linux-kernel - Re: linux-next: x86: Unalbe to run x32 processes on the x86

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Message-ID: <20170321064539.GB30094@gmail.com>
Date:   Tue, 21 Mar 2017 07:45:39 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Adam Borowski <kilobyte@...band.pl>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: linux-next: x86: Unalbe to run x32 processes on the x86_64 kernel


* Andrei Vagin <avagin@...il.com> wrote:

> Here is my bisect log:
> [avagin@...top linux-next]$ git bisect log
> # bad: [f921b263d9602fb7873710c2df70671f2ffcf658] Add linux-next
> specific files for 20170320
> # good: [4495c08e84729385774601b5146d51d9e5849f81] Linux 4.11-rc2
> git bisect start 'HEAD' 'v4.11-rc2'
> # good: [adeec71e7b8a11ff44103cfa4e1c6002c27ae9ac] Merge
> remote-tracking branch 'drm/drm-next'
> git bisect good adeec71e7b8a11ff44103cfa4e1c6002c27ae9ac
> # bad: [e5b1cee1bb9d8a66b28d64db9d8250ae2b0803d2] Merge
> remote-tracking branch 'tty/tty-next'
> git bisect bad e5b1cee1bb9d8a66b28d64db9d8250ae2b0803d2
> # good: [81cde6aecf21113c6bc65bc944587db4cbd3f64d] Merge
> remote-tracking branch 'mmc/next'
> git bisect good 81cde6aecf21113c6bc65bc944587db4cbd3f64d
> # bad: [69d76288005784205678520826a74c28e49a1703] Merge
> remote-tracking branch 'tip/auto-latest'
> git bisect bad 69d76288005784205678520826a74c28e49a1703
> # good: [6d32edf2be6c4e0f08b102e2d3227278ebb477c9] Merge
> remote-tracking branch 'spi/for-next'
> git bisect good 6d32edf2be6c4e0f08b102e2d3227278ebb477c9
> # good: [61f63e383784bd0ab6529cfc95ddc59c713afcc9] Merge tag
> 'perf-core-for-mingo-4.12-20170316' of
> git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux into
> perf/core
> git bisect good 61f63e383784bd0ab6529cfc95ddc59c713afcc9
> # good: [3069c5a046ec92dc552194686c95e27c8c9e919d] Merge branch 'x86/asm'
> git bisect good 3069c5a046ec92dc552194686c95e27c8c9e919d
> # bad: [5b804ec2fd3272385ba7530c3a67d7402fb6d8a0] Merge branch 'x86/mm'
> git bisect bad 5b804ec2fd3272385ba7530c3a67d7402fb6d8a0
> # good: [06c830a48346643e195801460dfe16d96ba4dff5] x86/power: Add
> 5-level paging support
> git bisect good 06c830a48346643e195801460dfe16d96ba4dff5
> # good: [2bbbd194fdca01f694e2ca3fb447a1acf9d19f76] Merge branch 'x86/cpu'
> git bisect good 2bbbd194fdca01f694e2ca3fb447a1acf9d19f76
> # good: [69218e47994da614e7af600bf06887750ab6657a] x86: Remap GDT
> tables in the fixmap section
> git bisect good 69218e47994da614e7af600bf06887750ab6657a
> # bad: [74c8ce958dbf0b64f198becb5d8aa93afb967438] Merge branch 'linus'
> into x86/mm, to pick up a bugfix
> git bisect bad 74c8ce958dbf0b64f198becb5d8aa93afb967438
> # bad: [45fc8757d1d2128e342b4e7ef39adedf7752faac] x86: Make the GDT
> remapping read-only on 64-bit
> git bisect bad 45fc8757d1d2128e342b4e7ef39adedf7752faac
> # first bad commit: [45fc8757d1d2128e342b4e7ef39adedf7752faac] x86:
> Make the GDT remapping read-only on 64-bit

Just wondering, does the following commit fix it:

  5b781c7e317f x86/tls: Forcibly set the accessed bit in TLS segments

?

Also attached below.

Thanks,

	Ingo

=================>
>From 5b781c7e317fcf9f74475dc82bfce2e359dfca13 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@...nel.org>
Date: Sat, 18 Mar 2017 22:17:24 -0700
Subject: [PATCH] x86/tls: Forcibly set the accessed bit in TLS segments

For mysterious historical reasons, struct user_desc doesn't indicate
whether segments are accessed.  set_thread_area() has always programmed
segments as non-accessed, so the first write will set the accessed bit.
This will fault if the GDT is read-only.

Fix it by making TLS segments start out accessed.

If this ends up breaking something, we could, in principle, leave TLS
segments non-accessed and fix them up when we get the page fault.  I'd be
surprised, though -- AFAIK all the nasty legacy segmented programs (DOSEMU,
Wine, things that run on DOSEMU and Wine, etc.) do their nasty segmented
things using the LDT and not the GDT.  I assume this is mainly because old
OSes (Linux and otherwise) didn't historically provide APIs to do nasty
things in the GDT.

Fixes: 45fc8757d1d2 ("x86: Make the GDT remapping read-only on 64-bit")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Thomas Garnier <thgarnie@...gle.com>
Link: http://lkml.kernel.org/r/62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/tls.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 6c8934406dc9..dcd699baea1b 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -92,10 +92,17 @@ static void set_tls_desc(struct task_struct *p, int idx,
 	cpu = get_cpu();
 
 	while (n-- > 0) {
-		if (LDT_empty(info) || LDT_zero(info))
+		if (LDT_empty(info) || LDT_zero(info)) {
 			desc->a = desc->b = 0;
-		else
+		} else {
 			fill_ldt(desc, info);
+
+			/*
+			 * Always set the accessed bit so that the CPU
+			 * doesn't try to write to the (read-only) GDT.
+			 */
+			desc->type |= 1;
+		}
 		++info;
 		++desc;
 	}