[PATCH v3 1/2] powerpc: fix KUAP warning in VMX usercopy path
Sayali Patil
sayalip at linux.ibm.com
Wed Mar 4 16:35:05 AEDT 2026
On powerpc with PREEMPT_FULL or PREEMPT_LAZY and function tracing enabled,
KUAP warnings can be triggered from the VMX usercopy path under memory
stress workloads.
KUAP requires that no subfunctions are called once userspace access has
been enabled. The existing VMX copy implementation violates this
requirement by invoking enter_vmx_usercopy() from the assembly path after
userspace access has already been enabled. If preemption occurs
in this window, the AMR state may not be preserved correctly,
leading to unexpected userspace access state and resulting in
KUAP warnings.
Fix this by restructuring the VMX usercopy flow so that VMX selection
and VMX state management are centralized in raw_copy_tofrom_user(),
which is invoked by the raw_copy_{to,from,in}_user() wrappers.
The new flow is:
- raw_copy_{to,from,in}_user() calls raw_copy_tofrom_user()
- raw_copy_tofrom_user() decides whether to use the VMX path
based on size and CPU capability
- Call enter_vmx_usercopy() before enabling userspace access
- Enable userspace access as per the copy direction
and perform the VMX copy
- Disable userspace access as per the copy direction
- Call exit_vmx_usercopy()
- Fall back to the base copy routine if the VMX copy faults
With this change, the VMX assembly routines no longer perform VMX state
management or call helper functions; they only implement the
copy operations.
The previous feature-section based VMX selection inside
__copy_tofrom_user_power7() is removed, and a dedicated
__copy_tofrom_user_power7_vmx() entry point is introduced.
This ensures correct KUAP ordering, avoids subfunction calls
while KUAP is unlocked, and eliminates the warnings while preserving
the VMX fast path.
Fixes: de78a9c42a79 ("powerpc: Add a framework for Kernel Userspace Access Protection")
Reported-by: Shrikanth Hegde <sshegde at linux.ibm.com>
Closes: https://lore.kernel.org/all/20260109064917.777587-2-sshegde@linux.ibm.com/
Suggested-by: Christophe Leroy <chleroy at kernel.org>
Co-developed-by: Aboorva Devarajan <aboorvad at linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad at linux.ibm.com>
Signed-off-by: Sayali Patil <sayalip at linux.ibm.com>
---
v2->v3
- Addressd as per review feedback by removing usercopy_mode enum
and using the copy direction directly for KUAP permission handling.
- Integrated __copy_tofrom_user_vmx() functionality into
raw_copy_tofrom_user() in uaccess.h as a static __always_inline
implementation.
- Exported enter_vmx_usercopy() and exit_vmx_usercopy()
to support VMX usercopy handling from the common path.
v2: https://lore.kernel.org/all/20260228135319.238985-1-sayalip@linux.ibm.com/
---
arch/powerpc/include/asm/uaccess.h | 66 ++++++++++++++++++++++--------
arch/powerpc/lib/copyuser_64.S | 1 +
arch/powerpc/lib/copyuser_power7.S | 45 +++++++-------------
arch/powerpc/lib/vmx-helper.c | 2 +
4 files changed, 66 insertions(+), 48 deletions(-)
