x86/fpu: Emulate XRSTOR's behavior if the xfeatures PKRU bit is not set

The hardware XRSTOR instruction resets the PKRU register to its hardware
init value (namely 0) if the PKRU bit is not set in the xfeatures mask.
Emulating that here restores the pre-5.14 behavior for PTRACE_SET_REGSET
with NT_X86_XSTATE, and makes sigreturn (which still uses XRSTOR) and
ptrace behave identically. KVM has never used XRSTOR and never had this
behavior, so KVM opts-out of this emulation by passing a NULL pkru pointer
to copy_uabi_to_xstate().

Fixes: e84ba47e313d ("x86/fpu: Hook up PKRU into ptrace()")
Signed-off-by: Kyle Huey <me@kylehuey.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20221115230932.7126-6-khuey%40kylehuey.com

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 46b935bc87c8561568a7d964d8c7a5741e22be49..8d0f6019c21d62c3411988ebb4c16d99a7e16ce0 100644 (file)
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -404,6 +404,14 @@ int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
        if (ustate->xsave.header.xfeatures & ~xcr0)
                return -EINVAL;
 
+       /*
+        * Nullify @vpkru to preserve its current value if PKRU's bit isn't set
+        * in the header.  KVM's odd ABI is to leave PKRU untouched in this
+        * case (all other components are eventually re-initialized).
+        */
+       if (!(ustate->xsave.header.xfeatures & XFEATURE_MASK_PKRU))
+               vpkru = NULL;
+
        return copy_uabi_from_kernel_to_xstate(kstate, ustate, vpkru);
 }
 EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate);

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a8cf604d8a25856f9102f4bcc529cd0668904eab..714166cc25f2f6bff10cffd2dee9d40a2fca9db8 100644 (file)
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1219,8 +1219,14 @@ static int copy_from_buffer(void *dst, unsigned int offset, unsigned int size,
  *     it is harmless.
  * 2.  When called from ptrace the PKRU register will be restored from the
  *     thread_struct's pkru field. A pointer to that is passed in @pkru.
+ *     The kernel will restore it manually, so the XRSTOR behavior that resets
+ *     the PKRU register to the hardware init value (0) if the corresponding
+ *     xfeatures bit is not set is emulated here.
  * 3.  When called from KVM the PKRU register will be restored from the vcpu's
- *     pkru field. A pointer to that is passed in @pkru.
+ *     pkru field. A pointer to that is passed in @pkru. KVM hasn't used
+ *     XRSTOR and hasn't had the PKRU resetting behavior described above. To
+ *     preserve that KVM behavior, it passes NULL for @pkru if the xfeatures
+ *     bit is not set.
  */
 static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
                               const void __user *ubuf, u32 *pkru)
@@ -1277,6 +1283,13 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
 
                xpkru = __raw_xsave_addr(xsave, XFEATURE_PKRU);
                *pkru = xpkru->pkru;
+       } else {
+               /*
+                * KVM may pass NULL here to indicate that it does not need
+                * PKRU updated.
+                */
+               if (pkru)
+                       *pkru = 0;
        }
 
        /*