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Subject: Re: [PATCH 05/13] efi/x86: don't map the entire kernel text RW for
 mixed mode
To:     Ard Biesheuvel <ardb@kernel.org>, linux-efi@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Young <dyoung@redhat.com>,
        Saravana Kannan <saravanak@google.com>
References: <20200113172245.27925-1-ardb@kernel.org>
 <20200113172245.27925-6-ardb@kernel.org>
From:   Jiri Slaby <jslaby@suse.cz>
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Message-ID: <63b125a4-6c62-fcdf-de22-d3bebe2dcbf5@suse.cz>
Date:   Wed, 8 Apr 2020 12:42:50 +0200
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On 13. 01. 20, 18:22, Ard Biesheuvel wrote:
> 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.

This patch causes unhandled page faults in mixed mode:

> BUG: unable to handle page fault for address: 000000001557ee88
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0003) - permissions violation
> PGD fd52063 P4D fd52063 PUD fd53063 PMD 154000e1
> Oops: 0003 [#1] SMP PTI
> CPU: 1 PID: 191 Comm: systemd-escape Not tainted
5.6.2-20.gb22bc26-default #1 openSUSE Tumbleweed (unreleased)
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0
02/06/2015
> RIP: 0008:0x3d2eed95
> Code: 8b 45 d4 8b 4d 10 8b 40 04 89 01 89 3b 50 6a 00 8b 55 0c 6a 00
8b 45 08 0f b6 4d e4 6a 01 31 f6 e8 ee c5 fc ff 83 c4 10 eb 07 <89> 03
be 05 00 00 80 a1 74 63 31 3d 83 c0 48 e8 44 d2 ff ff eb 05
> RSP: 0018:000000000fd66fa0 EFLAGS: 00010002
> RAX: 0000000000000001 RBX: 000000001557ee88 RCX: 000000003d1f1120
> RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000000000000001
> RBP: 000000000fd66fd8 R08: 000000001557ee88 R09: 0000000000000000
> R10: 0000000000000055 R11: 0000000000000000 R12: 0000000015bcf000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> FS:  00007f36ee9dc940(0000) GS:ffff9b903d700000(0000)
knlGS:0000000000000000
> CS:  0008 DS: 0018 ES: 0018 CR0: 0000000080050033
> CR2: 000000001557ee88 CR3: 000000000fd5e000 CR4: 00000000000006e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> Modules linked in: efivarfs
> CR2: 000000001557ee88

EFI apparently tries to write to now read-only memory.

See:
https://bugzilla.suse.com/show_bug.cgi?id=1168645

Reverting it on the top of 5.6 fixes the issue.

I am using
/usr/share/qemu/ovmf-ia32-code.bin
/usr/share/qemu/ovmf-ia32-vars.bin
from qemu-ovmf-ia32-202002-1.1.noarch rpm.

> Signed-off-by: Ard Biesheuvel <ardb@kernel.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;
> 

thanks,
-- 
js
suse labs