[PATCH v4 1/2] powerpc: fix KUAP warning in VMX usercopy path
Venkat Rao Bagalkote
venkat88 at linux.ibm.com
Thu Mar 5 15:38:43 AEDT 2026
On 04/03/26 5:52 pm, Sayali Patil wrote:
> 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 (CS GROUP) <chleroy at kernel.org>
> Reviewed-by: Christophe Leroy (CS GROUP) <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>
> ---
This patch fixes, reported issue.
Tested-by: Venkat Rao Bagalkote <venkat88 at linux.ibm.com>
Regards,
Venkat.
>
> v3->v4
> - Addressd as per review feedback for some editorial comments.
>
> v3: https://lore.kernel.org/all/20260304053506.118632-1-sayalip@linux.ibm.com/
>
> ---
> arch/powerpc/include/asm/uaccess.h | 65 +++++++++++++++++++++---------
> arch/powerpc/lib/copyuser_64.S | 1 +
> arch/powerpc/lib/copyuser_power7.S | 45 +++++++--------------
> arch/powerpc/lib/vmx-helper.c | 2 +
> 4 files changed, 63 insertions(+), 50 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index ba1d878c3f40..8b25be472cf0 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 +329,62 @@ 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;
> }
> -#endif /* __powerpc64__ */
>
> -static inline unsigned long raw_copy_from_user(void *to,
> - const void __user *from, unsigned long n)
> +#ifdef CONFIG_PPC64
> +static inline unsigned long
> +raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
> {
> - unsigned long ret;
> + barrier_nospec();
> + return raw_copy_tofrom_user(to, from, n, KUAP_READ_WRITE);
> +}
> +#endif /* CONFIG_PPC64 */
>
> - allow_user_access(NULL, KUAP_READ);
> - ret = __copy_tofrom_user((__force void __user *)to, from, n);
> - prevent_user_access(KUAP_READ);
> - return ret;
> +static inline unsigned long raw_copy_from_user(void *to, const void __user *from, unsigned long n)
> +{
> + 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)
> {
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