linux-kernel - [PATCH 19/20] efi/x86: don't map the entire kernel text RW for mixed mode

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Message-Id: <20200103113953.9571-20-ardb@kernel.org>
Date:   Fri,  3 Jan 2020 12:39:52 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux-efi@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Matthew Garrett <mjg59@...gle.com>
Subject: [PATCH 19/20] efi/x86: don't map the entire kernel text RW for mixed mode

The mixed mode thunking routine requires a part of it to be
mapped 1:1, and for this reason, we currently map the entire
kernel .text read/write in the EFI page tables, which is bad.

In fact, the kernel_map_pages_in_pgd() invocation that installs
this mapping is entirely redundant, since all of DRAM is already
1:1 mapped read/write in the EFI page tables when we reach this
point, which means that .rodata is mapped read-write as well.

So let's remap both .text and .rodata read-only in the EFI
page tables.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/platform/efi/efi_64.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index c13fa2150976..6ec58ff60b56 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -391,11 +391,11 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
 
 	efi_scratch.phys_stack = page_to_phys(page + 1); /* stack grows down */
 
-	npages = (_etext - _text) >> PAGE_SHIFT;
+	npages = (__end_rodata_aligned - _text) >> PAGE_SHIFT;
 	text = __pa(_text);
 	pfn = text >> PAGE_SHIFT;
 
-	pf = _PAGE_RW | _PAGE_ENC;
+	pf = _PAGE_ENC;
 	if (kernel_map_pages_in_pgd(pgd, pfn, text, npages, pf)) {
 		pr_err("Failed to map kernel text 1:1\n");
 		return 1;
-- 
2.20.1