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Date:   Sat, 28 Aug 2021 22:19:04 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Luck, Tony" <tony.luck@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org
Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable
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On Sat, Aug 28, 2021 at 10:11:36PM +0000, Al Viro wrote:
> On Sat, Aug 28, 2021 at 10:04:41PM +0000, Al Viro wrote:
> > On Sat, Aug 28, 2021 at 11:47:03PM +0200, Thomas Gleixner wrote:
> > 
> > >   /* Try to handle #PF, but anything else is fatal. */
> > >   if (ret != -EFAULT)
> > >      return -EINVAL;
> > 
> > > which all end up in user_insn(). user_insn() returns 0 or the negated
> > > trap number, which results in -EFAULT for #PF, but for #MC the negated
> > > trap number is -18 i.e. != -EFAULT. IOW, there is no endless loop.
> > > 
> > > This used to be a problem before commit:
> > > 
> > >   aee8c67a4faa ("x86/fpu: Return proper error codes from user access functions")
> > > 
> > > and as the changelog says the initial reason for this was #GP going into
> > > the fault path, but I'm pretty sure that I also discussed the #MC angle with
> > > Borislav back then. Should have added some more comments there
> > > obviously.
> > 
> > ... or at least have that check spelled
> > 
> > 	if (ret != -X86_TRAP_PF)
> > 		return -EINVAL;
> > 
> > Unless I'm misreading your explanation, that is...
> 
> BTW, is #MC triggered on stored to a poisoned cacheline?  Existence of CLZERO
> would seem to argue against that...

How about taking __clear_user() out of copy_fpregs_to_sigframe()
and replacing the call of fault_in_pages_writeable() with
	if (!clear_user(buf_fx, fpu_user_xstate_size))
		goto retry;
	return -EFAULT;
in the caller?