Re: [PATCH v2 1/2] powerpc: fix KUAP warning in VMX usercopy path

Tue, 03 Mar 2026 01:19:55 -0800 


On 02/03/26 16:42, Christophe Leroy (CS GROUP) wrote:

Hi Sayali,

Le 28/02/2026 à 14:53, Sayali Patil a écrit :
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.

Introduce a usercopy_mode enum to describe the copy direction
(IN, FROM, TO) and use it to derive the required KUAP permissions.
Userspace access is now enabled and disabled through common helpers
based on the selected mode, ensuring that the correct read/write
permissions are applied consistently.

  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 usercopy mode
     and perform the VMX copy
   - Disable userspace access as per the usercopy mode
   - 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 <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/ 
Suggested-by: Christophe Leroy <[email protected]>
Co-developed-by: Aboorva Devarajan <[email protected]>
Signed-off-by: Aboorva Devarajan <[email protected]>
Signed-off-by: Sayali Patil <[email protected]>
---

v1->v2
   - Updated as per the review comments.
   - Centralized VMX usercopy handling in __copy_tofrom_user_vmx() in
     arch/powerpc/lib/vmx-helper.c.
   - Introduced a usercopy_mode enum to describe the copy direction
     (IN, FROM, TO) and derive the required KUAP permissions, avoiding
     duplication across the different usercopy paths.
I like the reduction of duplication you propose but I can't see the added value of that enum, what about:
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h 
index 63d6eb8b004e..14a3219db838 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -329,12 +329,6 @@ do {                                \
 extern unsigned long __copy_tofrom_user(void __user *to,
         const void __user *from, unsigned long size);

-enum usercopy_mode {
-    USERCOPY_IN,
-    USERCOPY_FROM,
-    USERCOPY_TO,
-};
-
 unsigned long __copy_tofrom_user_vmx(void __user *to, const void __user *from, 
                 unsigned long size, enum usercopy_mode mode);

@@ -352,48 +346,18 @@ static inline bool will_use_vmx(unsigned long n)
         n > VMX_COPY_THRESHOLD;
 }
-static inline void raw_copy_allow(void __user *to, enum usercopy_mode mode) 
-{
-    switch (mode) {
-    case USERCOPY_IN:
-        allow_user_access(to, KUAP_READ_WRITE);
-        break;
-    case USERCOPY_FROM:
-        allow_user_access(NULL, KUAP_READ);
-        break;
-    case USERCOPY_TO:
-        allow_user_access(to, KUAP_WRITE);
-        break;
-    }
-}
-
-static inline void raw_copy_prevent(enum usercopy_mode mode)
-{
-    switch (mode) {
-    case USERCOPY_IN:
-        prevent_user_access(KUAP_READ_WRITE);
-        break;
-    case USERCOPY_FROM:
-        prevent_user_access(KUAP_READ);
-        break;
-    case USERCOPY_TO:
-        prevent_user_access(KUAP_WRITE);
-        break;
-    }
-}
-
 static inline unsigned long raw_copy_tofrom_user(void __user *to,
         const void __user *from, unsigned long n,
-        enum usercopy_mode mode)
+        unsigned long dir)
 {
     unsigned long ret;

     if (will_use_vmx(n))
         return __copy_tofrom_user_vmx(to, from,    n, mode);

-    raw_copy_allow(to, mode);
+    allow_user_access(to, dir);
     ret = __copy_tofrom_user(to, from, n);
-    raw_copy_prevent(mode);
+    prevent_user_access(dir);
     return ret;

 }
@@ -403,22 +367,20 @@ 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, USERCOPY_IN);
+    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)
 {
-    return raw_copy_tofrom_user((__force void __user *)to, from,
-                    n, USERCOPY_FROM);
+    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)
 {
-    return raw_copy_tofrom_user(to, (__force const void __user *)from,
-                    n, USERCOPY_TO);
+    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/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c 
index 35080885204b..4610f7153fd9 100644
--- a/arch/powerpc/lib/vmx-helper.c
+++ b/arch/powerpc/lib/vmx-helper.c
@@ -11,25 +11,25 @@
 #include <asm/switch_to.h>
 unsigned long __copy_tofrom_user_vmx(void __user *to, const void __user *from, 
-            unsigned long size, enum usercopy_mode mode)
+            unsigned long size, unsigned long dir)
 {
     unsigned long ret;

     if (!enter_vmx_usercopy()) {
-        raw_copy_allow(to, mode);
+        allow_user_access(to, dir);
         ret = __copy_tofrom_user(to, from, size);
-        raw_copy_prevent(mode);
+        prevent_user_access(dir);
         return ret;
     }

-    raw_copy_allow(to, mode);
+    allow_user_access(to, dir);
     ret = __copy_tofrom_user_power7_vmx(to, from, size);
-    raw_copy_prevent(mode);
+    prevent_user_access(dir);
     exit_vmx_usercopy();
     if (unlikely(ret)) {
-        raw_copy_allow(to, mode);
+        allow_user_access(to, dir);
         ret = __copy_tofrom_user_base(to, from, size);
-        raw_copy_prevent(mode);
+        prevent_user_access(dir);
     }

     return ret;



Christophe



Hi Christophe,
Thanks for the review.
With the suggested change, we are hitting a compilation error.

