linux-kernel - [tip:x86/urgent] x86/boot/64/clang: Use fixup_pointer() to access '__supported_pte

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Message-ID: <tip-4a09f0210c8b1221aae8afda8bd3a603fece0986@git.kernel.org>
Date:   Mon, 14 May 2018 05:38:47 -0700
From:   tip-bot for Alexander Potapenko <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, hpa@...or.com, tglx@...utronix.de,
        peterz@...radead.org, torvalds@...ux-foundation.org,
        glider@...gle.com, mingo@...nel.org, dave.hansen@...el.com
Subject: [tip:x86/urgent] x86/boot/64/clang: Use fixup_pointer() to access
 '__supported_pte_mask'

Commit-ID:  4a09f0210c8b1221aae8afda8bd3a603fece0986
Gitweb:     https://git.kernel.org/tip/4a09f0210c8b1221aae8afda8bd3a603fece0986
Author:     Alexander Potapenko <glider@...gle.com>
AuthorDate: Wed, 9 May 2018 11:18:22 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 14 May 2018 11:14:30 +0200

x86/boot/64/clang: Use fixup_pointer() to access '__supported_pte_mask'

Clang builds with defconfig started crashing after the following
commit:

  fb43d6cb91ef ("x86/mm: Do not auto-massage page protections")

This was caused by introducing a new global access in __startup_64().

Code in __startup_64() can be relocated during execution, but the compiler
doesn't have to generate PC-relative relocations when accessing globals
from that function. Clang actually does not generate them, which leads
to boot-time crashes. To work around this problem, every global pointer
must be adjusted using fixup_pointer().

Signed-off-by: Alexander Potapenko <glider@...gle.com>
Reviewed-by: Dave Hansen <dave.hansen@...el.com>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: dvyukov@...gle.com
Cc: kirill.shutemov@...ux.intel.com
Cc: linux-mm@...ck.org
Cc: md@...gle.com
Cc: mka@...omium.org
Fixes: fb43d6cb91ef ("x86/mm: Do not auto-massage page protections")
Link: http://lkml.kernel.org/r/20180509091822.191810-1-glider@google.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/head64.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 0c408f8c4ed4..2d29e47c056e 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -104,6 +104,12 @@ static bool __head check_la57_support(unsigned long physaddr)
 }
 #endif
 
+/* Code in __startup_64() can be relocated during execution, but the compiler
+ * doesn't have to generate PC-relative relocations when accessing globals from
+ * that function. Clang actually does not generate them, which leads to
+ * boot-time crashes. To work around this problem, every global pointer must
+ * be adjusted using fixup_pointer().
+ */
 unsigned long __head __startup_64(unsigned long physaddr,
 				  struct boot_params *bp)
 {
@@ -113,6 +119,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
 	p4dval_t *p4d;
 	pudval_t *pud;
 	pmdval_t *pmd, pmd_entry;
+	pteval_t *mask_ptr;
 	bool la57;
 	int i;
 	unsigned int *next_pgt_ptr;
@@ -196,7 +203,8 @@ unsigned long __head __startup_64(unsigned long physaddr,
 
 	pmd_entry = __PAGE_KERNEL_LARGE_EXEC & ~_PAGE_GLOBAL;
 	/* Filter out unsupported __PAGE_KERNEL_* bits: */
-	pmd_entry &= __supported_pte_mask;
+	mask_ptr = fixup_pointer(&__supported_pte_mask, physaddr);
+	pmd_entry &= *mask_ptr;
 	pmd_entry += sme_get_me_mask();
 	pmd_entry +=  physaddr;