linux-kernel - Re: [PATCH] x86/uaccess: restore get_user exception type to EX_TYPE

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Date:   Fri, 1 Apr 2022 20:45:19 +0800
From:   Youquan Song <youquan.song@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Youquan Song <youquan.song@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Zhiquan Li <zhiquan1.li@...el.com>,
        Youquan Song <youquan.song@...el.com>
Subject: Re: [PATCH] x86/uaccess: restore get_user exception type to
 EX_TYPE_UACCESS

On Thu, Mar 31, 2022 at 07:51:13PM +0200, Peter Zijlstra wrote:
> On Thu, Mar 31, 2022 at 07:31:25PM +0800, Youquan Song wrote:
> > > Did you do your testing on RHEL or something daft like that?
> > Tested on RHEL8.x
> 
> Right; the home of obsolete software.. :-)
> 
> > > Something like the below can also work, I suppose. But please, add
> > > coherent comments to the extable code with useful references to the MCE
> > > code that does this abuse.
> > Here is the full fix patch depending on your suggestion. Thanks a lot!
> 
> Did you verify this was indeed the only UACCESS I lost?

The full fix patch has included a change in error_context to identify
this case to be MCE_IN_KERNEL_COPYIN. I verfied it fix the issue.
In addition, LTP was run and no issues were reported.

@@ -286,6 +286,7 @@ static noinstr int error_context(struct mce *m, struct pt_regs *regs)
 	switch (fixup_type) {
 	case EX_TYPE_UACCESS:
 	case EX_TYPE_COPY:
+	case EX_TYPE_UA_IMM_REG:
 		if (!copy_user)
 			return IN_KERNEL;
 		m->kflags |= MCE_IN_KERNEL_COPYIN;

-Youquan