The issue is related to how KUAP enforces the access direction.
allow_user_access() contains:

BUILD_BUG_ON(!__builtin_constant_p(dir));

which requires that the access direction is a compile-time constant.
If we pass a runtime value (for example, an unsigned long), the
__builtin_constant_p() check fails and triggers the following build error.

Error:
In function 'allow_user_access', inlined from '__copy_tofrom_user_vmx' at arch/powerpc/lib/vmx-helper.c:19:3: 
BUILD_BUG_ON failed: !__builtin_constant_p(dir) 706
The previous implementation worked because allow_user_access() was invoked with enum constants (READ, WRITE, READ_WRITE), which satisfied the __builtin_constant_p() requirement. So in this case, the function must be called with a compile-time constant to satisfy KUAP. 

Please let me know if you would prefer a different approach.

Regards,
Sayali
v1: https://lore.kernel.org/all/[email protected]/ 

---
  arch/powerpc/include/asm/uaccess.h | 95 ++++++++++++++++++++++++------
  arch/powerpc/lib/copyuser_64.S     |  1 +
  arch/powerpc/lib/copyuser_power7.S | 45 +++++---------
  arch/powerpc/lib/vmx-helper.c      | 26 ++++++++
  4 files changed, 119 insertions(+), 48 deletions(-)
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h 
index ba1d878c3f40..63d6eb8b004e 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 +329,96 @@ 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) 
+enum usercopy_mode {
+    USERCOPY_IN,
+    USERCOPY_FROM,
+    USERCOPY_TO,
+};
+
+unsigned long __copy_tofrom_user_vmx(void __user *to, const void __user *from, 
+                unsigned long size, enum usercopy_mode mode);
+
+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 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 inline void raw_copy_allow(void __user *to, enum usercopy_mode mode) 
+{
+    switch (mode) {
+    case USERCOPY_IN:
+        allow_user_access(to, KUAP_READ_WRITE);
+        break;
+    case USERCOPY_FROM:
+        allow_user_access(NULL, KUAP_READ);
+        break;
+    case USERCOPY_TO:
+        allow_user_access(to, KUAP_WRITE);
+        break;
+    }
+}
+
+static inline void raw_copy_prevent(enum usercopy_mode mode)
+{
+    switch (mode) {
+    case USERCOPY_IN:
+        prevent_user_access(KUAP_READ_WRITE);
+        break;
+    case USERCOPY_FROM:
+        prevent_user_access(KUAP_READ);
+        break;
+    case USERCOPY_TO:
+        prevent_user_access(KUAP_WRITE);
+        break;
+    }
+}
+
+static inline unsigned long raw_copy_tofrom_user(void __user *to,
+        const void __user *from, unsigned long n,
+        enum usercopy_mode mode)
  {
      unsigned long ret;
  -    barrier_nospec();
-    allow_user_access(to, KUAP_READ_WRITE);
+    if (will_use_vmx(n))
+        return __copy_tofrom_user_vmx(to, from,    n, mode);
+
+    raw_copy_allow(to, mode);
      ret = __copy_tofrom_user(to, from, n);
-    prevent_user_access(KUAP_READ_WRITE);
+    raw_copy_prevent(mode);
      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, USERCOPY_IN);
  }
  #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, USERCOPY_FROM);
  }
    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, USERCOPY_TO);
  }
    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 <[email protected]>
   */
+#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..35080885204b 100644
--- a/arch/powerpc/lib/vmx-helper.c
+++ b/arch/powerpc/lib/vmx-helper.c
@@ -10,6 +10,32 @@
  #include <linux/hardirq.h>
  #include <asm/switch_to.h>
  +unsigned long __copy_tofrom_user_vmx(void __user *to, const void __user *from, 
+            unsigned long size, enum usercopy_mode mode)
+{
+    unsigned long ret;
+
+    if (!enter_vmx_usercopy()) {
+        raw_copy_allow(to, mode);
+        ret = __copy_tofrom_user(to, from, size);
+        raw_copy_prevent(mode);
+        return ret;
+    }
+
+    raw_copy_allow(to, mode);
+    ret = __copy_tofrom_user_power7_vmx(to, from, size);
+    raw_copy_prevent(mode);
+    exit_vmx_usercopy();
+    if (unlikely(ret)) {
+        raw_copy_allow(to, mode);
+        ret = __copy_tofrom_user_base(to, from, size);
+        raw_copy_prevent(mode);
+    }
+
+    return ret;
+}
+EXPORT_SYMBOL(__copy_tofrom_user_vmx);
+
  int enter_vmx_usercopy(void)
  {
      if (in_interrupt())

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