Re: [x86/uaccess] 9c5743dff4: WARNING:at_arch/x86/mm/extable.c:#ex_handler_fprestore

[Thomas Gleixner <tglx@linutronix.de>]

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On Fri, May 13 2022 at 09:52, Linus Torvalds wrote:
> On Fri, May 13, 2022 at 1:55 AM kernel test robot <oliver.sang@intel.com> wrote:
> But considering that the fail:runs thing is 41:52, I suspect it's
> something very timing-dependent and who knows how reliable the
> bisection has been.

This smells very much like the issue which got fixed with

     59f5ede3bc0f ("x86/fpu: Prevent FPU state corruption")

which resulted in the very same stack trace pattern because the restore
detects the fpstate corruption. The sigframe setup does:

     if (TIF_NEED_FPU_LOAD)
           restore();
     save_to_sigframe();

But yes, in theory it might be caused by ptrace as well. See below.

>>   24:   89 c2                   mov    %eax,%edx
>>   26:   48 0f ae 2f             xrstor64 (%rdi)
>>   2a:*  48 c7 c7 58 47 2b 8c    mov    $0xffffffff8c2b4758,%rdi         <-- trapping instruction
>
> Seems to be just the exception stack chain (ie notice how it's
> pointing to the instruction after the xrstor64, it's not that the
> immediate register move really trapped).

which is caused by ex_handler_fprestore() itself because it stupidly
fixes up regs->ip _before_ the warning. This should obviously be done
afterwards. Without that fixup it would point at xrstor64.

>>   28:   0f 05                   syscall
>>   2a:*  48 3d 00 f0 ff ff       cmp    $0xfffffffffffff000,%rax         <-- trapping instruction
>
> and again, it's just pointing back to after the 'syscall' instruction
> that caused this whole chain of events.
>
> Anyway, I *think* that what may be going on is some ptrace thing, but
> let's bring in other people. Because I don't think that "x86/uaccess:
> fix code generation in put_user()" commit is what triggered this, but
> who knows.. The x86 FP code can be very grotty.

Courtesy to the corresponding hardware...

The code which copies the ptrace supplied state has a pile of sanity
checks to catch invalid state, but I wouldn't bet my hat on it that it's
100% complete. We can be more defensive here, but I would be surprised.

Something like the untested below. I'll expose it to some testing to see
what explodes.

Thanks,

        tglx
---

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 39e1c8626ab9..c1228d6391c8 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1248,7 +1248,48 @@ static int copy_uabi_to_xstate(struct fpstate *fpstate, const void *kbuf,
  */
 int copy_uabi_from_kernel_to_xstate(struct fpstate *fpstate, const void *kbuf)
 {
-	return copy_uabi_to_xstate(fpstate, kbuf, NULL);
+	struct fpstate *tmpfps;
+	unsigned int fpsize;
+	int ret;
+
+	/* This cannot operate on current's fpstate */
+	if (WARN_ON_ONCE(fpstate == current->thread.fpu.fpstate))
+		return -EPERM;
+
+	/* Use a temporary fpstate for the xrstor validation below */
+	fpsize = fpstate->size + ALIGN(offsetof(struct fpstate, regs), 64);
+	tmpfps = vmalloc(fpsize);
+	if (!tmpfps)
+		return -ENOMEM;
+	memcpy(tmpfps, fpstate, fpstate->size);
+
+	ret = copy_uabi_to_xstate(tmpfps, kbuf, NULL);
+	if (ret)
+		goto out;
+	/*
+	 * Ensure right here that the user space provided xstate content is
+	 * correct. Save current's fpstate and invalidate the per-CPU FPU
+	 * state.
+	 */
+	kernel_fpu_begin_mask(0);
+	/*
+	 * Limit the restore attempt to the user features as fpstate
+	 * is not current's fpstate. So current's supervisor state
+	 * has to be preserved and the target's supervisor state was
+	 * not touched in copy_uabi_to_xstate().
+	 */
+	ret = os_xrstor_safe(tmpfps, tmpfps->user_xfeatures);
+	kernel_fpu_end();
+	/*
+	 * If the restore succeeded, copy the state. Otherwise
+	 * keep the previous content.
+	 */
+	if (!ret)
+		memcpy(fpstate, tmpfps, fpstate->size);
+
+out:
+	vfree(tmpfps);
+	return ret;
 }
 
 /*
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index dba2197c05c3..c0d852998d18 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -68,11 +68,10 @@ static bool ex_handler_sgx(const struct exception_table_entry *fixup,
 static bool ex_handler_fprestore(const struct exception_table_entry *fixup,
 				 struct pt_regs *regs)
 {
-	regs->ip = ex_fixup_addr(fixup);
-
 	WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing FPU registers.",
 		  (void *)instruction_pointer(regs));
 
+	regs->ip = ex_fixup_addr(fixup);
 	fpu_reset_from_exception_fixup();
 	return true;
 }