linux-kernel - [PATCH v2] x86/pat: fix x86_has_pat

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Message-Id: <20220620113441.23961-1-jgross@suse.com>
Date:   Mon, 20 Jun 2022 13:34:41 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Juergen Gross <jgross@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH v2] x86/pat: fix x86_has_pat_wp()

x86_has_pat_wp() is using a wrong test, as it relies on the normal
PAT configuration used by the kernel. In case the PAT MSR has been
setup by another entity (e.g. BIOS or Xen hypervisor) it might return
false even if the PAT configuration is allowing WP mappings.

The correct way to test for WP support is:

1. Get the PTE protection bits needed to select WP mode by reading
   __cachemode2pte_tbl[_PAGE_CACHE_MODE_WP] (depending on the PAT MSR
   setting this might return protection bits for a stronger mode, e.g.
   UC-)
2. Translate those bits back into the real cache mode selected by those
   PTE bits by reading __pte2cachemode_tbl[__pte2cm_idx(prot)]
3. Test for the cache mode to be _PAGE_CACHE_MODE_WP

Fixes: 1f6f655e01ad ("x86/mm: Add a x86_has_pat_wp() helper")
Signed-off-by: Juergen Gross <jgross@...e.com>
---
V2:
- fix indexing into __pte2cachemode_tbl[]
---
 arch/x86/mm/init.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index d8cfce221275..914ac5f29fca 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -80,7 +80,9 @@ static uint8_t __pte2cachemode_tbl[8] = {
 /* Check that the write-protect PAT entry is set for write-protect */
 bool x86_has_pat_wp(void)
 {
-	return __pte2cachemode_tbl[_PAGE_CACHE_MODE_WP] == _PAGE_CACHE_MODE_WP;
+	uint16_t prot = __cachemode2pte_tbl[_PAGE_CACHE_MODE_WP];
+
+	return __pte2cachemode_tbl[__pte2cm_idx(prot)] == _PAGE_CACHE_MODE_WP;
 }

 enum page_cache_mode pgprot2cachemode(pgprot_t pgprot)
-- 
2.35.3