x86/fpu: Do not inherit FPU context for kernel and IO worker threads

BugLink: https://bugs.launchpad.net/bugs/1967750
There is no reason why kernel and IO worker threads need a full clone of
the parent's FPU state. Both are kernel threads which are not supposed to
use FPU. So copying a large state or doing XSAVE() is pointless. Just clean
out the minimally required state for those tasks.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.839822981@linutronix.de
(cherry picked from commit 509e7a30cd0a9f38abac4114832d9f69ff0d73b4)
Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 191269edac9743721bcb95a7d98ab518f1fb80e4..9a6b195a8a001168225b72aad5a1c0c0b345a881 100644 (file)
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -212,6 +212,15 @@ static inline void fpstate_init_xstate(struct xregs_state *xsave)
        xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT | xfeatures_mask_all;
 }
 
+static inline unsigned int init_fpstate_copy_size(void)
+{
+       if (!use_xsave())
+               return fpu_kernel_xstate_size;
+
+       /* XSAVE(S) just needs the legacy and the xstate header part */
+       return sizeof(init_fpstate.xsave);
+}
+
 static inline void fpstate_init_fxstate(struct fxregs_state *fx)
 {
        fx->cwd = 0x37f;
@@ -259,6 +268,23 @@ int fpu_clone(struct task_struct *dst)
        if (!cpu_feature_enabled(X86_FEATURE_FPU))
                return 0;
 
+       /*
+        * Enforce reload for user space tasks and prevent kernel threads
+        * from trying to save the FPU registers on context switch.
+        */
+       set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);
+
+       /*
+        * No FPU state inheritance for kernel threads and IO
+        * worker threads.
+        */
+       if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
+               /* Clear out the minimal state */
+               memcpy(&dst_fpu->state, &init_fpstate,
+                      init_fpstate_copy_size());
+               return 0;
+       }
+
        /*
         * If the FPU registers are not owned by current just memcpy() the
         * state.  Otherwise save the FPU registers directly into the
@@ -272,8 +298,6 @@ int fpu_clone(struct task_struct *dst)
                save_fpregs_to_fpstate(dst_fpu);
        fpregs_unlock();
 
-       set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);
-
        trace_x86_fpu_copy_src(src_fpu);
        trace_x86_fpu_copy_dst(dst_fpu);
 
@@ -322,15 +346,6 @@ static inline void restore_fpregs_from_init_fpstate(u64 features_mask)
        pkru_write_default();
 }
 
-static inline unsigned int init_fpstate_copy_size(void)
-{
-       if (!use_xsave())
-               return fpu_kernel_xstate_size;
-
-       /* XSAVE(S) just needs the legacy and the xstate header part */
-       return sizeof(init_fpstate.xsave);
-}
-
 /*
  * Reset current->fpu memory state to the init values.
  */