *,int,__kernel_size_t);
void memcpy_flushcache(void *dest, const void *src, size_t size);
#ifdef CONFIG_KASAN
-/* __mem variants are used by KASAN to implement instrumented meminstrinsics.
*/
-#ifdef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
+/*
+ * powerpc requires CC_HAS_KASAN_MEMINTRINSIC_PREFIX whenever KASAN is
+ * enabled, so the compiler emits __asan_mem*() at instrumented sites.
+ * The raw mem* symbols are always safe to call directly.
+ */
#define __memset memset
#define __memcpy memcpy
#define __memmove memmove
-#else /* CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX */
-void *__memset(void *s, int c, __kernel_size_t count);
-void *__memcpy(void *to, const void *from, __kernel_size_t n);
-void *__memmove(void *to, const void *from, __kernel_size_t n);
-#ifndef __SANITIZE_ADDRESS__
-/*
- * For files that are not instrumented (e.g. mm/slub.c) we
- * should use not instrumented version of mem* functions.
- */
-#define memcpy(dst, src, len) __memcpy(dst, src, len)
-#define memmove(dst, src, len) __memmove(dst, src, len)
-#define memset(s, c, n) __memset(s, c, n)
-
-#ifndef __NO_FORTIFY
-#define __NO_FORTIFY /* FORTIFY_SOURCE uses __builtin_memcpy, etc. */
-#endif
-#endif /* !__SANITIZE_ADDRESS__ */
-#endif /* CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX */
#endif /* CONFIG_KASAN */
#ifdef CONFIG_PPC64
diff --git a/arch/powerpc/kernel/cputable.c b/arch/powerpc/kernel/cputable.c
index 6f6801da9dc1..44115f904c2c 100644
--- a/arch/powerpc/kernel/cputable.c
+++ b/arch/powerpc/kernel/cputable.c
@@ -36,8 +36,8 @@ void __init set_cur_cpu_spec(struct cpu_spec *s)
t = PTRRELOC(t);
/*
- * use memcpy() instead of *t = *s so that GCC replaces it
- * by __memcpy() when KASAN is active
+ * use memcpy() instead of *t = *s so that the compiler replaces it
+ * by __asan_memcpy() when KASAN is active
*/
memcpy(t, s, sizeof(*t));
@@ -55,7 +55,7 @@ static struct cpu_spec * __init setup_cpu_spec(unsigned long offset,
/*
* Copy everything, then do fixups. Use memcpy() instead of *t = *s
- * so that GCC replaces it by __memcpy() when KASAN is active
+ * so that the compiler replaces it by __asan_memcpy() when KASAN is
active
*/
memcpy(t, s, sizeof(*t));
diff --git a/arch/powerpc/kernel/prom_init.c b/arch/powerpc/kernel/prom_init.c
index eb9f556b0937..b763720c68b6 100644
--- a/arch/powerpc/kernel/prom_init.c
+++ b/arch/powerpc/kernel/prom_init.c
@@ -1365,8 +1365,8 @@ static void __init prom_check_platform_support(void)
/*
* First copy the architecture vec template
*
- * use memcpy() instead of *vec = *vec_template so that GCC replaces it
- * by __memcpy() when KASAN is active
+ * use memcpy() instead of *vec = *vec_template so that the compiler
+ * replaces it by __asan_memcpy() when KASAN is active
*/
memcpy(&ibm_architecture_vec, &ibm_architecture_vec_template,
sizeof(ibm_architecture_vec));