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index ba1d878c3f40..8fd412671025 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -15,6 +15,9 @@
#define TASK_SIZE_MAX TASK_SIZE_USER64
#endif
+/* Threshold above which VMX copy path is used */
+#define VMX_COPY_THRESHOLD 3328
+
#include <asm-generic/access_ok.h>
/*
@@ -326,40 +329,67 @@ do { \
extern unsigned long __copy_tofrom_user(void __user *to,
const void __user *from, unsigned long size);
-#ifdef __powerpc64__
-static inline unsigned long
-raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
+unsigned long __copy_tofrom_user_base(void __user *to,
+ const void __user *from, unsigned long size);
+
+unsigned long __copy_tofrom_user_power7_vmx(void __user *to,
+ const void __user *from, unsigned long size);
+
+
+static __always_inline bool will_use_vmx(unsigned long n)
+{
+ return IS_ENABLED(CONFIG_ALTIVEC) &&
+ cpu_has_feature(CPU_FTR_VMX_COPY) &&
+ n > VMX_COPY_THRESHOLD;
+}
+
+static __always_inline unsigned long raw_copy_tofrom_user(void __user *to,
+ const void __user *from, unsigned long n,
+ unsigned long dir)
{
unsigned long ret;
- barrier_nospec();
- allow_user_access(to, KUAP_READ_WRITE);
+ if (will_use_vmx(n) && enter_vmx_usercopy()) {
+ allow_user_access(to, dir);
+ ret = __copy_tofrom_user_power7_vmx(to, from, n);
+ prevent_user_access(dir);
+ exit_vmx_usercopy();
+
+ if (unlikely(ret)) {
+ allow_user_access(to, dir);
+ ret = __copy_tofrom_user_base(to, from, n);
+ prevent_user_access(dir);
+ }
+ return ret;
+ }
+
+ allow_user_access(to, dir);
ret = __copy_tofrom_user(to, from, n);
- prevent_user_access(KUAP_READ_WRITE);
+ prevent_user_access(dir);
return ret;
}
+
+#ifdef __powerpc64__
+static inline unsigned long
+raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
+{
+ barrier_nospec();
+ return raw_copy_tofrom_user(to, from, n, KUAP_READ_WRITE);
+}
#endif /* __powerpc64__ */
static inline unsigned long raw_copy_from_user(void *to,
const void __user *from, unsigned long n)
{
- unsigned long ret;
-
- allow_user_access(NULL, KUAP_READ);
- ret = __copy_tofrom_user((__force void __user *)to, from, n);
- prevent_user_access(KUAP_READ);
- return ret;
+ return raw_copy_tofrom_user((__force void __user *)to, from,
+ n, KUAP_READ);
}
static inline unsigned long
raw_copy_to_user(void __user *to, const void *from, unsigned long n)
{
- unsigned long ret;
-
- allow_user_access(to, KUAP_WRITE);
- ret = __copy_tofrom_user(to, (__force const void __user *)from, n);
- prevent_user_access(KUAP_WRITE);
- return ret;
+ return raw_copy_tofrom_user(to, (__force const void __user *)from,
+ n, KUAP_WRITE);
}
unsigned long __arch_clear_user(void __user *addr, unsigned long size);
diff --git a/arch/powerpc/lib/copyuser_64.S b/arch/powerpc/lib/copyuser_64.S
index 9af969d2cc0c..25a99108caff 100644
--- a/arch/powerpc/lib/copyuser_64.S
+++ b/arch/powerpc/lib/copyuser_64.S
@@ -562,3 +562,4 @@ exc; std r10,32(3)
li r5,4096
b .Ldst_aligned
EXPORT_SYMBOL(__copy_tofrom_user)
+EXPORT_SYMBOL(__copy_tofrom_user_base)
diff --git a/arch/powerpc/lib/copyuser_power7.S b/arch/powerpc/lib/copyuser_power7.S
index 8474c682a178..17dbcfbae25f 100644
