Re: [patch V2 11/52] x86/fpu: Rewrite xfpregs_set()

Borislav Petkov
2021-06-16T15:22:38Z

On Mon, Jun 14, 2021 at 05:44:19PM +0200, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@kernel.org>
>
> xfpregs_set() was incomprehensible. Almost all of the complexity was due
> to trying to support nonsensically sized writes or -EFAULT errors that
> would have partially or completely overwritten the destination before
> failing. Nonsensically sized input would only have been possible using
> PTRACE_SETREGSET on REGSET_XFP. Fortunately, it appears (based on Debian
> code search results) that no one uses that API at all, let alone with the
> wrong sized buffer. Failed user access can be handled more cleanly by
> first copying to kernel memory.
>
> Just rewrite it to require sensible input.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> V2: New patch picked up from Andy
> ---
> arch/x86/kernel/fpu/regset.c | 40 +++++++++++++++++++++++++---------------
> 1 file changed, 25 insertions(+), 15 deletions(-)
>
> --- a/arch/x86/kernel/fpu/regset.c
> +++ b/arch/x86/kernel/fpu/regset.c
> @@ -47,30 +47,40 @@ int xfpregs_set(struct task_struct *targ
> const void *kbuf, const void __user *ubuf)
> {
> struct fpu *fpu = &target->thread.fpu;
> + struct user32_fxsr_struct newstate;
> int ret;
>
> - if (!boot_cpu_has(X86_FEATURE_FXSR))
> + BUILD_BUG_ON(sizeof(newstate) != sizeof(struct fxregs_state));
> +
> + if (!static_cpu_has(X86_FEATURE_FXSR))

cpu_feature_enabled

> return -ENODEV;
>
> - fpu__prepare_write(fpu);
> - fpstate_sanitize_xstate(fpu);
> + /* No funny business with partial or oversized writes is permitted. */
> + if (pos != 0 || count != sizeof(newstate))
> + return -EINVAL;
>
> ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
> - &fpu->state.fxsave, 0, -1);
> + &newstate, 0, -1);

Let that line stick out.

> + if (ret)
> + return ret;
> +
> + /* Mask invalid MXCSR bits (for historical reasons). */

security reasons became historical reasons huh? :-)

With those fixed:

Reviewed-by: Borislav Petkov <bp@suse.de>

Thx.

--
Regards/Gruss,
Boris.

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