linux-kernel - [PATCH 3.19.y-ckt 018/155] x86/asm/entry: Create and use a 'TOP_OF_KERNEL_STACK

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1447370309-357-19-git-send-email-kamal@canonical.com>
Date:	Thu, 12 Nov 2015 15:16:12 -0800
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.19.y-ckt 018/155] x86/asm/entry: Create and use a 'TOP_OF_KERNEL_STACK_PADDING' macro

3.19.8-ckt10 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@...capital.net>

commit 3ee4298f440c81638cbb5ec06f2497fb7a9a9eb4 upstream.

x86_32, unlike x86_64, pads the top of the kernel stack, because the
hardware stack frame formats are variable in size.

Document this padding and give it a name.

This should make no change whatsoever to the compiled kernel
image. It also doesn't fix any of the current bugs in this area.

Signed-off-by: Andy Lutomirski <luto@...capital.net>
Acked-by: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/02bf2f54b8dcb76a62a142b6dfe07d4ef7fc582e.1426009661.git.luto@amacapital.net
[ Fixed small details, such as a missed magic constant in entry_32.S pointed out by Denys Vlasenko. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[ kamal: 3.19-stable prereq for
  eddd382 x86/process: Add proper bound checks in 64bit get_wchan() ]
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 arch/x86/include/asm/processor.h   |  3 ++-
 arch/x86/include/asm/thread_info.h | 27 +++++++++++++++++++++++++++
 arch/x86/kernel/entry_32.S         |  2 +-
 3 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index a092a0c..401a988 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -886,7 +886,8 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
 #define task_pt_regs(task)                                             \
 ({                                                                     \
        struct pt_regs *__regs__;                                       \
-       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
+       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
+				     TOP_OF_KERNEL_STACK_PADDING);     \
        __regs__ - 1;                                                   \
 })
 
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 547e344..c4d9694 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -13,6 +13,33 @@
 #include <asm/types.h>
 
 /*
+ * TOP_OF_KERNEL_STACK_PADDING is a number of unused bytes that we
+ * reserve at the top of the kernel stack.  We do it because of a nasty
+ * 32-bit corner case.  On x86_32, the hardware stack frame is
+ * variable-length.  Except for vm86 mode, struct pt_regs assumes a
+ * maximum-length frame.  If we enter from CPL 0, the top 8 bytes of
+ * pt_regs don't actually exist.  Ordinarily this doesn't matter, but it
+ * does in at least one case:
+ *
+ * If we take an NMI early enough in SYSENTER, then we can end up with
+ * pt_regs that extends above sp0.  On the way out, in the espfix code,
+ * we can read the saved SS value, but that value will be above sp0.
+ * Without this offset, that can result in a page fault.  (We are
+ * careful that, in this case, the value we read doesn't matter.)
+ *
+ * In vm86 mode, the hardware frame is much longer still, but we neither
+ * access the extra members from NMI context, nor do we write such a
+ * frame at sp0 at all.
+ *
+ * x86_64 has a fixed-length stack frame.
+ */
+#ifdef CONFIG_X86_32
+# define TOP_OF_KERNEL_STACK_PADDING 8
+#else
+# define TOP_OF_KERNEL_STACK_PADDING 0
+#endif
+
+/*
  * low level task data that entry.S needs immediate access to
  * - this struct should fit entirely inside of one cache line
  * - this struct shares the supervisor stack pages
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 000d419..3890e21 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -398,7 +398,7 @@ sysenter_past_esp:
 	 * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
 	 * pushed above; +8 corresponds to copy_thread's esp0 setting.
 	 */
-	pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+8+4*4)(%esp)
+	pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+TOP_OF_KERNEL_STACK_PADDING+4*4)(%esp)
 	CFI_REL_OFFSET eip, 0
 
 	pushl_cfi %eax
-- 
1.9.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/