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To:     linux-tip-commits@vger.kernel.org
Subject: [tip:x86/pti] x86/entry/32: Load task stack from x86_tss.sp1 in
 SYSENTER handler
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Commit-ID:  a6b744f3ce9d017dd86b28355de2d8e0d36496d4
Gitweb:     https://git.kernel.org/tip/a6b744f3ce9d017dd86b28355de2d8e0d36496d4
Author:     Joerg Roedel <jroedel@suse.de>
AuthorDate: Wed, 18 Jul 2018 11:40:40 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 20 Jul 2018 01:11:36 +0200

x86/entry/32: Load task stack from x86_tss.sp1 in SYSENTER handler

x86_tss.sp0 will be used to point to the entry stack later to use it as a
trampoline stack for other kernel entry points besides SYSENTER.

So store the real task stack pointer in x86_tss.sp1, which is otherwise
unused by the hardware, as Linux doesn't make use of Ring 1.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Pavel Machek <pavel@ucw.cz>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Waiman Long <llong@redhat.com>
Cc: "David H . Gutteridge" <dhgutteridge@sympatico.ca>
Cc: joro@8bytes.org
Link: https://lkml.kernel.org/r/1531906876-13451-4-git-send-email-joro@8bytes.org

---
 arch/x86/kernel/asm-offsets_32.c | 9 +++++++--
 arch/x86/kernel/process_32.c     | 2 ++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
index 15b3f45b69cd..82826f2275cc 100644
--- a/arch/x86/kernel/asm-offsets_32.c
+++ b/arch/x86/kernel/asm-offsets_32.c
@@ -46,9 +46,14 @@ void foo(void)
 	OFFSET(saved_context_gdt_desc, saved_context, gdt_desc);
 	BLANK();
 
-	/* Offset from the entry stack to task stack stored in TSS */
+	/*
+	 * Offset from the entry stack to task stack stored in TSS. Kernel entry
+	 * happens on the per-cpu entry-stack, and the asm code switches to the
+	 * task-stack pointer stored in x86_tss.sp1, which is a copy of
+	 * task->thread.sp0 where entry code can find it.
+	 */
 	DEFINE(TSS_entry2task_stack,
-	       offsetof(struct cpu_entry_area, tss.x86_tss.sp0) -
+	       offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
 	       offsetofend(struct cpu_entry_area, entry_stack_page.stack));
 
 #ifdef CONFIG_STACKPROTECTOR
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 0ae659de21eb..ec62cc77388d 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -290,6 +290,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	this_cpu_write(cpu_current_top_of_stack,
 		       (unsigned long)task_stack_page(next_p) +
 		       THREAD_SIZE);
+	/* SYSENTER reads the task-stack from tss.sp1 */
+	this_cpu_write(cpu_tss_rw.x86_tss.sp1, next_p->thread.sp0);
 
 	/*
 	 * Restore %gs if needed (which is common)