linux-kernel - Re: [PATCH] [backport for 5.10] powerpc/603: Fix protection of user pages mapped with PROT

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Date:   Fri, 12 Mar 2021 11:03:09 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] [backport for 5.10] powerpc/603: Fix protection of user
 pages mapped with PROT_NONE

On Thu, Mar 11, 2021 at 06:24:30PM +0000, Christophe Leroy wrote:
> (cherry picked from commit c119565a15a628efdfa51352f9f6c5186e506a1c)
> 
> On book3s/32, page protection is defined by the PP bits in the PTE
> which provide the following protection depending on the access
> keys defined in the matching segment register:
> - PP 00 means RW with key 0 and N/A with key 1.
> - PP 01 means RW with key 0 and RO with key 1.
> - PP 10 means RW with both key 0 and key 1.
> - PP 11 means RO with both key 0 and key 1.
> 
> Since the implementation of kernel userspace access protection,
> PP bits have been set as follows:
> - PP00 for pages without _PAGE_USER
> - PP01 for pages with _PAGE_USER and _PAGE_RW
> - PP11 for pages with _PAGE_USER and without _PAGE_RW
> 
> For kernelspace segments, kernel accesses are performed with key 0
> and user accesses are performed with key 1. As PP00 is used for
> non _PAGE_USER pages, user can't access kernel pages not flagged
> _PAGE_USER while kernel can.
> 
> For userspace segments, both kernel and user accesses are performed
> with key 0, therefore pages not flagged _PAGE_USER are still
> accessible to the user.
> 
> This shouldn't be an issue, because userspace is expected to be
> accessible to the user. But unlike most other architectures, powerpc
> implements PROT_NONE protection by removing _PAGE_USER flag instead of
> flagging the page as not valid. This means that pages in userspace
> that are not flagged _PAGE_USER shall remain inaccessible.
> 
> To get the expected behaviour, just mimic other architectures in the
> TLB miss handler by checking _PAGE_USER permission on userspace
> accesses as if it was the _PAGE_PRESENT bit.
> 
> Note that this problem only is only for 603 cores. The 604+ have
> an hash table, and hash_page() function already implement the
> verification of _PAGE_USER permission on userspace pages.
> 
> Fixes: f342adca3afc ("powerpc/32s: Prepare Kernel Userspace Access Protection")
> Change-Id: I68bc5e5ff4542bdfcdcd12923fa96a5811707475
> Cc: stable@...r.kernel.org # v5.2+
> Reported-by: Christoph Plattner <christoph.plattner@...lesgroup.com>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
> Link: https://lore.kernel.org/r/4a0c6e3bb8f0c162457bf54d9bc6fd8d7b55129f.1612160907.git.christophe.leroy@csgroup.eu
> ---
>  arch/powerpc/kernel/head_book3s_32.S | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)

Both backports applied, thanks.

greg k-h