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    Subject[tip: x86/urgent] x86/process: Check PF_KTHREAD and not current->mm for kernel threads
    The following commit has been merged into the x86/urgent branch of tip:

    Commit-ID: 12f7764ac61200e32c916f038bdc08f884b0b604
    Gitweb: https://git.kernel.org/tip/12f7764ac61200e32c916f038bdc08f884b0b604
    Author: Thomas Gleixner <tglx@linutronix.de>
    AuthorDate: Tue, 08 Jun 2021 16:36:20 +02:00
    Committer: Borislav Petkov <bp@suse.de>
    CommitterDate: Wed, 09 Jun 2021 10:39:04 +02:00

    x86/process: Check PF_KTHREAD and not current->mm for kernel threads

    switch_fpu_finish() checks current->mm as indicator for kernel threads.
    That's wrong because kernel threads can temporarily use a mm of a user
    process via kthread_use_mm().

    Check the task flags for PF_KTHREAD instead.

    Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state")
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
    Acked-by: Rik van Riel <riel@surriel.com>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/20210608144345.912645927@linutronix.de
    ---
    arch/x86/include/asm/fpu/internal.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
    index ceeba9f..18382ac 100644
    --- a/arch/x86/include/asm/fpu/internal.h
    +++ b/arch/x86/include/asm/fpu/internal.h
    @@ -578,7 +578,7 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)
    * PKRU state is switched eagerly because it needs to be valid before we
    * return to userland e.g. for a copy_to_user() operation.
    */
    - if (current->mm) {
    + if (!(current->flags & PF_KTHREAD)) {
    pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
    if (pk)
    pkru_val = pk->pkru;
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