--- a/arch/powerpc/lib/copyuser_power7.S
+++ b/arch/powerpc/lib/copyuser_power7.S
@@ -5,13 +5,9 @@
*
* Author: Anton Blanchard <anton at au.ibm.com>
*/
+#include <linux/export.h>
#include <asm/ppc_asm.h>
-#ifndef SELFTEST_CASE
-/* 0 == don't use VMX, 1 == use VMX */
-#define SELFTEST_CASE 0
-#endif
-
#ifdef __BIG_ENDIAN__
#define LVS(VRT,RA,RB) lvsl VRT,RA,RB
#define VPERM(VRT,VRA,VRB,VRC) vperm VRT,VRA,VRB,VRC
@@ -47,10 +43,14 @@
ld r15,STK_REG(R15)(r1)
ld r14,STK_REG(R14)(r1)
.Ldo_err3:
- bl CFUNC(exit_vmx_usercopy)
+ ld r6,STK_REG(R31)(r1) /* original destination pointer */
+ ld r5,STK_REG(R29)(r1) /* original number of bytes */
+ subf r7,r6,r3 /* #bytes copied */
+ subf r3,r7,r5 /* #bytes not copied in r3 */
ld r0,STACKFRAMESIZE+16(r1)
mtlr r0
- b .Lexit
+ addi r1,r1,STACKFRAMESIZE
+ blr
#endif /* CONFIG_ALTIVEC */
.Ldo_err2:
@@ -74,7 +74,6 @@
_GLOBAL(__copy_tofrom_user_power7)
cmpldi r5,16
- cmpldi cr1,r5,3328
std r3,-STACKFRAMESIZE+STK_REG(R31)(r1)
std r4,-STACKFRAMESIZE+STK_REG(R30)(r1)
@@ -82,12 +81,6 @@ _GLOBAL(__copy_tofrom_user_power7)
blt .Lshort_copy
-#ifdef CONFIG_ALTIVEC
-test_feature = SELFTEST_CASE
-BEGIN_FTR_SECTION
- bgt cr1,.Lvmx_copy
-END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
-#endif
.Lnonvmx_copy:
/* Get the source 8B aligned */
@@ -263,23 +256,14 @@ err1; stb r0,0(r3)
15: li r3,0
blr
-.Lunwind_stack_nonvmx_copy:
- addi r1,r1,STACKFRAMESIZE
- b .Lnonvmx_copy
-
-.Lvmx_copy:
#ifdef CONFIG_ALTIVEC
+_GLOBAL(__copy_tofrom_user_power7_vmx)
mflr r0
std r0,16(r1)
stdu r1,-STACKFRAMESIZE(r1)
- bl CFUNC(enter_vmx_usercopy)
- cmpwi cr1,r3,0
- ld r0,STACKFRAMESIZE+16(r1)
- ld r3,STK_REG(R31)(r1)
- ld r4,STK_REG(R30)(r1)
- ld r5,STK_REG(R29)(r1)
- mtlr r0
+ std r3,STK_REG(R31)(r1)
+ std r5,STK_REG(R29)(r1)
/*
* We prefetch both the source and destination using enhanced touch
* instructions. We use a stream ID of 0 for the load side and
@@ -300,8 +284,6 @@ err1; stb r0,0(r3)
DCBT_SETUP_STREAMS(r6, r7, r9, r10, r8)
- beq cr1,.Lunwind_stack_nonvmx_copy
-
/*
* If source and destination are not relatively aligned we use a
* slower permute loop.
@@ -478,7 +460,8 @@ err3; lbz r0,0(r4)
err3; stb r0,0(r3)
15: addi r1,r1,STACKFRAMESIZE
- b CFUNC(exit_vmx_usercopy) /* tail call optimise */
+ li r3,0
+ blr
.Lvmx_unaligned_copy:
/* Get the destination 16B aligned */
@@ -681,5 +664,7 @@ err3; lbz r0,0(r4)
err3; stb r0,0(r3)
15: addi r1,r1,STACKFRAMESIZE
- b CFUNC(exit_vmx_usercopy) /* tail call optimise */
+ li r3,0
+ blr
+EXPORT_SYMBOL(__copy_tofrom_user_power7_vmx)
#endif /* CONFIG_ALTIVEC */
diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
index 54340912398f..554b248002b4 100644
--- a/arch/powerpc/lib/vmx-helper.c
+++ b/arch/powerpc/lib/vmx-helper.c
@@ -27,6 +27,7 @@ int enter_vmx_usercopy(void)
return 1;
}
+EXPORT_SYMBOL(enter_vmx_usercopy);
/*
* This function must return 0 because we tail call optimise when calling
@@ -49,6 +50,7 @@ int exit_vmx_usercopy(void)
set_dec(1);
return 0;
}
+EXPORT_SYMBOL(exit_vmx_usercopy);
int enter_vmx_ops(void)
{
--
2.52.0
More information about the Linuxppc-dev
mailing list