linux-kernel - Re: [PATCH v4 0/7] mm/mprotect: Fix dax puds

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Message-ID: <ZrTXXJ8r_WpPgcU4@x1n>
Date: Thu, 8 Aug 2024 10:34:04 -0400
From: Peter Xu <peterx@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Oscar Salvador <osalvador@...e.de>,
	Dan Williams <dan.j.williams@...el.com>,
	James Houghton <jthoughton@...gle.com>,
	Matthew Wilcox <willy@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,
	Rik van Riel <riel@...riel.com>, Dave Jiang <dave.jiang@...el.com>,
	x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
	Rick P Edgecombe <rick.p.edgecombe@...el.com>,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	linuxppc-dev@...ts.ozlabs.org,
	Mel Gorman <mgorman@...hsingularity.net>,
	Hugh Dickins <hughd@...gle.com>, Borislav Petkov <bp@...en8.de>,
	David Hildenbrand <david@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH v4 0/7] mm/mprotect: Fix dax puds

On Wed, Aug 07, 2024 at 02:44:54PM -0700, Andrew Morton wrote:
> On Wed, 7 Aug 2024 17:34:10 -0400 Peter Xu <peterx@...hat.com> wrote:
> 
> > The problem is mprotect() will skip the dax 1G PUD while it shouldn't;
> > meanwhile it'll dump some bad PUD in dmesg.  Both of them look like (corner
> > case) bugs to me.. where:
> > 
> >   - skipping the 1G pud means mprotect() will succeed even if the pud won't
> >     be updated with the correct permission specified. Logically that can
> >     cause e.g. in mprotect(RO) then write the page can cause data corrupt,
> >     as the pud page will still be writable.
> > 
> >   - the bad pud will generate a pr_err() into dmesg, with no limit so far I
> >     can see.  So I think it means an userspace can DoS the kernel log if it
> >     wants.. simply by creating the PUD and keep mprotect-ing it
> > 
> 
> I edited this important info into the [0/n] text, thanks.
> 
> So current kernels can be made to spew into the kernel logs?  That's

I suppose yes to this one.

> considered serious.  Can unprivileged userspace code do this?

AFAIU, /dev/dax* require root privilege by default, so looks not.  But
anyone more familiar with real life dax usages please correct me otherwise.

Thanks,

-- 
Peter Xu